Brachiopods are a phylum of marine animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Articulate brachiopods have toothed hinges and simple opening and closing muscles, while inarticulate brachiopods have untoothed hinges and more complex muscles. In a typical brachiopod a stalk-like
pedicleIn zoology, the pedicle refers to a fleshy line brachiopods and some bivalve mollusks use to attach and anchor to a substrate. In brachiopods, the pedicle emerges from a pedicle opening. Some brachiopods do not have a functional pedicle, and thus do not have this pedicle opening....
projects from an opening in the hinge or from a hole in the larger valve, attaching the animal to the seabed but clear of silt that would obstruct the opening.
Brachiopods have a mantle that
secreteSecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
and lines the shell, and encloses the internal organs. The body occupies typically about one-third of the internal space inside the shell, nearest the hinge. The rest of the mantle encloses a water-filled space containing the
lophophoreThe lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...
, a crown of tentacles that
filtersFilter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
food particles out of the water. In all species the lophophore is supported by
cartilageCartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
and by a
hydrostatic skeletonA hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an...
. The lophophore filters food, mostly
phytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
, out of the water. From there the food is transported in succession to: the grooves along the bases of the lophophore's tentacles; the mouth;
pharynxThe human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
; oesophagus; and finally the
stomachThe stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
, where the food is
digestedDigestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....
. Nutrients are transported from the stomach throughout the
coelomThe coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
(main body cavity), include the mantle lobes, by cilia. The wastes produced by
metabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
are broken into
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, which is eliminated by
diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
through the mantle and lophophore. The lophophore and mantle are the only surfaces that absorb
oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and eliminate
carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. Oxygen seems to be distributed by the fluid of the coelom. The heart is above the stomach, and the blood vessels connect it to the major organs. However, the main function of the blood may be to deliver nutrients. The maximum oxygen consumption of brachiopods is low, and their minimum requirement is not measurable.
The "brain" of adult articulates consists of two
gangliaIn anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
, one above and the other below the oesophagus. Adult inarticulates have only the lower ganglion. Nerves run to the lophophore, the mantle lobes and the muscles that operate the valves. Many brachiopods close their valves if shadows appear above them, but the cells responsible for this are unknown. Some brachiopods have
statocystThe statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
s which detect changes in the animals'
balanceEquilibrioception or sense of balance is one of the physiological senses. It helps prevent humans and animals from falling over when walking or standing still. Balance is the result of a number of body systems working together: the eyes , ears and the body's sense of where it is in space ideally...
.
Lifespans range from 3 to over 30 years. Ripe
gameteA gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s (
ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
or
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
) float from the
gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s into the main coelom and then exit into the mantle cavity. The
larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e of inarticulate brachiopods are miniature adults, with lophophores that enable the larvae to feed and swimming for months, until the animals become heavy enough to settle to the seabed. Larvae of articulate species are different from the adult forms, live only on yolk, remain only among the plankton for only a few days, and then
metamorphosingMetamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
.
In addition to the traditional classification into inarticulate and articulate brachiopods, two approaches appeared in the 1990s: grouping the inarticulate Craniida with articulate brachiopods, as both used the same material in the
mineralA mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
layers of the shell; and making the Craniida a third group, as their outer
organicOrganic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
layer is different from that of either the others. However, some taxonomists believe it is premature to suggest higher levels of classification such as
orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, and recommend a bottom-up approach that identifies genera and then groups these into intermediate groups. Traditionally brachiopods have been regarded as members of or as a sister group to the
deuterostomeDeuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s, a super-phylum which includes
chordateChordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
s and
echinodermEchinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s. One type of analysis of brachiopods' evolutionary relationships has always placed brachiopods as
protostomeProtostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s, while another type has split between placing brachiopods among the protostomes or the deuterostomes.
In 2003 it was suggested that brachiopods evolved from an ancestor similar to
HalkieriaHalkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages...
, a
slugSlug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
-like animal with "
chain mail"Chain Mail" is a single by Mancunian band James, released in March 1986 by Sire Records, the first after the band defected from Factory Records. The record was released in two different versions, as 7" single and 12" EP, with different artworks by John Carroll and, confusingly, under different...
" on its back and a shell at the front and rear end, and that the ancestral brachiopod converted its shells into a pair of valves by folding the rear part of its body under its front. However, new fossils found in 2007 to 2008 showed that
tommotiidTommotiids are Cambrian shelly fossils of uncertain affinity.Unlike most of the small shelly fauna, tommotiids are mineralised with calcium phosphate rather than calcium carbonate...
s' "chain mail" formed the tube of a
sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
animal, and that one resembled
phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s, which are close relatives or a sub-group of brachiopod, while the other tommotiid bearing two symmetrical plates that might be an early form of brachiopod valves. Lineages that have both fossil and extant brachiopods appeared in the early
CambrianThe Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
,
OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
and
CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
periods respectively. Other lineages have arisen and then become extinct, sometimes during severe mass extinctions. At their peak in the
PaleozoicThe Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
era the brachiopods were among the most abundant filter-feeders and reef-builders, and occupied other
ecological nicheIn ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s, including swimming in the jet-propulsion style of
scallopA scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...
s. Brachiopod fossils have been useful indicators of climate changes during the
PaleozoicThe Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
era. However, after the Permian–Triassic extinction event, brachiopods recovered only a third of their former diversity. A study in 2007 concluded that brachiopods were especially vulnerable to the Permian–Triassic extinction, as they built calcareous hard parts (made of
calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
) and had low metabolic rates and weak respiratory systems. It was often thought that brachiopods were in decline after the Permian–Triassic extinction, and were out-competed by bivalves. However, a study in 1980 concluded that: both brachiopods and bivalves increased all the way from the Paleozoic to modern times, but bivalves increased faster; after the Permian–Triassic extinction, brachiopods for the first time were less diverse than bivalves.
Brachiopods live only in the sea, and most species avoid locations with strong currents or waves. Articulate species have larvae that settle in quickly and form dense populations in well-defined areas, while inarticulate larvae swimming for up to a month and have wide ranges. Brachiopods now live mainly in cold and low-light conditions. Fish and crustaceans seem to find brachiopod flesh distasteful and seldom attack them. Among brachiopods only the lingulids have been fished commercially, on a very small scale. One brachiopod species may be a measure of environmental conditions around an oil terminal being built in Russia on the shore of the
Sea of JapanThe Sea of Japan is a marginal sea of the western Pacific Ocean, between the Asian mainland, the Japanese archipelago and Sakhalin. It is bordered by Japan, North Korea, Russia and South Korea. Like the Mediterranean Sea, it has almost no tides due to its nearly complete enclosure from the Pacific...
.
Name
The scientific name "Brachiopod" is formed from the
Ancient GreekAncient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
words βραχίων ("arm") and πούς ("foot"). They are often known as "lamp shells", since the curved shells of the
classIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
TerebratulidaTerebratulids are one of only three living orders of articulate brachiopods, the others being the Rhynchonellida and the Thecideida. Craniida and Lingulida include living brachiopods, but are inarticulates. The name, Terebratula, may be derived from the Latin "terebra", meaning "hole-borer"...
look rather like pottery oil-lamps.
Shells and their mechanisms
Modern brachiopods range from 1 to 100 mm (0.0393700787401575 to 3.9 in) long, and most species are about 10 to 30 mm (0.393700787401575 to 1.2 in). Each has two valves (shell sections) which are biomineralized. The valves cover the dorsal and ventral surface of the animal, unlike bivalve molluscs whose shells cover the lateral surfaces. The brachial valve bears on its inner surface the brachia ("arms") from which the phylum gets its name, and which supports the
lophophoreThe lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...
, used for
filteringFilter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
and
respirationAquatic respiration is the process whereby an aquatic animal obtains oxygen from water.-Fish:In most fish respiration takes place through gills. Lungfish, however, possess one or two lungs...
. The other is known as the pedicle valve, as its inner surface bears the stalk-like pedicle by which most brachiopods attach themselves to surfaces. The brachial and pedicle valves are often called the dorsal ("upper") and ventral ("lower"), but some
paleontologistsPaleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
regard "dorsal" and "ventral" as incorrect terms, since they believe that the "ventral" valve was formed by folding of the upper surface under the body. Irrespective of this debate, the valves of brachiopods are differently arranged of those of bivalve molluscs, which lie on the left and right sides of the body. In most brachiopod species both valves are convex, the surfaces often bear growth lines or other ornaments, and the pedicle valve is larger than the brachial. However, the lingulids, which burrow into the seabed, have valves that are smoother, flatter and of similar size and shape.
Brachiopod valves have a
hingeA hinge is a type of bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation. Hinges may be made of flexible material or of moving components...
, in which the rearmost end of the brachial valve rocks on an internal projection of the pedicle valve. The internal projections of articulate ("jointed") brachiopods have teeth which fit into sockets on the brachial valve, an arrangement that locks the valves together. Inarticulate brachiopods have no matching teeth and sockets, and their valves are held together only by muscles.
All brachiopods have adductor muscles, that are set on the inside of the pedicle valve and close the valves by pulling on the part of the brachial valve ahead of the hinge. These muscles have both "quick" fibers that close the valves in emergencies and "catch" fibers that are slower but can keep the valves closed for long periods. Articulate brachiopods open the valves by means of abductor muscles, also known as diductors, which lie further to the rear and pull on the part of the brachial valve behind the hinge. Inarticulate brachiopods use a different opening mechanism, in which muscles reduce the length of the
coelomThe coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
(main body cavity) and make it bulge outwards, pushing the valves apart. Both
classesIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
open the valves to an angle of about 10°. The more complex set of muscles employed by inarticulate brachiopods can also operate the valves as scissors, a mechanism that lingulids use to burrow.
Each valve consists of three layers, an outer
periostracumThe periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including mollusks and brachiopods. Among mollusks it is primarily seen in snails and clams, i.e. in bivalves and gastropods, but it is also found in cephalopods such as the...
made of
organic compoundAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s and two biomineralized layers. Articulated brachiopods have a periostracum made of
proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, a "primary layer" of
calciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
(a form of
calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
) under that, and finally a mixture of proteins and calcite. Inarticulate brachiopod shells have a similar sequence of layers, but their composition is different from that articulated brachiopods and also varies between the
classesIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of inarticulate brachiopods. Linguids and discinids, which have pedicles, have a
matrixIn biology, matrix is the material between animal or plant cells, in which more specialized structures are embedded, and a specific part of the mitochondrion that is the site of oxidation of organic molecules. The internal structure of connective tissues is an extracellular matrix...
of
glycosaminoglycanGlycosaminoglycans or mucopolysaccharides are long unbranched polysaccharides consisting of a repeating disaccharide unit. The repeating unit consists of a hexose or a hexuronic acid, linked to a hexosamine .-Production:Protein cores made in the rough endoplasmic reticulum are posttranslationally...
s (long, unbranched
polysaccharidePolysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
s), in which other material are embedded:
chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
in the periostracum;
apatiteApatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
containing
calcium phosphateCalcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
in the primary biomineralized layer; and a complex mixture in the innermost layer, containing
collagenCollagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
and other proteins, chitinophosphate and apatite.
CraniidsThe Craniidae are a family of brachiopods . They are the only members of the order Craniida and the monotypic suborder Craniidina and superfamily Cranioidea; consequently, the latter two taxa are presently redundant and not used very often...
, which have no pedicle and cement themselves directly to hard surfaces, have a periostracum of
chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
and mineralized layers of calcite.
Mantle
Like molluscs, brachiopods have a mantle, an
epitheliumEpithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
that lines the shell and encloses the internal organs. The brachiopod body occupies only about one-third of the internal space inside the shell, nearest the hinge. The rest of the space is lined with the mantle
lobesIn anatomy, a lobe is a clear anatomical division or extension that can be determined without the use of a microscope This is in contrast to a lobule, which is a clear division only visible histologically....
, extensions that enclose a water-filled space in which sits the lophophore. The coelom extends into each lobe as a network of canals, which carry nutrients to the edges of the mantle.
Relatively new cells in a groove on the edges of the mantle secrete material that extends the periostracum. These cells are gradually displaced to the underside of the mantle by more recent cells in the groove, and switch to secreting the mineralized material of the shell valves. In other words, on the edge of the valve the periostracum is extended first, and then reinforced by extension of the mineralized layers under the periostracum. In most species the edge of the mantle also bears movable bristles, often called
chaetaA chaeta or cheta is a chitinous bristle or seta found on an insect, arthropod or annelid worms such as the earthworm, although the term is also frequently used to describe similar structures in other invertebrates. The plural form is chaetae or chetae.In the Polychaeta, they are located on the...
e or setae, that may help defend the animals and may act as
sensorA sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s. In some brachiopods groups of chaetae help to channel the flow of water into and out of the mantle cavity.
In most brachiopods,
diverticulaA diverticulum is medical or biological term for an outpouching of a hollow structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false....
(hollow extensions) of the mantle penetrate through the mineralized layers of the valves into the periostraca. The function of these diverticula is uncertain and it is suggested that they may be storage chambers for chemicals such as
glycogenGlycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
, may
secreteSecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
repellents to deter organisms that stick to the shell or may help in
respirationBreathing is the process that moves air in and out of the lungs. Aerobic organisms require oxygen to release energy via respiration, in the form of the metabolism of energy-rich molecules such as glucose. Breathing is only one process that delivers oxygen to where it is needed in the body and...
. Experiments show that a brachiopod's
oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
consumption drops if
petroleum jellyPetroleum jelly, petrolatum, white petrolatum or soft paraffin, CAS number 8009-03-8, is a semi-solid mixture of hydrocarbons , originally promoted as a topical ointment for its healing properties...
is smeared on the shell, clogging the diverticula.
Lophophore
Like bryozoans and
phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s, brachiopods have a lophophore, a crown of tentacles whose cilia (fine hairs) create a water current that enables them to
filterFilter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
food particles out of the water. However a bryozoan or phoronid lophophore is a ring of tentacles mounted on a single, retracted stalk, while the basic form of the brachiopod lophophore is U-shaped, forming the brachia ("arms") from which the phylum gets its name. Brachiopod lophophores are non-retractable and occupy up to two-thirds of the internal space, in the frontmost area where the valves gape when opened. To provide enough filtering capacity in this restricted space, lophophores of larger brachiopods are folded in moderately to very complex shapes – loops and coils are common, and some species' lophophores resemble a hand with the fingers splayed. In all species the lophophore is supported by
cartilageCartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
and by a
hydrostatic skeletonA hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an...
(in other words by the pressure of its internal fluid), and the fluid extends into the tentacles. Some articulate brachiopods also have a brachidium, a calcareous support for the lophophore attached to the inside of the brachial valve.
The tentacles bear cilia (fine mobile hairs) on their edges and along the center. The beating of the outer cilia drives a water current from the tips of the tentacles to their bases, where it exits. Food particles that collide with the tentacles are trapped by
mucusIn vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
, and the cillia down the middle drive this mixture to the base of the tentacles. A brachial groove runs round the bases of the tentacles, and its own cilia pass food along the groove towards the mouth. The method used by brachiopods is known as "upstream collecting", as food particles are captured as they enter the field of cilia that creates the feeding current. This method is used by the related
phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s and bryozoans, and also by pterobranchs. Entoprocts use a similar-looking crown of tentacles, but it is solid and the flow runs from bases to tips, forming a "downstream collecting" system that catches food particles as they are about to exit.
Attachment to substrate
Most modern species attach to hard surfaces by means of a cylindrical pedicle ("stalk"), an extension of the body wall. This has a chitinous
cuticleA cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
(non-cellular "skin") and protrudes through an opening in the hinge. However, some
generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
such as the inarticulate Crania and the articulate Lacazella have no pedicle, and cement the rear of the "pedicle" valve to a surface so that the front is slightly inclined up away from the surface. In a few articulate genera such as Neothyris and Anakinetica, the pedicles wither as the adults grow and finally lie loosely on the surface. In these genera the shells are thickened and shaped so that the opening of the gaping valves is kept free of the sediment.
Pedicles of inarticulate species are extensions of the main coelom, which houses the internal organs. A layer of longitudinal muscles lines the
epidermisIn anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...
of the pedicle. Members of the
orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Lingulida have long pedicles, which they use to burrow into soft substrates, to raise the shell to the opening of the burrow to feed, and to retract the shell when disturbed. A lingulid moves its body up and down the top two-thirds of the burrow, while the remaining third is occupied only by the pedicle, with a bulb on the end that builds a "concrete" anchor. However, the pedicles of the order Discinida are short and attach to hard surfaces.
An articulate pedicle has no coelom, is constructed from a different part of the
larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
l body, and has a core composed of
connective tissue"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...
. Muscles at the rear of the body can straighten, bend or even rotate the pedicle. The far end of the pedicle generally has rootlike extensions or short papillae ("bumps") which attach to hard surfaces. However, articulate brachiopods of genus Chlidonophora use a branched pedicle to anchor in
sedimentSediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
. The pedicle emerges from the pedicle valve, either through a notch in the hinge or, in species where the pedicle valve is longer than the brachial, from a hole where the pedicle valve doubles back to touch the brachial valve. Some species stand with the front end upwards, while others lie horizontal with the pedicle valve uppermost.
Feeding and excretion
The water flow enters the lophophore from the sides of the open valves and exits at the front of the animal. In lingulids the entrance and exit channels are formed by groups of chaetae that function as funnels. In other brachiopods the entry and exit channels are organized by the shape of the lophophore. The lophophore captures food particles, especially
phytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
(tiny photosynthetic organisms), and deliver them to the mouth via the brachial grooves along the bases of the tentacles.
The mouth is at the base of the lophophore. Food passes through the mouth, muscular
pharynxThe human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
("throat") and oesophagus ("gullet"), all of which are lined with cilia and cells that secrete
mucusIn vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
and digestive enzymes. The
stomachThe stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
wall has branched ceca ("pouches") where food is digested, mainly within the cells.
Nutrients are transported throughout the coelom, including the mantle lobes, by cilia. The wastes produced by
metabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
are broken into
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, which is eliminated by
diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
through the mantle and lophophore. Brachiopods have metanephridia, used by many
phylaIn biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
to excrete ammonia and other dissolved wastes. However, brachiopods have no sign of the podocytes which perform the first phase of excretion in this process, and brachiopod metanephridia appear to be used only to emit
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
and
ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
.
The majority of food consumed by brachiopods is digestible, with very little solid waste produced. The cilia of the lophophore can change direction to eject isolated particles of indigestible matter. If the animal encounters larger lumps of undesired matter, the cilia lining the entry channels pause and the tentacles in contact with the lumps move apart to form large gaps and then slowly use their cilia to dump the lumps onto the lining of the mantle. This has its own cilia, which wash the lumps out through the opening between the valves. If the lophophore is clogged, the adductors snap the valves sharply, which creates a "sneeze" that clears the obstructions. In some inarticulate brachiopods the digestive tract is U-shaped and ends with an anus that eliminates solids from the front of the body wall. Other inarticulate brachiopods and all articulate brachiopods have a curved gut that ends blindly, with no anus. These animals bundle solid waste with mucus and periodically "sneeze" it out, using sharp contractions of the gut muscles.
Circulation and respiration
The lophophore and mantle are the only surfaces that absorb
oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and eliminate
carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. Oxygen seems to be distributed by the fluid of the coelom, which is circulated through the mantle and driven either by contractions of the lining of the coelom or by beating of its cilia. In some species oxygen is partly carried by the
respiratory pigmentA respiratory pigment is a molecule, such as hemoglobin in humans, that increases the oxygen-carrying capacity of the blood. The four most common invertebrate respiratory pigments are hemoglobin, haemocyanin, haemerythrin and chlorocruorin...
hemerythrinHemerythrin is an oligomeric protein responsible for oxygen transport in the marine invertebrate phyla of sipunculids, priapulids, brachiopods, and in a single annelid worm, magelona. Recently, hemerythrin was discovered in methanotrophic bacterium Methylococcus capsulatus...
, which is transported in coelomocyte cells.
Brachiopods also have colorless
bloodBlood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
, circulated by a muscular heart which lies in the dorsal part of the body above the stomach. The blood passes through vessels that extend to the front and back of the body, and branch to organs including the lophophore at the front and the gut, muscles, gonads and nephridia at the rear. The blood circulation seems not to be completely closed, and the coelomic and blood fluids must mix to a degree. The main function of the blood may be to deliver nutrients.
Nervous system and senses
The "brain" of adult articulates consists of two
gangliaIn anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
, one above and the other below the oesophagus. Adult inarticulates have only the lower ganglion. From the ganglia and the
commissureA commissure is the place where two things are joined. The term is used especially in the fields of anatomy and biology.In anatomy, commissure refers to a bundle of nerve fibers that cross the midline at their level of origin or entry .* The most common usage of the term refers to the brain's...
s where they join, nerves run to the lophophore, the mantle lobes and the muscles that operate the valves. The edge of the mantle has probably the greatest concentration of sensors. Although not directly connected to sensory neurons, the mantle's
chaetaA chaeta or cheta is a chitinous bristle or seta found on an insect, arthropod or annelid worms such as the earthworm, although the term is also frequently used to describe similar structures in other invertebrates. The plural form is chaetae or chetae.In the Polychaeta, they are located on the...
e probably send tactile signals to receptors in the
epidermisIn anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...
of the mantle. Many brachiopods close their valves if shadows appear above them, but the cells responsible for this are unknown. Some brachiopods have
statocystThe statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
s which detect changes in the animals' position.
Reproduction and lifecycle
Lifespans range from 3 to over 30 years. Adults of most species are of one sex throughout their lives. The
gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s are masses of developing
gameteA gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s (
ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
or
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
), and most species have four gonads, two in each valve. Those of articulates lie in the channels of the mantle lobes, while those of inarticulates lie near the gut. Ripe gametes float into the main coelom and then exit into the mantle cavity via the metanephridia, which open on either side of the mouth. Most species release both ova and sperm into the water, but females of some species keep the
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s in brood chambers until the larvae hatch.
The
cell divisionIn embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
in the embryo is radial (cells form in stacks of rings directly above each other), holoblastic (cells are separate, although adjoining) and regulative (the type of tissue into which a cell develops is controlled by interactions between adjacent cells, rather than rigidly within in each cell). While some animals develop the mouth and
anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
by deepening the
blastoporeA blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...
, a "dent" in the surface of the early embryo, the blastopore of brachiopods closes up, and their mouth and anus develop from new openings.
The
larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e of inarticulates swim as
planktonPlankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
for months and are like miniature adults, with valves, mantle lobes, a pedicle that coils in the mantle cavity, and a small lophophore, which is used for both feeding and swimming – except that
CraniidsThe Craniidae are a family of brachiopods . They are the only members of the order Craniida and the monotypic suborder Craniidina and superfamily Cranioidea; consequently, the latter two taxa are presently redundant and not used very often...
have no pedicle. As the shell becomes heavier, the juvenile sinks to the bottom and becomes a sessile adult. The larvae of articulate species live only on yolk, and remain among the plankton for only a few days. This type of larva has a ciliated frontmost lobe that becomes the body and lophophore, a rear lobe that becomes the pedicle, and a mantle like a skirt, with the hem towards the rear. On
metamorphosingMetamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
into an adult, the pedicle attaches to a surface, the front lobe develops the lophophore and other organs, and the mantle rolls up over the front lobe and starts to
secreteSecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
the shell.
The maximum oxygen consumption of brachiopods is low, and their minimal requirement is not measurable. In cold seas, brachiopod growth is seasonal and the animals often lose weight in winter. These variations in growth often form growth lines in the shells. Members of some
generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
have survived for a year in aquaria without food.
Taxonomy
The fossil brachiopod
generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
have great diversity but only a few
skeletalThe skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
characteristics, while the modern genera have much lower diversity but provide soft-bodied characteristics as well as skeletal ones – and both sets of specimens have limitations that make it difficult to produce a comprehensive classification of brachiopods. The phylum also has experienced significant
convergent evolutionConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
and reversals (in which a more recent group seems to have lost a characteristic that is seen in an intermediate group, reverting to a characteristic last seen in an older group). Hence some brachiopod
taxonomistsTaxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
believe it is premature to define higher levels of classification such as
orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, and recommend instead a bottom-up approach that identifies genera and then groups these into intermediate groups.
However, other taxonomists believe that some patterns of characteristics are sufficiently stable to make higher-level classifications worthwhile, although there are different views about what the higher-level classifications should be.
The "traditional" classification was defined in 1869; two further approaches were established in the 1990s:
- In the "traditional" classification, the Articulata have toothed hinges between the valves, while the hinges of the Inarticulata are held together only by muscles.
- A classification devised in the 1990s, based on the materials of which the shells are based, united the Craniida and the "articulate" brachiopods in the Calciata, which have calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
shells. The LingulidaLingulida is an order of brachiopods....
and Discinida, combined in the LingulataLingulata is a class of brachiopod, among the oldest of all brachiopods having existed since the Cambrian period . They are also among the most morphologically conservative of the brachiopods, having lasted from their earliest appearance to the present with very little change in shape...
, have shells made of chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
and calcium phosphateCalcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
.
- A three-part scheme, also from the 1990s, places the Craniida in a separate group of its own, the Craniformea. The Lingulida and Discinida are grouped as Linguliformea
Linguliformea is a subphylum of inarticulate brachiopods. These were the earliest of brachiopods, ranging from the Cambrian into the Holocene. They rapidly diversified during the Cambrian into the Ordovician, but most families went extinct by the end of the Devonian.The articulation in these...
, and the Rhynchonellida and Terebratulida as RhynchonelliformeaRhynchonelliformea is the name now given to the articulate brachiopods, Class Articulata, revised as a subphylum. Articulate brachiopods are those with hard, articulated, shells with a simple set of opening and closing muscles....
.
Three high-level classifications of brachiopods
| "Traditional" classification | Inarticulata | Articulata |
| "Calciata" approach | Lingulata Lingulata is a class of brachiopod, among the oldest of all brachiopods having existed since the Cambrian period . They are also among the most morphologically conservative of the brachiopods, having lasted from their earliest appearance to the present with very little change in shape...
| Calciata |
Three-part approach
{{Distinguish|Branchiopoda}}
{{automatic taxobox
| taxon = Brachiopoda
| fossil_range = {{fossil range|Lower Cambrian|Recent}}
| image = PlatystrophiaOrdovician.jpg
| image_caption = Vinlandostrophia ponderosa (OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
). Scale bar is 5.0 mm.
| display parents = 3
| authority = DumérilAndré Marie Constant Duméril was a French zoologist. He was professor of anatomy at the Muséum National d'Histoire Naturelle from 1801 to 1812, when he became professor of herpetology and ichthyology...
, 1806{{sfn|Zvyagintsev etc: Brachio fouling|(2007)||}}
| subdivision_ranks = SubphylaIn life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum.Not all phyla are divided into subphyla...
and classIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
es
| subdivision = See taxonomy
| diversity_link = List of brachiopod genera
| diversity = About 100 living generaGenera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...
.
About 5,000 fossil genera.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
}}
Brachiopods are a phylum of marine animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Articulate brachiopods have toothed hinges and simple opening and closing muscles, while inarticulate brachiopods have untoothed hinges and more complex muscles. In a typical brachiopod a stalk-like pedicleIn zoology, the pedicle refers to a fleshy line brachiopods and some bivalve mollusks use to attach and anchor to a substrate. In brachiopods, the pedicle emerges from a pedicle opening. Some brachiopods do not have a functional pedicle, and thus do not have this pedicle opening....
projects from an opening in the hinge or from a hole in the larger valve, attaching the animal to the seabed but clear of silt that would obstruct the opening.
Brachiopods have a mantle that secreteSecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
and lines the shell, and encloses the internal organs. The body occupies typically about one-third of the internal space inside the shell, nearest the hinge. The rest of the mantle encloses a water-filled space containing the lophophoreThe lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...
, a crown of tentacles that filtersFilter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
food particles out of the water. In all species the lophophore is supported by cartilageCartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
and by a hydrostatic skeletonA hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an...
. The lophophore filters food, mostly phytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
, out of the water. From there the food is transported in succession to: the grooves along the bases of the lophophore's tentacles; the mouth; pharynxThe human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
; oesophagus; and finally the stomachThe stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
, where the food is digestedDigestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....
. Nutrients are transported from the stomach throughout the coelomThe coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
(main body cavity), include the mantle lobes, by cilia. The wastes produced by metabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
are broken into ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, which is eliminated by diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
through the mantle and lophophore. The lophophore and mantle are the only surfaces that absorb oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and eliminate carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. Oxygen seems to be distributed by the fluid of the coelom. The heart is above the stomach, and the blood vessels connect it to the major organs. However, the main function of the blood may be to deliver nutrients. The maximum oxygen consumption of brachiopods is low, and their minimum requirement is not measurable.
The "brain" of adult articulates consists of two gangliaIn anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
, one above and the other below the oesophagus. Adult inarticulates have only the lower ganglion. Nerves run to the lophophore, the mantle lobes and the muscles that operate the valves. Many brachiopods close their valves if shadows appear above them, but the cells responsible for this are unknown. Some brachiopods have statocystThe statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
s which detect changes in the animals' balanceEquilibrioception or sense of balance is one of the physiological senses. It helps prevent humans and animals from falling over when walking or standing still. Balance is the result of a number of body systems working together: the eyes , ears and the body's sense of where it is in space ideally...
.
Lifespans range from 3 to over 30 years. Ripe gameteA gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s (ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
or spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
) float from the gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s into the main coelom and then exit into the mantle cavity. The larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e of inarticulate brachiopods are miniature adults, with lophophores that enable the larvae to feed and swimming for months, until the animals become heavy enough to settle to the seabed. Larvae of articulate species are different from the adult forms, live only on yolk, remain only among the plankton for only a few days, and then metamorphosingMetamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
.
In addition to the traditional classification into inarticulate and articulate brachiopods, two approaches appeared in the 1990s: grouping the inarticulate Craniida with articulate brachiopods, as both used the same material in the mineralA mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
layers of the shell; and making the Craniida a third group, as their outer organicOrganic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
layer is different from that of either the others. However, some taxonomists believe it is premature to suggest higher levels of classification such as orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, and recommend a bottom-up approach that identifies genera and then groups these into intermediate groups. Traditionally brachiopods have been regarded as members of or as a sister group to the deuterostomeDeuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s, a super-phylum which includes chordateChordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
s and echinodermEchinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s. One type of analysis of brachiopods' evolutionary relationships has always placed brachiopods as protostomeProtostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s, while another type has split between placing brachiopods among the protostomes or the deuterostomes.
In 2003 it was suggested that brachiopods evolved from an ancestor similar to HalkieriaHalkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages...
, a slugSlug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
-like animal with "chain mail"Chain Mail" is a single by Mancunian band James, released in March 1986 by Sire Records, the first after the band defected from Factory Records. The record was released in two different versions, as 7" single and 12" EP, with different artworks by John Carroll and, confusingly, under different...
" on its back and a shell at the front and rear end, and that the ancestral brachiopod converted its shells into a pair of valves by folding the rear part of its body under its front. However, new fossils found in 2007 to 2008 showed that tommotiidTommotiids are Cambrian shelly fossils of uncertain affinity.Unlike most of the small shelly fauna, tommotiids are mineralised with calcium phosphate rather than calcium carbonate...
s' "chain mail" formed the tube of a sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
animal, and that one resembled phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s, which are close relatives or a sub-group of brachiopod, while the other tommotiid bearing two symmetrical plates that might be an early form of brachiopod valves. Lineages that have both fossil and extant brachiopods appeared in the early CambrianThe Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
, OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
and CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
periods respectively. Other lineages have arisen and then become extinct, sometimes during severe mass extinctions. At their peak in the PaleozoicThe Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
era the brachiopods were among the most abundant filter-feeders and reef-builders, and occupied other ecological nicheIn ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s, including swimming in the jet-propulsion style of scallopA scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...
s. Brachiopod fossils have been useful indicators of climate changes during the PaleozoicThe Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
era. However, after the Permian–Triassic extinction event, brachiopods recovered only a third of their former diversity. A study in 2007 concluded that brachiopods were especially vulnerable to the Permian–Triassic extinction, as they built calcareous hard parts (made of calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
) and had low metabolic rates and weak respiratory systems. It was often thought that brachiopods were in decline after the Permian–Triassic extinction, and were out-competed by bivalves. However, a study in 1980 concluded that: both brachiopods and bivalves increased all the way from the Paleozoic to modern times, but bivalves increased faster; after the Permian–Triassic extinction, brachiopods for the first time were less diverse than bivalves.
Brachiopods live only in the sea, and most species avoid locations with strong currents or waves. Articulate species have larvae that settle in quickly and form dense populations in well-defined areas, while inarticulate larvae swimming for up to a month and have wide ranges. Brachiopods now live mainly in cold and low-light conditions. Fish and crustaceans seem to find brachiopod flesh distasteful and seldom attack them. Among brachiopods only the lingulids have been fished commercially, on a very small scale. One brachiopod species may be a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of JapanThe Sea of Japan is a marginal sea of the western Pacific Ocean, between the Asian mainland, the Japanese archipelago and Sakhalin. It is bordered by Japan, North Korea, Russia and South Korea. Like the Mediterranean Sea, it has almost no tides due to its nearly complete enclosure from the Pacific...
.
Name
The scientific name "Brachiopod" is formed from the Ancient GreekAncient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
words βραχίων ("arm") and πούς ("foot").{{sfn|Shorter Oxford English Dictionary|(2002)|loc=entry "Brachiopod"}} They are often known as "lamp shells", since the curved shells of the classIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
TerebratulidaTerebratulids are one of only three living orders of articulate brachiopods, the others being the Rhynchonellida and the Thecideida. Craniida and Lingulida include living brachiopods, but are inarticulates. The name, Terebratula, may be derived from the Latin "terebra", meaning "hole-borer"...
look rather like pottery oil-lamps.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
Shells and their mechanisms
Modern brachiopods range from 1 to 100 mm (0.0393700787401575 to 3.9 in) long, and most species are about 10 to 30 mm (0.393700787401575 to 1.2 in).{{sfn|Cohen: Brachiopoda ELS|(2002)|}} Each has two valves (shell sections) which are biomineralized. The valves cover the dorsal and ventral surface of the animal, unlike bivalve molluscs whose shells cover the lateral surfaces. The brachial valve bears on its inner surface the brachia ("arms") from which the phylum gets its name, and which supports the lophophoreThe lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...
, used for filteringFilter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
and respirationAquatic respiration is the process whereby an aquatic animal obtains oxygen from water.-Fish:In most fish respiration takes place through gills. Lungfish, however, possess one or two lungs...
. The other is known as the pedicle valve, as its inner surface bears the stalk-like pedicle by which most brachiopods attach themselves to surfaces.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} The brachial and pedicle valves are often called the dorsal ("upper") and ventral ("lower"),{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} but some paleontologistsPaleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
regard "dorsal" and "ventral" as incorrect terms, since they believe that the "ventral" valve was formed by folding of the upper surface under the body.{{sfn|Cohen: Brachiopoda ELS|(2002)|}} Irrespective of this debate, the valves of brachiopods are differently arranged of those of bivalve molluscs, which lie on the left and right sides of the body. In most brachiopod species both valves are convex, the surfaces often bear growth lines or other ornaments, and the pedicle valve is larger than the brachial. However, the lingulids, which burrow into the seabed, have valves that are smoother, flatter and of similar size and shape.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Brachiopod valves have a hingeA hinge is a type of bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation. Hinges may be made of flexible material or of moving components...
, in which the rearmost end of the brachial valve rocks on an internal projection of the pedicle valve. The internal projections of articulate ("jointed") brachiopods have teeth which fit into sockets on the brachial valve, an arrangement that locks the valves together. Inarticulate brachiopods have no matching teeth and sockets, and their valves are held together only by muscles.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
All brachiopods have adductor muscles, that are set on the inside of the pedicle valve and close the valves by pulling on the part of the brachial valve ahead of the hinge. These muscles have both "quick" fibers that close the valves in emergencies and "catch" fibers that are slower but can keep the valves closed for long periods. Articulate brachiopods open the valves by means of abductor muscles, also known as diductors, which lie further to the rear and pull on the part of the brachial valve behind the hinge. Inarticulate brachiopods use a different opening mechanism, in which muscles reduce the length of the coelomThe coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
(main body cavity) and make it bulge outwards, pushing the valves apart. Both classesIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
open the valves to an angle of about 10°. The more complex set of muscles employed by inarticulate brachiopods can also operate the valves as scissors, a mechanism that lingulids use to burrow.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
{{Annotated image | float=right | image=Lingula anatina.jpg | width=200 | image-width=200 | height=100 | image-top=-50
| caption=The inarticulate species Lingula anatina, showing the long pedicle, flattened shells and prominent chaetaA chaeta or cheta is a chitinous bristle or seta found on an insect, arthropod or annelid worms such as the earthworm, although the term is also frequently used to describe similar structures in other invertebrates. The plural form is chaetae or chetae.In the Polychaeta, they are located on the...
e around the front edge of the shells
|annotations=
}}
Each valve consists of three layers, an outer periostracumThe periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including mollusks and brachiopods. Among mollusks it is primarily seen in snails and clams, i.e. in bivalves and gastropods, but it is also found in cephalopods such as the...
made of organic compoundAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s and two biomineralized layers. Articulated brachiopods have a periostracum made of proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, a "primary layer" of calciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
(a form of calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
) under that, and finally a mixture of proteins and calcite.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Inarticulate brachiopod shells have a similar sequence of layers, but their composition is different from that articulated brachiopods and also varies between the classesIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of inarticulate brachiopods. Linguids and discinids, which have pedicles, have a matrixIn biology, matrix is the material between animal or plant cells, in which more specialized structures are embedded, and a specific part of the mitochondrion that is the site of oxidation of organic molecules. The internal structure of connective tissues is an extracellular matrix...
of glycosaminoglycanGlycosaminoglycans or mucopolysaccharides are long unbranched polysaccharides consisting of a repeating disaccharide unit. The repeating unit consists of a hexose or a hexuronic acid, linked to a hexosamine .-Production:Protein cores made in the rough endoplasmic reticulum are posttranslationally...
s (long, unbranched polysaccharidePolysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
s), in which other material are embedded: chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
in the periostracum;{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} apatiteApatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
containing calcium phosphateCalcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
in the primary biomineralized layer; and a complex mixture in the innermost layer, containing collagenCollagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
and other proteins, chitinophosphate and apatite.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} CraniidsThe Craniidae are a family of brachiopods . They are the only members of the order Craniida and the monotypic suborder Craniidina and superfamily Cranioidea; consequently, the latter two taxa are presently redundant and not used very often...
, which have no pedicle and cement themselves directly to hard surfaces, have a periostracum of chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
and mineralized layers of calcite.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}{{sfn|Parkinson etc: Brachiopod shells|(2005)|}}
Mantle
Like molluscs, brachiopods have a mantle, an epitheliumEpithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
that lines the shell and encloses the internal organs. The brachiopod body occupies only about one-third of the internal space inside the shell, nearest the hinge. The rest of the space is lined with the mantle lobesIn anatomy, a lobe is a clear anatomical division or extension that can be determined without the use of a microscope This is in contrast to a lobule, which is a clear division only visible histologically....
, extensions that enclose a water-filled space in which sits the lophophore.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} The coelom extends into each lobe as a network of canals, which carry nutrients to the edges of the mantle.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}}
Relatively new cells in a groove on the edges of the mantle secrete material that extends the periostracum. These cells are gradually displaced to the underside of the mantle by more recent cells in the groove, and switch to secreting the mineralized material of the shell valves. In other words, on the edge of the valve the periostracum is extended first, and then reinforced by extension of the mineralized layers under the periostracum.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} In most species the edge of the mantle also bears movable bristles, often called chaetaA chaeta or cheta is a chitinous bristle or seta found on an insect, arthropod or annelid worms such as the earthworm, although the term is also frequently used to describe similar structures in other invertebrates. The plural form is chaetae or chetae.In the Polychaeta, they are located on the...
e or setae, that may help defend the animals and may act as sensorA sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s. In some brachiopods groups of chaetae help to channel the flow of water into and out of the mantle cavity.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
In most brachiopods, diverticulaA diverticulum is medical or biological term for an outpouching of a hollow structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false....
(hollow extensions) of the mantle penetrate through the mineralized layers of the valves into the periostraca. The function of these diverticula is uncertain and it is suggested that they may be storage chambers for chemicals such as glycogenGlycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
, may secreteSecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
repellents to deter organisms that stick to the shell or may help in respirationBreathing is the process that moves air in and out of the lungs. Aerobic organisms require oxygen to release energy via respiration, in the form of the metabolism of energy-rich molecules such as glucose. Breathing is only one process that delivers oxygen to where it is needed in the body and...
.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Experiments show that a brachiopod's oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
consumption drops if petroleum jellyPetroleum jelly, petrolatum, white petrolatum or soft paraffin, CAS number 8009-03-8, is a semi-solid mixture of hydrocarbons , originally promoted as a topical ointment for its healing properties...
is smeared on the shell, clogging the diverticula.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}}
Lophophore
Like bryozoans and phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s, brachiopods have a lophophore, a crown of tentacles whose cilia (fine hairs) create a water current that enables them to filterFilter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
food particles out of the water. However a bryozoan or phoronid lophophore is a ring of tentacles mounted on a single, retracted stalk,{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Bryozoa"|pp=829–845}}{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Phoronida"|pp=817–821}} while the basic form of the brachiopod lophophore is U-shaped, forming the brachia ("arms") from which the phylum gets its name.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Brachiopod lophophores are non-retractable and occupy up to two-thirds of the internal space, in the frontmost area where the valves gape when opened. To provide enough filtering capacity in this restricted space, lophophores of larger brachiopods are folded in moderately to very complex shapes – loops and coils are common, and some species' lophophores resemble a hand with the fingers splayed.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} In all species the lophophore is supported by cartilageCartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
and by a hydrostatic skeletonA hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an...
(in other words by the pressure of its internal fluid),{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} and the fluid extends into the tentacles.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Some articulate brachiopods also have a brachidium, a calcareous support for the lophophore attached to the inside of the brachial valve.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}}
The tentacles bear cilia (fine mobile hairs) on their edges and along the center. The beating of the outer cilia drives a water current from the tips of the tentacles to their bases, where it exits. Food particles that collide with the tentacles are trapped by mucusIn vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
, and the cillia down the middle drive this mixture to the base of the tentacles.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Introduction"|p=817}} A brachial groove runs round the bases of the tentacles, and its own cilia pass food along the groove towards the mouth.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} The method used by brachiopods is known as "upstream collecting", as food particles are captured as they enter the field of cilia that creates the feeding current. This method is used by the related phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s and bryozoans, and also by pterobranchs. Entoprocts use a similar-looking crown of tentacles, but it is solid and the flow runs from bases to tips, forming a "downstream collecting" system that catches food particles as they are about to exit.{{sfn|Riisgård etc: Downstream|(2000)|}}
Attachment to substrate
Most modern species attach to hard surfaces by means of a cylindrical pedicle ("stalk"), an extension of the body wall. This has a chitinous cuticleA cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
(non-cellular "skin") and protrudes through an opening in the hinge.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} However, some generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
such as the inarticulate Crania and the articulate Lacazella have no pedicle, and cement the rear of the "pedicle" valve to a surface so that the front is slightly inclined up away from the surface.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} In a few articulate genera such as Neothyris and Anakinetica, the pedicles wither as the adults grow and finally lie loosely on the surface. In these genera the shells are thickened and shaped so that the opening of the gaping valves is kept free of the sediment.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
Pedicles of inarticulate species are extensions of the main coelom, which houses the internal organs. A layer of longitudinal muscles lines the epidermisIn anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...
of the pedicle.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Members of the orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Lingulida have long pedicles, which they use to burrow into soft substrates, to raise the shell to the opening of the burrow to feed, and to retract the shell when disturbed.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} A lingulid moves its body up and down the top two-thirds of the burrow, while the remaining third is occupied only by the pedicle, with a bulb on the end that builds a "concrete" anchor.{{sfn|Emig: Inart Brach|(2001)|}} However, the pedicles of the order Discinida are short and attach to hard surfaces.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
An articulate pedicle has no coelom, is constructed from a different part of the larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
l body, and has a core composed of connective tissue"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...
. Muscles at the rear of the body can straighten, bend or even rotate the pedicle. The far end of the pedicle generally has rootlike extensions or short papillae ("bumps") which attach to hard surfaces. However, articulate brachiopods of genus Chlidonophora use a branched pedicle to anchor in sedimentSediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
. The pedicle emerges from the pedicle valve, either through a notch in the hinge or, in species where the pedicle valve is longer than the brachial, from a hole where the pedicle valve doubles back to touch the brachial valve. Some species stand with the front end upwards, while others lie horizontal with the pedicle valve uppermost.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Feeding and excretion
The water flow enters the lophophore from the sides of the open valves and exits at the front of the animal. In lingulids the entrance and exit channels are formed by groups of chaetae that function as funnels.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} In other brachiopods the entry and exit channels are organized by the shape of the lophophore.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} The lophophore captures food particles, especially phytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
(tiny photosynthetic organisms), and deliver them to the mouth via the brachial grooves along the bases of the tentacles.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
The mouth is at the base of the lophophore.{{sfn|Cohen etc: Brachiopod fold|(2003)|}} Food passes through the mouth, muscular pharynxThe human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
("throat") and oesophagus ("gullet"),{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} all of which are lined with cilia and cells that secrete mucusIn vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
and digestive enzymes.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} The stomachThe stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
wall has branched ceca ("pouches") where food is digested, mainly within the cells.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Nutrients are transported throughout the coelom, including the mantle lobes, by cilia.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} The wastes produced by metabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
are broken into ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, which is eliminated by diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
through the mantle and lophophore.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Brachiopods have metanephridia, used by many phylaIn biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
to excrete ammonia and other dissolved wastes. However, brachiopods have no sign of the podocytes which perform the first phase of excretion in this process,{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Bilateria" sect. "Excretion"|pp=212–214}} and brachiopod metanephridia appear to be used only to emit spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
and ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
The majority of food consumed by brachiopods is digestible, with very little solid waste produced.{{sfn|Cowen: History of life|(2000)|loc=ch. "Invert Paleo"|p=408}} The cilia of the lophophore can change direction to eject isolated particles of indigestible matter. If the animal encounters larger lumps of undesired matter, the cilia lining the entry channels pause and the tentacles in contact with the lumps move apart to form large gaps and then slowly use their cilia to dump the lumps onto the lining of the mantle. This has its own cilia, which wash the lumps out through the opening between the valves. If the lophophore is clogged, the adductors snap the valves sharply, which creates a "sneeze" that clears the obstructions.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} In some inarticulate brachiopods the digestive tract is U-shaped and ends with an anus that eliminates solids from the front of the body wall.{{sfn|Cohen etc: Brachiopod fold|(2003)|}} Other inarticulate brachiopods and all articulate brachiopods have a curved gut that ends blindly, with no anus.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} These animals bundle solid waste with mucus and periodically "sneeze" it out, using sharp contractions of the gut muscles.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}}
Circulation and respiration
The lophophore and mantle are the only surfaces that absorb oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and eliminate carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. Oxygen seems to be distributed by the fluid of the coelom, which is circulated through the mantle and driven either by contractions of the lining of the coelom or by beating of its cilia. In some species oxygen is partly carried by the respiratory pigmentA respiratory pigment is a molecule, such as hemoglobin in humans, that increases the oxygen-carrying capacity of the blood. The four most common invertebrate respiratory pigments are hemoglobin, haemocyanin, haemerythrin and chlorocruorin...
hemerythrinHemerythrin is an oligomeric protein responsible for oxygen transport in the marine invertebrate phyla of sipunculids, priapulids, brachiopods, and in a single annelid worm, magelona. Recently, hemerythrin was discovered in methanotrophic bacterium Methylococcus capsulatus...
, which is transported in coelomocyte cells.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Brachiopods also have colorless bloodBlood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
, circulated by a muscular heart which lies in the dorsal part of the body above the stomach.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} The blood passes through vessels that extend to the front and back of the body, and branch to organs including the lophophore at the front and the gut, muscles, gonads and nephridia at the rear. The blood circulation seems not to be completely closed, and the coelomic and blood fluids must mix to a degree.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} The main function of the blood may be to deliver nutrients.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Nervous system and senses
The "brain" of adult articulates consists of two gangliaIn anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
, one above and the other below the oesophagus. Adult inarticulates have only the lower ganglion.{{sfn|Nielsen: Brachio brains|(2005)|}} From the ganglia and the commissureA commissure is the place where two things are joined. The term is used especially in the fields of anatomy and biology.In anatomy, commissure refers to a bundle of nerve fibers that cross the midline at their level of origin or entry .* The most common usage of the term refers to the brain's...
s where they join, nerves run to the lophophore, the mantle lobes and the muscles that operate the valves. The edge of the mantle has probably the greatest concentration of sensors. Although not directly connected to sensory neurons, the mantle's chaetaA chaeta or cheta is a chitinous bristle or seta found on an insect, arthropod or annelid worms such as the earthworm, although the term is also frequently used to describe similar structures in other invertebrates. The plural form is chaetae or chetae.In the Polychaeta, they are located on the...
e probably send tactile signals to receptors in the epidermisIn anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...
of the mantle. Many brachiopods close their valves if shadows appear above them, but the cells responsible for this are unknown. Some brachiopods have statocystThe statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
s which detect changes in the animals' position.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Reproduction and lifecycle
Lifespans range from 3 to over 30 years.{{sfn|Cohen: Brachiopoda ELS|(2002)|}} Adults of most species are of one sex throughout their lives. The gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s are masses of developing gameteA gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s (ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
or spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
), and most species have four gonads, two in each valve.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Those of articulates lie in the channels of the mantle lobes, while those of inarticulates lie near the gut.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} Ripe gametes float into the main coelom and then exit into the mantle cavity via the metanephridia, which open on either side of the mouth. Most species release both ova and sperm into the water, but females of some species keep the embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s in brood chambers until the larvae hatch.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
The cell divisionIn embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
in the embryo is radial (cells form in stacks of rings directly above each other), holoblastic (cells are separate, although adjoining) and regulative (the type of tissue into which a cell develops is controlled by interactions between adjacent cells, rather than rigidly within in each cell).{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Bilateria" sect. "Reproduction"|pp=214–219}}{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} While some animals develop the mouth and anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
by deepening the blastoporeA blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...
, a "dent" in the surface of the early embryo, the blastopore of brachiopods closes up, and their mouth and anus develop from new openings.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
The larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e of inarticulates swim as planktonPlankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
for months{{sfn|Cohen: Brachiopoda ELS|(2002)|}} and are like miniature adults, with valves, mantle lobes, a pedicle that coils in the mantle cavity, and a small lophophore, which is used for both feeding and swimming{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} – except that CraniidsThe Craniidae are a family of brachiopods . They are the only members of the order Craniida and the monotypic suborder Craniidina and superfamily Cranioidea; consequently, the latter two taxa are presently redundant and not used very often...
have no pedicle.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} As the shell becomes heavier, the juvenile sinks to the bottom and becomes a sessile adult.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} The larvae of articulate species live only on yolk, and remain among the plankton for only a few days.{{sfn|Cohen: Brachiopoda ELS|(2002)|}} This type of larva has a ciliated frontmost lobe that becomes the body and lophophore, a rear lobe that becomes the pedicle, and a mantle like a skirt, with the hem towards the rear. On metamorphosingMetamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
into an adult, the pedicle attaches to a surface, the front lobe develops the lophophore and other organs, and the mantle rolls up over the front lobe and starts to secreteSecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
the shell.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
The maximum oxygen consumption of brachiopods is low, and their minimal requirement is not measurable. In cold seas, brachiopod growth is seasonal and the animals often lose weight in winter. These variations in growth often form growth lines in the shells. Members of some generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
have survived for a year in aquaria without food.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
{{anchor|TaxonomyAnchor}}Taxonomy
The fossil brachiopod generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
have great diversity but only a few skeletalThe skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
characteristics, while the modern genera have much lower diversity but provide soft-bodied characteristics as well as skeletal ones – and both sets of specimens have limitations that make it difficult to produce a comprehensive classification of brachiopods. The phylum also has experienced significant convergent evolutionConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
and reversals (in which a more recent group seems to have lost a characteristic that is seen in an intermediate group, reverting to a characteristic last seen in an older group). Hence some brachiopod taxonomistsTaxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
believe it is premature to define higher levels of classification such as orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, and recommend instead a bottom-up approach that identifies genera and then groups these into intermediate groups.{{sfn|Carlson: Ghosts|(2001)|}}
However, other taxonomists believe that some patterns of characteristics are sufficiently stable to make higher-level classifications worthwhile, although there are different views about what the higher-level classifications should be.{{sfn|Carlson: Ghosts|(2001)|}}
The "traditional" classification was defined in 1869;{{sfn|ITIS: Brachiopoda||}} two further approaches were established in the 1990s:{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
- In the "traditional" classification, the Articulata have toothed hinges between the valves, while the hinges of the Inarticulata are held together only by muscles.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
- A classification devised in the 1990s, based on the materials of which the shells are based, united the Craniida and the "articulate" brachiopods in the Calciata, which have calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
shells. The LingulidaLingulida is an order of brachiopods....
and Discinida, combined in the LingulataLingulata is a class of brachiopod, among the oldest of all brachiopods having existed since the Cambrian period . They are also among the most morphologically conservative of the brachiopods, having lasted from their earliest appearance to the present with very little change in shape...
, have shells made of chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
and calcium phosphateCalcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
.{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
- A three-part scheme, also from the 1990s, places the Craniida in a separate group of its own, the Craniformea. The Lingulida and Discinida are grouped as Linguliformea
Linguliformea is a subphylum of inarticulate brachiopods. These were the earliest of brachiopods, ranging from the Cambrian into the Holocene. They rapidly diversified during the Cambrian into the Ordovician, but most families went extinct by the end of the Devonian.The articulation in these...
, and the Rhynchonellida and Terebratulida as RhynchonelliformeaRhynchonelliformea is the name now given to the articulate brachiopods, Class Articulata, revised as a subphylum. Articulate brachiopods are those with hard, articulated, shells with a simple set of opening and closing muscles....
.{{sfn|Milsom etc: 3-part taxonomy|(2009)||}}{{sfn|Williams etc: Suprafamilial Classif|(2000)|loc=Preface|pp=xxxix-xlv}}
Three high-level classifications of brachiopods{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
| "Traditional" classification{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} | Inarticulata | Articulata |
| "Calciata" approach{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} | Lingulata Lingulata is a class of brachiopod, among the oldest of all brachiopods having existed since the Cambrian period . They are also among the most morphologically conservative of the brachiopods, having lasted from their earliest appearance to the present with very little change in shape...
| Calciata |
Three-part approach
{{Distinguish|Branchiopoda}}
{{automatic taxobox
| taxon = Brachiopoda
| fossil_range = {{fossil range|Lower Cambrian|Recent}}
| image = PlatystrophiaOrdovician.jpg
| image_caption = Vinlandostrophia ponderosa (OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
). Scale bar is 5.0 mm.
| display parents = 3
| authority = DumérilAndré Marie Constant Duméril was a French zoologist. He was professor of anatomy at the Muséum National d'Histoire Naturelle from 1801 to 1812, when he became professor of herpetology and ichthyology...
, 1806{{sfn|Zvyagintsev etc: Brachio fouling|(2007)||}}
| subdivision_ranks = SubphylaIn life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum.Not all phyla are divided into subphyla...
and classIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
es
| subdivision = See taxonomy
| diversity_link = List of brachiopod genera
| diversity = About 100 living generaGenera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...
.
About 5,000 fossil genera.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
}}
Brachiopods are a phylum of marine animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Articulate brachiopods have toothed hinges and simple opening and closing muscles, while inarticulate brachiopods have untoothed hinges and more complex muscles. In a typical brachiopod a stalk-like pedicleIn zoology, the pedicle refers to a fleshy line brachiopods and some bivalve mollusks use to attach and anchor to a substrate. In brachiopods, the pedicle emerges from a pedicle opening. Some brachiopods do not have a functional pedicle, and thus do not have this pedicle opening....
projects from an opening in the hinge or from a hole in the larger valve, attaching the animal to the seabed but clear of silt that would obstruct the opening.
Brachiopods have a mantle that secreteSecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
and lines the shell, and encloses the internal organs. The body occupies typically about one-third of the internal space inside the shell, nearest the hinge. The rest of the mantle encloses a water-filled space containing the lophophoreThe lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...
, a crown of tentacles that filtersFilter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
food particles out of the water. In all species the lophophore is supported by cartilageCartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
and by a hydrostatic skeletonA hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an...
. The lophophore filters food, mostly phytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
, out of the water. From there the food is transported in succession to: the grooves along the bases of the lophophore's tentacles; the mouth; pharynxThe human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
; oesophagus; and finally the stomachThe stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
, where the food is digestedDigestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....
. Nutrients are transported from the stomach throughout the coelomThe coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
(main body cavity), include the mantle lobes, by cilia. The wastes produced by metabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
are broken into ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, which is eliminated by diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
through the mantle and lophophore. The lophophore and mantle are the only surfaces that absorb oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and eliminate carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. Oxygen seems to be distributed by the fluid of the coelom. The heart is above the stomach, and the blood vessels connect it to the major organs. However, the main function of the blood may be to deliver nutrients. The maximum oxygen consumption of brachiopods is low, and their minimum requirement is not measurable.
The "brain" of adult articulates consists of two gangliaIn anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
, one above and the other below the oesophagus. Adult inarticulates have only the lower ganglion. Nerves run to the lophophore, the mantle lobes and the muscles that operate the valves. Many brachiopods close their valves if shadows appear above them, but the cells responsible for this are unknown. Some brachiopods have statocystThe statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
s which detect changes in the animals' balanceEquilibrioception or sense of balance is one of the physiological senses. It helps prevent humans and animals from falling over when walking or standing still. Balance is the result of a number of body systems working together: the eyes , ears and the body's sense of where it is in space ideally...
.
Lifespans range from 3 to over 30 years. Ripe gameteA gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s (ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
or spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
) float from the gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s into the main coelom and then exit into the mantle cavity. The larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e of inarticulate brachiopods are miniature adults, with lophophores that enable the larvae to feed and swimming for months, until the animals become heavy enough to settle to the seabed. Larvae of articulate species are different from the adult forms, live only on yolk, remain only among the plankton for only a few days, and then metamorphosingMetamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
.
In addition to the traditional classification into inarticulate and articulate brachiopods, two approaches appeared in the 1990s: grouping the inarticulate Craniida with articulate brachiopods, as both used the same material in the mineralA mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
layers of the shell; and making the Craniida a third group, as their outer organicOrganic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
layer is different from that of either the others. However, some taxonomists believe it is premature to suggest higher levels of classification such as orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, and recommend a bottom-up approach that identifies genera and then groups these into intermediate groups. Traditionally brachiopods have been regarded as members of or as a sister group to the deuterostomeDeuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s, a super-phylum which includes chordateChordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
s and echinodermEchinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s. One type of analysis of brachiopods' evolutionary relationships has always placed brachiopods as protostomeProtostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s, while another type has split between placing brachiopods among the protostomes or the deuterostomes.
In 2003 it was suggested that brachiopods evolved from an ancestor similar to HalkieriaHalkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages...
, a slugSlug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
-like animal with "chain mail"Chain Mail" is a single by Mancunian band James, released in March 1986 by Sire Records, the first after the band defected from Factory Records. The record was released in two different versions, as 7" single and 12" EP, with different artworks by John Carroll and, confusingly, under different...
" on its back and a shell at the front and rear end, and that the ancestral brachiopod converted its shells into a pair of valves by folding the rear part of its body under its front. However, new fossils found in 2007 to 2008 showed that tommotiidTommotiids are Cambrian shelly fossils of uncertain affinity.Unlike most of the small shelly fauna, tommotiids are mineralised with calcium phosphate rather than calcium carbonate...
s' "chain mail" formed the tube of a sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
animal, and that one resembled phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s, which are close relatives or a sub-group of brachiopod, while the other tommotiid bearing two symmetrical plates that might be an early form of brachiopod valves. Lineages that have both fossil and extant brachiopods appeared in the early CambrianThe Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
, OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
and CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
periods respectively. Other lineages have arisen and then become extinct, sometimes during severe mass extinctions. At their peak in the PaleozoicThe Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
era the brachiopods were among the most abundant filter-feeders and reef-builders, and occupied other ecological nicheIn ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s, including swimming in the jet-propulsion style of scallopA scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...
s. Brachiopod fossils have been useful indicators of climate changes during the PaleozoicThe Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
era. However, after the Permian–Triassic extinction event, brachiopods recovered only a third of their former diversity. A study in 2007 concluded that brachiopods were especially vulnerable to the Permian–Triassic extinction, as they built calcareous hard parts (made of calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
) and had low metabolic rates and weak respiratory systems. It was often thought that brachiopods were in decline after the Permian–Triassic extinction, and were out-competed by bivalves. However, a study in 1980 concluded that: both brachiopods and bivalves increased all the way from the Paleozoic to modern times, but bivalves increased faster; after the Permian–Triassic extinction, brachiopods for the first time were less diverse than bivalves.
Brachiopods live only in the sea, and most species avoid locations with strong currents or waves. Articulate species have larvae that settle in quickly and form dense populations in well-defined areas, while inarticulate larvae swimming for up to a month and have wide ranges. Brachiopods now live mainly in cold and low-light conditions. Fish and crustaceans seem to find brachiopod flesh distasteful and seldom attack them. Among brachiopods only the lingulids have been fished commercially, on a very small scale. One brachiopod species may be a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of JapanThe Sea of Japan is a marginal sea of the western Pacific Ocean, between the Asian mainland, the Japanese archipelago and Sakhalin. It is bordered by Japan, North Korea, Russia and South Korea. Like the Mediterranean Sea, it has almost no tides due to its nearly complete enclosure from the Pacific...
.
Name
The scientific name "Brachiopod" is formed from the Ancient GreekAncient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
words βραχίων ("arm") and πούς ("foot").{{sfn|Shorter Oxford English Dictionary|(2002)|loc=entry "Brachiopod"}} They are often known as "lamp shells", since the curved shells of the classIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
TerebratulidaTerebratulids are one of only three living orders of articulate brachiopods, the others being the Rhynchonellida and the Thecideida. Craniida and Lingulida include living brachiopods, but are inarticulates. The name, Terebratula, may be derived from the Latin "terebra", meaning "hole-borer"...
look rather like pottery oil-lamps.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
Shells and their mechanisms
Modern brachiopods range from 1 to 100 mm (0.0393700787401575 to 3.9 in) long, and most species are about 10 to 30 mm (0.393700787401575 to 1.2 in).{{sfn|Cohen: Brachiopoda ELS|(2002)|}} Each has two valves (shell sections) which are biomineralized. The valves cover the dorsal and ventral surface of the animal, unlike bivalve molluscs whose shells cover the lateral surfaces. The brachial valve bears on its inner surface the brachia ("arms") from which the phylum gets its name, and which supports the lophophoreThe lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...
, used for filteringFilter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
and respirationAquatic respiration is the process whereby an aquatic animal obtains oxygen from water.-Fish:In most fish respiration takes place through gills. Lungfish, however, possess one or two lungs...
. The other is known as the pedicle valve, as its inner surface bears the stalk-like pedicle by which most brachiopods attach themselves to surfaces.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} The brachial and pedicle valves are often called the dorsal ("upper") and ventral ("lower"),{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} but some paleontologistsPaleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
regard "dorsal" and "ventral" as incorrect terms, since they believe that the "ventral" valve was formed by folding of the upper surface under the body.{{sfn|Cohen: Brachiopoda ELS|(2002)|}} Irrespective of this debate, the valves of brachiopods are differently arranged of those of bivalve molluscs, which lie on the left and right sides of the body. In most brachiopod species both valves are convex, the surfaces often bear growth lines or other ornaments, and the pedicle valve is larger than the brachial. However, the lingulids, which burrow into the seabed, have valves that are smoother, flatter and of similar size and shape.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Brachiopod valves have a hingeA hinge is a type of bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation. Hinges may be made of flexible material or of moving components...
, in which the rearmost end of the brachial valve rocks on an internal projection of the pedicle valve. The internal projections of articulate ("jointed") brachiopods have teeth which fit into sockets on the brachial valve, an arrangement that locks the valves together. Inarticulate brachiopods have no matching teeth and sockets, and their valves are held together only by muscles.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
All brachiopods have adductor muscles, that are set on the inside of the pedicle valve and close the valves by pulling on the part of the brachial valve ahead of the hinge. These muscles have both "quick" fibers that close the valves in emergencies and "catch" fibers that are slower but can keep the valves closed for long periods. Articulate brachiopods open the valves by means of abductor muscles, also known as diductors, which lie further to the rear and pull on the part of the brachial valve behind the hinge. Inarticulate brachiopods use a different opening mechanism, in which muscles reduce the length of the coelomThe coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
(main body cavity) and make it bulge outwards, pushing the valves apart. Both classesIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
open the valves to an angle of about 10°. The more complex set of muscles employed by inarticulate brachiopods can also operate the valves as scissors, a mechanism that lingulids use to burrow.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
{{Annotated image | float=right | image=Lingula anatina.jpg | width=200 | image-width=200 | height=100 | image-top=-50
| caption=The inarticulate species Lingula anatina, showing the long pedicle, flattened shells and prominent chaetaA chaeta or cheta is a chitinous bristle or seta found on an insect, arthropod or annelid worms such as the earthworm, although the term is also frequently used to describe similar structures in other invertebrates. The plural form is chaetae or chetae.In the Polychaeta, they are located on the...
e around the front edge of the shells
|annotations=
}}
Each valve consists of three layers, an outer periostracumThe periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including mollusks and brachiopods. Among mollusks it is primarily seen in snails and clams, i.e. in bivalves and gastropods, but it is also found in cephalopods such as the...
made of organic compoundAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s and two biomineralized layers. Articulated brachiopods have a periostracum made of proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, a "primary layer" of calciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
(a form of calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
) under that, and finally a mixture of proteins and calcite.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Inarticulate brachiopod shells have a similar sequence of layers, but their composition is different from that articulated brachiopods and also varies between the classesIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of inarticulate brachiopods. Linguids and discinids, which have pedicles, have a matrixIn biology, matrix is the material between animal or plant cells, in which more specialized structures are embedded, and a specific part of the mitochondrion that is the site of oxidation of organic molecules. The internal structure of connective tissues is an extracellular matrix...
of glycosaminoglycanGlycosaminoglycans or mucopolysaccharides are long unbranched polysaccharides consisting of a repeating disaccharide unit. The repeating unit consists of a hexose or a hexuronic acid, linked to a hexosamine .-Production:Protein cores made in the rough endoplasmic reticulum are posttranslationally...
s (long, unbranched polysaccharidePolysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
s), in which other material are embedded: chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
in the periostracum;{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} apatiteApatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
containing calcium phosphateCalcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
in the primary biomineralized layer; and a complex mixture in the innermost layer, containing collagenCollagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
and other proteins, chitinophosphate and apatite.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} CraniidsThe Craniidae are a family of brachiopods . They are the only members of the order Craniida and the monotypic suborder Craniidina and superfamily Cranioidea; consequently, the latter two taxa are presently redundant and not used very often...
, which have no pedicle and cement themselves directly to hard surfaces, have a periostracum of chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
and mineralized layers of calcite.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}{{sfn|Parkinson etc: Brachiopod shells|(2005)|}}
Mantle
Like molluscs, brachiopods have a mantle, an epitheliumEpithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
that lines the shell and encloses the internal organs. The brachiopod body occupies only about one-third of the internal space inside the shell, nearest the hinge. The rest of the space is lined with the mantle lobesIn anatomy, a lobe is a clear anatomical division or extension that can be determined without the use of a microscope This is in contrast to a lobule, which is a clear division only visible histologically....
, extensions that enclose a water-filled space in which sits the lophophore.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} The coelom extends into each lobe as a network of canals, which carry nutrients to the edges of the mantle.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}}
Relatively new cells in a groove on the edges of the mantle secrete material that extends the periostracum. These cells are gradually displaced to the underside of the mantle by more recent cells in the groove, and switch to secreting the mineralized material of the shell valves. In other words, on the edge of the valve the periostracum is extended first, and then reinforced by extension of the mineralized layers under the periostracum.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} In most species the edge of the mantle also bears movable bristles, often called chaetaA chaeta or cheta is a chitinous bristle or seta found on an insect, arthropod or annelid worms such as the earthworm, although the term is also frequently used to describe similar structures in other invertebrates. The plural form is chaetae or chetae.In the Polychaeta, they are located on the...
e or setae, that may help defend the animals and may act as sensorA sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s. In some brachiopods groups of chaetae help to channel the flow of water into and out of the mantle cavity.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
In most brachiopods, diverticulaA diverticulum is medical or biological term for an outpouching of a hollow structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false....
(hollow extensions) of the mantle penetrate through the mineralized layers of the valves into the periostraca. The function of these diverticula is uncertain and it is suggested that they may be storage chambers for chemicals such as glycogenGlycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
, may secreteSecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
repellents to deter organisms that stick to the shell or may help in respirationBreathing is the process that moves air in and out of the lungs. Aerobic organisms require oxygen to release energy via respiration, in the form of the metabolism of energy-rich molecules such as glucose. Breathing is only one process that delivers oxygen to where it is needed in the body and...
.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Experiments show that a brachiopod's oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
consumption drops if petroleum jellyPetroleum jelly, petrolatum, white petrolatum or soft paraffin, CAS number 8009-03-8, is a semi-solid mixture of hydrocarbons , originally promoted as a topical ointment for its healing properties...
is smeared on the shell, clogging the diverticula.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}}
Lophophore
Like bryozoans and phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s, brachiopods have a lophophore, a crown of tentacles whose cilia (fine hairs) create a water current that enables them to filterFilter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
food particles out of the water. However a bryozoan or phoronid lophophore is a ring of tentacles mounted on a single, retracted stalk,{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Bryozoa"|pp=829–845}}{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Phoronida"|pp=817–821}} while the basic form of the brachiopod lophophore is U-shaped, forming the brachia ("arms") from which the phylum gets its name.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Brachiopod lophophores are non-retractable and occupy up to two-thirds of the internal space, in the frontmost area where the valves gape when opened. To provide enough filtering capacity in this restricted space, lophophores of larger brachiopods are folded in moderately to very complex shapes – loops and coils are common, and some species' lophophores resemble a hand with the fingers splayed.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} In all species the lophophore is supported by cartilageCartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
and by a hydrostatic skeletonA hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an...
(in other words by the pressure of its internal fluid),{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} and the fluid extends into the tentacles.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Some articulate brachiopods also have a brachidium, a calcareous support for the lophophore attached to the inside of the brachial valve.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}}
The tentacles bear cilia (fine mobile hairs) on their edges and along the center. The beating of the outer cilia drives a water current from the tips of the tentacles to their bases, where it exits. Food particles that collide with the tentacles are trapped by mucusIn vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
, and the cillia down the middle drive this mixture to the base of the tentacles.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Introduction"|p=817}} A brachial groove runs round the bases of the tentacles, and its own cilia pass food along the groove towards the mouth.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} The method used by brachiopods is known as "upstream collecting", as food particles are captured as they enter the field of cilia that creates the feeding current. This method is used by the related phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s and bryozoans, and also by pterobranchs. Entoprocts use a similar-looking crown of tentacles, but it is solid and the flow runs from bases to tips, forming a "downstream collecting" system that catches food particles as they are about to exit.{{sfn|Riisgård etc: Downstream|(2000)|}}
Attachment to substrate
Most modern species attach to hard surfaces by means of a cylindrical pedicle ("stalk"), an extension of the body wall. This has a chitinous cuticleA cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
(non-cellular "skin") and protrudes through an opening in the hinge.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} However, some generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
such as the inarticulate Crania and the articulate Lacazella have no pedicle, and cement the rear of the "pedicle" valve to a surface so that the front is slightly inclined up away from the surface.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} In a few articulate genera such as Neothyris and Anakinetica, the pedicles wither as the adults grow and finally lie loosely on the surface. In these genera the shells are thickened and shaped so that the opening of the gaping valves is kept free of the sediment.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
Pedicles of inarticulate species are extensions of the main coelom, which houses the internal organs. A layer of longitudinal muscles lines the epidermisIn anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...
of the pedicle.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Members of the orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Lingulida have long pedicles, which they use to burrow into soft substrates, to raise the shell to the opening of the burrow to feed, and to retract the shell when disturbed.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} A lingulid moves its body up and down the top two-thirds of the burrow, while the remaining third is occupied only by the pedicle, with a bulb on the end that builds a "concrete" anchor.{{sfn|Emig: Inart Brach|(2001)|}} However, the pedicles of the order Discinida are short and attach to hard surfaces.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
An articulate pedicle has no coelom, is constructed from a different part of the larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
l body, and has a core composed of connective tissue"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...
. Muscles at the rear of the body can straighten, bend or even rotate the pedicle. The far end of the pedicle generally has rootlike extensions or short papillae ("bumps") which attach to hard surfaces. However, articulate brachiopods of genus Chlidonophora use a branched pedicle to anchor in sedimentSediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
. The pedicle emerges from the pedicle valve, either through a notch in the hinge or, in species where the pedicle valve is longer than the brachial, from a hole where the pedicle valve doubles back to touch the brachial valve. Some species stand with the front end upwards, while others lie horizontal with the pedicle valve uppermost.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Feeding and excretion
The water flow enters the lophophore from the sides of the open valves and exits at the front of the animal. In lingulids the entrance and exit channels are formed by groups of chaetae that function as funnels.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} In other brachiopods the entry and exit channels are organized by the shape of the lophophore.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} The lophophore captures food particles, especially phytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
(tiny photosynthetic organisms), and deliver them to the mouth via the brachial grooves along the bases of the tentacles.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
The mouth is at the base of the lophophore.{{sfn|Cohen etc: Brachiopod fold|(2003)|}} Food passes through the mouth, muscular pharynxThe human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
("throat") and oesophagus ("gullet"),{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} all of which are lined with cilia and cells that secrete mucusIn vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
and digestive enzymes.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} The stomachThe stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
wall has branched ceca ("pouches") where food is digested, mainly within the cells.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Nutrients are transported throughout the coelom, including the mantle lobes, by cilia.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} The wastes produced by metabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
are broken into ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, which is eliminated by diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
through the mantle and lophophore.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Brachiopods have metanephridia, used by many phylaIn biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
to excrete ammonia and other dissolved wastes. However, brachiopods have no sign of the podocytes which perform the first phase of excretion in this process,{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Bilateria" sect. "Excretion"|pp=212–214}} and brachiopod metanephridia appear to be used only to emit spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
and ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
The majority of food consumed by brachiopods is digestible, with very little solid waste produced.{{sfn|Cowen: History of life|(2000)|loc=ch. "Invert Paleo"|p=408}} The cilia of the lophophore can change direction to eject isolated particles of indigestible matter. If the animal encounters larger lumps of undesired matter, the cilia lining the entry channels pause and the tentacles in contact with the lumps move apart to form large gaps and then slowly use their cilia to dump the lumps onto the lining of the mantle. This has its own cilia, which wash the lumps out through the opening between the valves. If the lophophore is clogged, the adductors snap the valves sharply, which creates a "sneeze" that clears the obstructions.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} In some inarticulate brachiopods the digestive tract is U-shaped and ends with an anus that eliminates solids from the front of the body wall.{{sfn|Cohen etc: Brachiopod fold|(2003)|}} Other inarticulate brachiopods and all articulate brachiopods have a curved gut that ends blindly, with no anus.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} These animals bundle solid waste with mucus and periodically "sneeze" it out, using sharp contractions of the gut muscles.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}}
Circulation and respiration
The lophophore and mantle are the only surfaces that absorb oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and eliminate carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. Oxygen seems to be distributed by the fluid of the coelom, which is circulated through the mantle and driven either by contractions of the lining of the coelom or by beating of its cilia. In some species oxygen is partly carried by the respiratory pigmentA respiratory pigment is a molecule, such as hemoglobin in humans, that increases the oxygen-carrying capacity of the blood. The four most common invertebrate respiratory pigments are hemoglobin, haemocyanin, haemerythrin and chlorocruorin...
hemerythrinHemerythrin is an oligomeric protein responsible for oxygen transport in the marine invertebrate phyla of sipunculids, priapulids, brachiopods, and in a single annelid worm, magelona. Recently, hemerythrin was discovered in methanotrophic bacterium Methylococcus capsulatus...
, which is transported in coelomocyte cells.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Brachiopods also have colorless bloodBlood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
, circulated by a muscular heart which lies in the dorsal part of the body above the stomach.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} The blood passes through vessels that extend to the front and back of the body, and branch to organs including the lophophore at the front and the gut, muscles, gonads and nephridia at the rear. The blood circulation seems not to be completely closed, and the coelomic and blood fluids must mix to a degree.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} The main function of the blood may be to deliver nutrients.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Nervous system and senses
The "brain" of adult articulates consists of two gangliaIn anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
, one above and the other below the oesophagus. Adult inarticulates have only the lower ganglion.{{sfn|Nielsen: Brachio brains|(2005)|}} From the ganglia and the commissureA commissure is the place where two things are joined. The term is used especially in the fields of anatomy and biology.In anatomy, commissure refers to a bundle of nerve fibers that cross the midline at their level of origin or entry .* The most common usage of the term refers to the brain's...
s where they join, nerves run to the lophophore, the mantle lobes and the muscles that operate the valves. The edge of the mantle has probably the greatest concentration of sensors. Although not directly connected to sensory neurons, the mantle's chaetaA chaeta or cheta is a chitinous bristle or seta found on an insect, arthropod or annelid worms such as the earthworm, although the term is also frequently used to describe similar structures in other invertebrates. The plural form is chaetae or chetae.In the Polychaeta, they are located on the...
e probably send tactile signals to receptors in the epidermisIn anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...
of the mantle. Many brachiopods close their valves if shadows appear above them, but the cells responsible for this are unknown. Some brachiopods have statocystThe statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
s which detect changes in the animals' position.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
Reproduction and lifecycle
Lifespans range from 3 to over 30 years.{{sfn|Cohen: Brachiopoda ELS|(2002)|}} Adults of most species are of one sex throughout their lives. The gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s are masses of developing gameteA gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s (ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
or spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
), and most species have four gonads, two in each valve.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} Those of articulates lie in the channels of the mantle lobes, while those of inarticulates lie near the gut.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} Ripe gametes float into the main coelom and then exit into the mantle cavity via the metanephridia, which open on either side of the mouth. Most species release both ova and sperm into the water, but females of some species keep the embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s in brood chambers until the larvae hatch.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
The cell divisionIn embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
in the embryo is radial (cells form in stacks of rings directly above each other), holoblastic (cells are separate, although adjoining) and regulative (the type of tissue into which a cell develops is controlled by interactions between adjacent cells, rather than rigidly within in each cell).{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Bilateria" sect. "Reproduction"|pp=214–219}}{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} While some animals develop the mouth and anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
by deepening the blastoporeA blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...
, a "dent" in the surface of the early embryo, the blastopore of brachiopods closes up, and their mouth and anus develop from new openings.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
The larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e of inarticulates swim as planktonPlankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
for months{{sfn|Cohen: Brachiopoda ELS|(2002)|}} and are like miniature adults, with valves, mantle lobes, a pedicle that coils in the mantle cavity, and a small lophophore, which is used for both feeding and swimming{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} – except that CraniidsThe Craniidae are a family of brachiopods . They are the only members of the order Craniida and the monotypic suborder Craniidina and superfamily Cranioidea; consequently, the latter two taxa are presently redundant and not used very often...
have no pedicle.{{sfn|Doherty: Lophophorates|(2001)|loc=sect. "Introduction", "Brachiopoda"|pp=341–342, 356-363}} As the shell becomes heavier, the juvenile sinks to the bottom and becomes a sessile adult.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} The larvae of articulate species live only on yolk, and remain among the plankton for only a few days.{{sfn|Cohen: Brachiopoda ELS|(2002)|}} This type of larva has a ciliated frontmost lobe that becomes the body and lophophore, a rear lobe that becomes the pedicle, and a mantle like a skirt, with the hem towards the rear. On metamorphosingMetamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
into an adult, the pedicle attaches to a surface, the front lobe develops the lophophore and other organs, and the mantle rolls up over the front lobe and starts to secreteSecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
the shell.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
The maximum oxygen consumption of brachiopods is low, and their minimal requirement is not measurable. In cold seas, brachiopod growth is seasonal and the animals often lose weight in winter. These variations in growth often form growth lines in the shells. Members of some generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
have survived for a year in aquaria without food.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
{{anchor|TaxonomyAnchor}}Taxonomy
The fossil brachiopod generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
have great diversity but only a few skeletalThe skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
characteristics, while the modern genera have much lower diversity but provide soft-bodied characteristics as well as skeletal ones – and both sets of specimens have limitations that make it difficult to produce a comprehensive classification of brachiopods. The phylum also has experienced significant convergent evolutionConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
and reversals (in which a more recent group seems to have lost a characteristic that is seen in an intermediate group, reverting to a characteristic last seen in an older group). Hence some brachiopod taxonomistsTaxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
believe it is premature to define higher levels of classification such as orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, and recommend instead a bottom-up approach that identifies genera and then groups these into intermediate groups.{{sfn|Carlson: Ghosts|(2001)|}}
However, other taxonomists believe that some patterns of characteristics are sufficiently stable to make higher-level classifications worthwhile, although there are different views about what the higher-level classifications should be.{{sfn|Carlson: Ghosts|(2001)|}}
The "traditional" classification was defined in 1869;{{sfn|ITIS: Brachiopoda||}} two further approaches were established in the 1990s:{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
- In the "traditional" classification, the Articulata have toothed hinges between the valves, while the hinges of the Inarticulata are held together only by muscles.{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
- A classification devised in the 1990s, based on the materials of which the shells are based, united the Craniida and the "articulate" brachiopods in the Calciata, which have calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
shells. The LingulidaLingulida is an order of brachiopods....
and Discinida, combined in the LingulataLingulata is a class of brachiopod, among the oldest of all brachiopods having existed since the Cambrian period . They are also among the most morphologically conservative of the brachiopods, having lasted from their earliest appearance to the present with very little change in shape...
, have shells made of chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
and calcium phosphateCalcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
.{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
- A three-part scheme, also from the 1990s, places the Craniida in a separate group of its own, the Craniformea. The Lingulida and Discinida are grouped as Linguliformea
Linguliformea is a subphylum of inarticulate brachiopods. These were the earliest of brachiopods, ranging from the Cambrian into the Holocene. They rapidly diversified during the Cambrian into the Ordovician, but most families went extinct by the end of the Devonian.The articulation in these...
, and the Rhynchonellida and Terebratulida as RhynchonelliformeaRhynchonelliformea is the name now given to the articulate brachiopods, Class Articulata, revised as a subphylum. Articulate brachiopods are those with hard, articulated, shells with a simple set of opening and closing muscles....
.{{sfn|Milsom etc: 3-part taxonomy|(2009)||}}{{sfn|Williams etc: Suprafamilial Classif|(2000)|loc=Preface|pp=xxxix-xlv}}
Three high-level classifications of brachiopods{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
| "Traditional" classification{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} | Inarticulata | Articulata |
| "Calciata" approach{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} | Lingulata Lingulata is a class of brachiopod, among the oldest of all brachiopods having existed since the Cambrian period . They are also among the most morphologically conservative of the brachiopods, having lasted from their earliest appearance to the present with very little change in shape...
| Calciata |
| Three-part approach{{sfn|Milsom etc: 3-part taxonomy|(2009) |
}{{sfn|Williams etc: Suprafamilial Classif|(2000)|loc=Preface|pp=xxxix-xlv}}
!! style="background:#ffc;" colspan="2"| Linguliformea !! style="background:#ffc;"| Craniformea !! style="background:#ffc;" colspan="2"| Rhynchonelliformea
|- style="text-align:center;"
! OrdersIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
!! LingulidaLingulida is an order of brachiopods....
{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} !! Discinida{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} !! Craniida{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} !! TerebratulidaTerebratulids are one of only three living orders of articulate brachiopods, the others being the Rhynchonellida and the Thecideida. Craniida and Lingulida include living brachiopods, but are inarticulates. The name, Terebratula, may be derived from the Latin "terebra", meaning "hole-borer"...
{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}} !! RhynchonellidaThe taxonomic order Rhynchonellida is one of the two main groups of living articulate brachiopods, the other being the order Terebratulida. They are recognized by their strongly ribbed wedge-shaped or nut-like shells, and the very short hinge line....
{{sfn|Ruppert etc: Invert Zoo|(2004)|loc=ch. "Lophophorata" sect. "Brachiopoda"|pp=821–829}}
|- style="text-align:center;"
! Hinge
| colspan="3" | No teeth || colspan="2" | Teeth and sockets
|- style="text-align:center;"
! Anus
| colspan="3" | On front of body, at end of U-shaped gut || colspan="2" | None
|- style="text-align:center;"
! rowspan="2" | Pedicle
| colspan="2" | Contains coelomThe coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
with muscles running through || No pedicle || colspan="2" | No coelom, muscles where joins body
|- style="text-align:center;"
| Long, burrows || Short, attached to hard surfaces || None, cemented to surface || colspan="2" | Short, attached to hard surfaces{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
|- style="text-align:center;"
! PeriostracumThe periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including mollusks and brachiopods. Among mollusks it is primarily seen in snails and clams, i.e. in bivalves and gastropods, but it is also found in cephalopods such as the...
| colspan="2" | Glycosaminoglycans and chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
|| Chitin || colspan="2" | ProteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s
|- style="text-align:center;"
! Primary (middle) mineralized layer of shell
| colspan="2" | Glycosaminoglycans and apatiteApatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
(calcium phosphate) || colspan="3" | CalciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
(a form of calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
)
|- style="text-align:center;"
! Inner mineralized layer of shell
| colspan="2" | CollagenCollagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
and other proteins, chitinophosphate and apatite (calcium phosphate) || Calcite || colspan="2" | Proteins and calcite
|- style="text-align:center;"
! ChaetaA chaeta or cheta is a chitinous bristle or seta found on an insect, arthropod or annelid worms such as the earthworm, although the term is also frequently used to describe similar structures in other invertebrates. The plural form is chaetae or chetae.In the Polychaeta, they are located on the...
e around opening of shells
| colspan="2" | Yes{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} || No{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} || colspan="2" | Yes{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
|- style="text-align:center;"
! CoelomThe coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
fully divided
| colspan="2" | Yes{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} || No{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} || colspan="2" | Yes{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}
|}
About 330 living species are recognized,{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}} grouped into over 100 generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
. The great majority of modern brachiopods are rhynchonelliforms (Articulata, but excluding Craniida).{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
Distribution and habitat
Brachiopods live only in the sea. Most species avoid locations with strong currents or waves, and typical sites include rocky overhangs, crevices and caves, steep slopes of continental shelvesThe continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
, and in the bottoms of deep oceans. However, some articulate species attach to kelpKelps are large seaweeds belonging to the brown algae in the order Laminariales. There are about 30 different genera....
or in exceptionally sheltered sites in intertidal zoneThe intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...
s. The smallest living brachiopod, Gwynia, is only about 1 millimetre (0.0393700787401575 in) long, and lives in gravelGravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
Rhynchonelliforms (Articulata excluding Craniida), whose larvae consume only their yolks and settle and develop quickly, specialize in specific areas and form dense populations that can reach thousands per meter. Young adults often attach to the shells of more mature ones. On the other hand, inarticulate brachipods, whose larva swim for up to a month before settling, have wide ranges. Members of the discinoid genus PelagodiscusPelagodiscus is a genus of brachiopods....
have a cosmopolitan distributionIn biogeography, a taxon is said to have a cosmopolitan distribution if its range extends across all or most of the world in appropriate habitats. For instance, the killer whale has a cosmopolitan distribution, extending over most of the world's oceans. Other examples include humans, the lichen...
.{{sfn|Cohen: Brachiopoda ELS|(2002)|}}
Interactions with other organisms
Brachiopod metabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
s are 3 to 10 times slower than those of bivalves. While brachiopods were abundant in warm, shallow seas during the CretaceousThe Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period, they have been out-bred by bivalves, and now live mainly in cold and low-light conditions.{{sfn|Vermeij: Directionality|(1999)||}}
Brachiopod shells occasionally show evidence of damage by predators, and sometimes of subsequent repair. Fish and crustaceans seem to find brachiopod flesh distasteful.{{sfn|Cohen: Brachiopoda ELS|(2002)|}} The fossil record shows that drilling predators like gastropods attacked molluscs and echinoids 10 to 20 times more often than they did brachiopods, suggesting that such predators attacked brachiopods by mistake or when other prey was scarce.{{sfn|Kowalewski etc: 2nd-choice prey|(2002)||}} In waters where food is scarce, the snail Capulus ungaricusCapulus ungaricus, common name the bonnet shell, is a species of small sea snail, a marine gastropod mollusk in the family Capulidae, the cap snails.-Taxonomy:...
steals food from bivalves, snails, tube worms, and brachiopods.{{sfn|Iyengar: Kleptoparasitism|(2008)||}}
Among brachiopods only the lingulids have been fished commercially, and only on a very small scale.{{sfn|UCMP: Lingulata||}} Brachiopods seldom settle on artificial surfaces, probably because they are vulnerable to pollution. This may make the population of Coptothyrus adamsi useful as a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of JapanThe Sea of Japan is a marginal sea of the western Pacific Ocean, between the Asian mainland, the Japanese archipelago and Sakhalin. It is bordered by Japan, North Korea, Russia and South Korea. Like the Mediterranean Sea, it has almost no tides due to its nearly complete enclosure from the Pacific...
.{{sfn|Zvyagintsev etc: Brachio fouling|(2007)||}}
Fossil record
{{Main|Evolutionary history of brachiopods}}
Over 12,000 fossil species are recognized,{{sfn|Ax: Multicellular Animals|(2003)|loc=ch."Brachiopoda"|pp=87–93}}grouped into over 5,000 generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
. While the largest modern brachiopods are 100 millimetres (3.9 in) long,{{sfn|Cohen: Brachiopoda ELS|(2002)|}} a few fossils measure up to 200 millimetres (7.9 in) wide.{{sfn|Fortey: Fossils the key|(2008)|loc=ch. "How to recognize" sect. "Brachiopods"}} The earliest confirmed brachiopods have been found in the early CambrianThe Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
, inarticulate forms appearing first, followed soon after by articulate forms.{{sfn|Ushatinskaya: Earliest brachiopods|(2008)|}} Three unmineralized species have also been found in the Cambrian, and apparently represent two distinct groups that evolved from mineralized ancestors.{{sfn|Balthasar etc: Brachios stem Phoronids|(2009)||}} The inarticulate LingulaLingula is a genus of brachiopods within the class Lingulata. Lingula is known since the Tertiary.-Species:The following species are recognised:*Lingula adamsi Dall, 1873*Lingula anatina Lamarck, 1801*Lingula dregeri Andreae, 1893...
is often called a "living fossilLiving fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...
", as very similar generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
have been all the way back to the OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
. On the other hand, articulate brachiopods have produced major diversifications, and severe mass extinctions{{sfn|Fortey: Fossils the key|(2008)|loc=ch.
"How to recognize" sect. "Brachiopods"}} – but the articulate Rhynchonellida and Terebratulida, the most diverse present-day groups, appeared at the start of the Ordovician and CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
respectively.{{sfn|UCMP: Brachio Fossil Range||}}
Since 1991 Nielsen has proposed a hypothesis about the development of brachiopods, adapted in 2003 by Cohen and colleagues as a hypothesis about the earliest evolution of brachiopods. This "brachiopod fold" hypothesis suggests that brachiopods evolved from an ancestor similar to HalkieriaHalkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages...
,{{sfn|Cohen etc: Brachiopod fold|(2003)|}} a slugSlug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
-like animal with "chain mail"Chain Mail" is a single by Mancunian band James, released in March 1986 by Sire Records, the first after the band defected from Factory Records. The record was released in two different versions, as 7" single and 12" EP, with different artworks by John Carroll and, confusingly, under different...
" on its back and a shell at the front and rear end.{{sfn|Conway Morris etc: Articulated Halkieriids|(1995)}} The hypothesis proposes that the first brachiopod converted its shells into a pair of valves by folding the rear part of its body under its front.{{sfn|Cohen etc: Brachiopod fold|(2003)|}}
However, fossils from 2007 onwards have supported a new interpretation of the Early-Cambrian tommotiidTommotiids are Cambrian shelly fossils of uncertain affinity.Unlike most of the small shelly fauna, tommotiids are mineralised with calcium phosphate rather than calcium carbonate...
s and a new hypothesis that brachiopods evolved from tommotiids. The "armor mail" of tommotiids was well-known but not in an assembled form, and it was generally assumed that tommotiids were slug-like animals similar to Halkieria, except that tommotiids' armor was made of organophosphatic compounds while that of Halkieria was made of calciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
. However fossils of a new tommotiid, EccentrothecaEccentrotheca is a genus of sclerite-bearing marine organism known from Cambrian deposits. Its sclerite form rings that are stacked to produce a widening-upwards conical scleritome. Individual plates have been homologized with the valves of brachiopods, and a relationship with the phoronids is...
, showed an assembled mail coat that formed a tube, which would indicate a sessile animal rather than a creeping slug-like one. Eccentrothecas organophosphatic tube resembled that of phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s, sessile animals that feed by lophophoreThe lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...
s and are regarded either very close relatives or a sub-group of brachiopods.{{sfn|Cohen: Phoronids in Brachios|(2000|)}} Paterimitra, another mostly assembled fossil found in 2008 and described in 2009, had two symmetrical plates at the bottom, like brachiopod valves but not fully enclosing the animal's body.
At their peak in the PaleozoicThe Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
the brachiopods were among the most abundant filter-feeders and reef-builders,{{sfn|Barry: Great Dying|(2002)||}} and occupied other ecological nicheIn ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s, including swimming in the jet-propulsion style of scallopA scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...
s.{{sfn|Cohen: Brachiopoda ELS|(2002)|}} However, after the Permian–Triassic extinction event, informally known as the "Great Dying",{{sfn|Barry: Great Dying|(2002)||}} brachiopods recovered only a third of their former diversity.{{sfn|Barry: Great Dying|(2002)||}} It was often thought that brachiopods were actually decline in diversity, and that in some way bivalves out-competed them. However, in 1980 GouldStephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
and Calloway produced a statistical analysis that concluded that: both brachiopods and bivalves increased all the way from the Paleozoic to modern times, but bivalves increased faster; the Permian–Triassic extinction was moderately severe for bivalves but devastating for brachiopods, so that brachiopods for the first time were less diverse than bivalves and their diversity after the Permian increased from a very low base; there is no evidence that bivalves out-competed brachiopods, and short-term increases or decreases for both groups appeared at the same times.{{sfn|Gould etc: Clams and Brachios|(1980)}} In 2007 Knoll and Bambach concluded that brachiopods were one of several groups that were most vulnerable to the Permian–Triassic extinction, as all had calcareous hard parts (made of calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
) and had low metabolic rates and weak respiratory systems.{{sfn|Knoll etc: P-Tr extinction|(2007)|}}
Brachiopod fossils have been useful indicators of climate changes during the PaleozoicThe Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
era. When global temperatures were low, as in much of the OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
, the large difference in temperatures between equator and poles created different collections of fossils at different latitudeIn geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s. On the other hand, warmer periods, such much of the SilurianThe Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
, created smaller difference in temperatures, and all seas at the low to middle latitudes were colonized by the same few brachiopod species.{{sfn|Gaines etc: Inverteb proxies|(2009)|}}
Deuterostomes or protostomes
From about the 1940s to the 1990s, family trees based on embryologicalEmbryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
and morphologicalIn biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
features placed brachiopods among or as a sister group to the deuterostomeDeuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s.{{sfn|Halanych: New phylogeny|(2004)|}} a super-phylum which includes chordateChordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
s and echinodermEchinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s.{{sfn|UCMP: Deuterostomia|}} Closer examination has found difficulties in the grounds on which brachiopods were affiliated with deuterostomes:
- Radial cleavage in the earliest divisions
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
of the egg appears to be the original condition for the ancestral bilaterians, in the earliest Ecdysozoa and possibly in the earliest Eutrochozoa, a major sub-group of the Lophotrochozoa.{{sfn|Valentine: Cleavage patterns|}} Hence radial cleavage does not imply that brachiopods are affiliated with deuterostomes.
- The traditional view is that the coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
(s) in deuterostomes and protostomes form by different process, called enterocoelyEnterocoely is a process by which some animal embryos develop. In enterocoely, a mesoderm is formed in a developing embryo, in which the coelom forms from pouches "pinched" off of the digestive tract...
and schizocoely respectively. However, research since the early 1990s has found significant exceptions.{{sfn|Valentine: Cleavage patterns|(1997)|}} Both types of coelom construction appear among brachiopods, and therefore do not imply that brachiopods are deuterostomes.
- The terms "deuterostomes" and "protostomes" originally defined distinct ways of forming the mouth from the blastopore
A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...
, a depression that appears in an early stage of the embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. However, some "protostomes" forming the mouth using a process more like that typical of deuterostomes. Hence forming the mouth via a deuterostome-like process does not imply that brachiopods are affiliated with deuterostomes.
Nielsen views the brachiopods and closely related phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s as affiliated with the deuterostome pterobranchs because their lophophores are driven by one cilium per cell, while those of bryozoans, which he regards as protosomes, have multiple cilia per cell.{{sfn|Nielsen: Phylog pos of Brachios|(2002)|}} However, pterobranchs are hemichordates and probably closely related to echinodermEchinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s, and there is no evidence that the latest common ancestor of pterobranchs and other hemichordates or the latest common ancestor of hemichordates and echinoderms was sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
and fed by means of tentacles.
From 1988 onwards analyses based on molecular phylogenyMolecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree...
, which compares biochemicalBiochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
features such as similarities in DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
, have placed brachiopods among the LophotrochozoaThe Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
, a protostomeProtostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
super-phylum that includes molluscs, annelidThe annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s and flatwormThe flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s but excludes the other protostome super-phylum EcdysozoaEcdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...
, whose members include arthropodAn arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s.{{sfn|Halanych: New phylogeny |(2004)|}} This conclusion is unanimous among molecular phylogeny studies, which use a wide selection of genes: rDNARibosomal DNA codes for ribosomal RNA. The ribosome is an intracellular macromolecule that produces proteins or polypeptide chains. The ribosome itself consists of a composite of proteins and RNA. As shown in the figure, rDNA consists of a tandem repeat of a unit segment, an operon, composed of...
, Hox genes, mitochondrial proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
genes, single nuclearIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
genes and sets of nuclear protein genes.{{sfn|Helmkampf etc: Lophotrochozoa concept|(2008)|}}
Some combined studies in 2000 and 2001, using both molecular and morphological data, support brachiopods as Lophotrochozoa,{{sfn|Giribet etc: Combined phylogeny|(2000)|}}{{sfn|Peterson etc: Combined phylogeny|(2001)|}} while others in 1998 and 2004 concluded that brachiopods were deuterostomes.{{sfn|Helmkampf etc: Lophotrochozoa concept|(2008)|}}
Relationship with other lophotrochozoans
The phoronidPhoronids are a phylum of marine animals that filter-feed with a lophophore , and build upright tubes of chitin to support and protect their soft bodies. They live in all the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about...
s feed with a lophophore, burrow or encrust on surfaces, and build three-layered tubes made of polysaccharidePolysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
, possibly chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
, mixed with particles with seabed material. Traditionally they have been regarded as a separate phylum, but increasingly detailed molecular phylogeny studies between 1997 and 2000 have concluded that phoronids are a sub-group of brachiopods.{{sfn|Cohen: Phoronids in Brachios|(2000|)}} However, an analysis in 2005 concluded that phoronids are a sub-group of bryozoaThe Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
ns.{{sfn|Wood etc: Phylactolaemate Phylog|(2005)|}}
While all molecular phylogeny studies and half the combined studies until 2008 conclude that brachiopods are lophotrochozoaThe Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
ns, they could not identify which lophotrochozoan phylum were the closest relatives of brachiopods - except phoronids, which are a sub-group of brachiopods.{{sfn|Cohen: Phoronids in Brachios|(2000|)}}{{sfn|Helmkampf etc: Lophotrochozoa concept|(2008)|}} However in 2008 two analyses found that brachiopods' closest lophotrochozoan relatives were nemertineNemertea is a phylum of invertebrate animals also known as "ribbon worms" or "proboscis worms". Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela. Although most are less than long, one specimen has been estimated at , which would make it the longest animal ever...
s. The authors found this surprising, since nemertines have spiral cleavage in the early stages of cell divisionCell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
and form a trochophoreA trochophore is a type of free-swimming planktonic marine larva with several bands of cilia.By moving their cilia rapidly, a water eddy is created. In this way they control the direction of their movement...
larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
, while brachiopods have radial cleavage and a larva that shows no sign of having evolved from a trochophore.{{sfn|Dunn etc: Close to Nemertines|(2008)|}}{{sfn|Bourlat etc: Close to Nemertines|(2008)|}} Another study in 2008 also concluded that brachiopods are closely related to nemertines, casting doubt on the idea that brachiopods are part of a cladeA clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
Lophophorata of lophophore-feeding animals within the lophotrochozoans.{{sfn|Helmkampf etc: Lophotrochozoa concept|(2008)|}}
External links
{{Commons category|Brachiopoda}}
{{Animalia}}
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