The
Ctenophora , commonly known as
comb jellies, are a
phylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia — adults of various species range from a few millimeters to in size. Like
cnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
ns, their bodies consist of a mass of jelly with one layer of
cellsThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
on the outside and another lining the internal cavity. In ctenophores these layers are two cells deep while those in cnidarians are only one cell deep. Ctenophores also resemble cnidarians in having a decentralized
nerve netA nerve net is a type of simple nervous system that is found in members of the cnidaria and echinodermata phyla . Nerve nets consist of interconnected neurons lacking a brain or any form of cephalization. This nervous system allows cnidarians to respond to physical contact. They may then detect...
rather than a brain. Some authors combined ctenophores and cnidarians in one phylum,
CoelenterataCoelenterata is an obsolete long term encompassing two animal phyla, the Ctenophora and the Cnidaria . The name comes from the Greek "koilos" , referring to the hollow body cavity common to these two phyla...
, as both groups rely on water flow through the body cavity for both digestion and respiration. Increasing awareness of the differences persuaded more recent authors to
classifyBiological classification or scientific classification in biology, is a method by which biologists group and categorize organisms by biological type, such as genus or species. Biological classification is a form of scientific taxonomy, but should be distinguished from folk taxonomy, which lacks...
them in separate phyla.
Almost all ctenophores are predators, taking prey ranging from microscopic
larvaA larva is a young form of animal with indirect development, going through or undergoing metamorphosis ....
e and
rotiferThe rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
s to the adults of small crustaceans; the exceptions are juveniles of two species, which live as parasites on the
salpA salp is a barrel-shaped, free-floating tunicate. It moves by contracting, thus pumping water through its gelatinous body...
s on which adults of their species feed. In favorable circumstances ctenophores can eat ten times their own weight in a day. There are only 100–150 valid species and possibly another 25 that have not been fully described and named. The textbook examples are cydippids with egg-shaped bodies and a pair of retractable tentacles fringed with tentilla ("little tentacles") that are covered with
colloblastColloblasts are a cell type of ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. On contact, vesicles containing a gluey substance rupture, and thus entangle the prey animal in the tentilla, the fine threads attached to the tentacle.The colloblasts are...
s, sticky cells that capture prey. The phylum has a wide range of body forms, including the flattened deep-sea platyctenids, in which the adults of most species lack combs, and the coastal beroids, which lack tentacles and prey on other ctenophores by using huge mouths armed with groups of large, stiffened cilia that act as "teeth". These variations enable different species to build huge populations in the same area, because they specialize in different types of prey, which they capture by as wide a range of methods as
spiderSpiders are air-breathing chelicerate arthropods that have eight legs, and chelicerae modified into fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s use.
Most species are hermaphrodites, in other words a single animal can produce both eggs and sperm; if they are both produced at the same time, the animal is a simultaneous hermaphrodite, and if the eggs and sperm mature at different times, the animal is a sequential hermaphrodite. Fertilization is generally
externalExternal fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....
, although platyctenids' eggs are fertilized inside their parents' bodies and kept there until they hatch. The young are generally
planktonPlankton consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their phylogenetic or taxonomic classification...
ic and in most species look like miniature cydippids, gradually changing into the adult shape as they grow. The exceptions are the beroids, whose young are miniature beroids with large mouths and no tentacles, and the platyctenids, whose young live as cydippid-like plankton until they reach near-adult size, but then sink to the bottom and rapidly
metamorphoseMetamorphosis 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 the adult form. In at least some species, juveniles are capable of reproduction before reaching the adult size and shape. The combination of hermaphroditism and early reproduction enables small populations to grow at an explosive rate.
Predation by ctenophores controls the populations of small planktonic populations such as
copepodCopepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Many species are planktonic , but more are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s, which might otherwise wipe out the
phytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτον , or "plant", and πλαγκτος , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
(planktonic plants), which are a vital part of marine
food chainFood chains describe the eating relationships between species within an ecosystem or a particular living place. Many types of food chains or webs are applicable depending on habitat or environmental factors...
s. One ctenophore,
Mnemiopsis, has accidentally been
introducedAn introduced, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its native distributional range, which has arrived there by human activity, either deliberate or accidental...
into the
Black Seaur a loser!The Black Sea is an inland sea bounded by Europe, Anatolia and the Caucasus and is ultimately connected to the Atlantic Ocean via the Mediterranean and Aegean Seas and various straits. The Bosporus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects it to...
, where it is blamed for causing fish stocks to collapse by eating both fish larvae and organisms that would otherwise have fed the fish. The situation was aggravated by other factors, such as over-fishing and environmental changes that promoted the growth of the
Mnemiopsis population. The later accidental introduction of
Beroe helped to mitigate the problem, as
Beroe preys on other ctenophores.
Despite their soft, gelatinous bodies,
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
s thought to represent ctenophores, apparently with no tentacles but many more comb-rows than modern forms, have been found in
lagerstätteA Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness. Palaeontologists distinguish two kinds....
n as far back as the early
CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
, about . The position of the ctenophores in the evolutionary family tree of animals has long been debated, and the majority view at present, based on molecular phylogenetics, is that cnidarians and
bilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum of animals, including the majority...
ns are more closely related to each other than either is to ctenophores. A recent molecular phylogenetics analysis concluded that the common ancestor of all modern ctenophores was cydippid-like, and that all the modern groups appeared relatively recently, probably after the Cretaceous-Tertiary extinction . Evidence accumulating since the 1980s indicates that the "cydippids" are not monophyletic, in other words do not include all and only the descendants of a single common ancestor, because all the other traditional ctenophore groups are descendants of various cydippids.
Distinguishing features
Ctenophores form an
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
phylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
that is more complex than sponges, about as complex as
cnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
ns (
jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa Jellyfish (also known as jellies or sea jellies) are free-swimming members of...
,
sea anemoneSea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower...
s, etc.), and less complex than
bilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum of animals, including the majority...
ns, which include almost all other animals. Unlike sponges, both ctenophores and cnidarians have: cells bound by inter-cell connections and carpet-like
basement membraneThe basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
s;
muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s;
nervous systemThe nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells that aid in the...
s; and some have
sensorySensory may refer to:relating to senses or smellIn biology:* Sensory preference* Sensory system, part of the nervous system of organisms* Sensory neuron, nerve cell responsible for transmitting information about external stimuli...
organs. Ctenophores are distinguished from all other animals by having
colloblastColloblasts are a cell type of ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. On contact, vesicles containing a gluey substance rupture, and thus entangle the prey animal in the tentilla, the fine threads attached to the tentacle.The colloblasts are...
s that capture prey by squirting glue on them, although a few ctenophore species lack them.
Like sponges and cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the
mesogleaMesoglea is the translucent, inert, jellylike substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phylum Cnidaria. It acts as the creatures' structural support in water, as they lack bones, cartilage or other more common means of support...
in cnidarians; more complex
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
s have three main cell layers and no intermediate jelly-like layer. Hence ctenophores and cnidarians have traditionally been labelled diploblastic, along with sponges. Both ctenophores and cnidarians have a type of
muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
that, in more complex animals, arises from the
middle cell layerIn humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the...
, and as a result some recent text books classify ctenophores as triploblastic, while others still regard them as diploblastic.
Ranging from about to in size, ctenophores are the largest non-colonial animals that use cilia ("hairs") as their main method of locomotion. Most species have up to eight strips that run the length of their bodies and bear comb-like bands of cilia running at intervals across the strips. The name "ctenophore" means "comb-bearing", from the Greek κτείς (stem-form κτεν-) meaning "comb" and the Greek suffix -φορος meaning "carrying".
Comparison with other major animal groups
| | Sponges | Cnidarians | Ctenophores | BilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum of animals, including the majority...
|
|---|
| Cnidocyte A cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple lacking a skeleton and usually being sessile, cnidarians prey on...
s |
No |
Yes |
No |
| Colloblast Colloblasts are a cell type of ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. On contact, vesicles containing a gluey substance rupture, and thus entangle the prey animal in the tentilla, the fine threads attached to the tentacle.The colloblasts are...
s |
No |
In most species |
No |
| Digestive and circulatory - [Headline text]--75.137.171.242 01:21, 21 October 2009 :bThe circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, nitrogen waste products, etc. to and from cells in the body to help fight diseases and help stabilize body temperature and pHb to maintain...
organIn biology and anatomy, an organ is a collection of tissues joined in structural unit to serve a common function ....
s |
No |
Yes |
| Number of main cell layers |
Two, with jelly-like layer between them |
Debate about whether two or three |
three |
| Cells in each layer bound together |
No, except that HomoscleromorphaHomoscleromorpha is a subclass of marine demosponges containing a single order, Homosclerophorida and a single family, Plakinidae. These sponges are massive or encrusting in form and have a very simple structure with very little variation in spicule form . Reproduction is viviparous and the larva...
have basement membranes. |
Yes: inter-cell connections; basement membranes |
| Sensory Sensory may refer to:relating to senses or smellIn biology:* Sensory preference* Sensory system, part of the nervous system of organisms* Sensory neuron, nerve cell responsible for transmitting information about external stimuli...
organs |
No |
Yes |
| Number of cells in middle "jelly" layer |
Many |
Few |
(not applicable) |
| Cells in outer layers can move inwards and change functions |
Yes |
No |
(not applicable) |
| Nervous system |
No |
Yes, simple |
Simple to complex |
MuscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s |
None |
Mostly epitheliomuscular |
Mostly myoepithelial |
Mostly myocyte A myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long chains of sarcomeres, the contractile units of the cell....
s |
Description
For a phylum with relatively few species, ctenophores have a wide range of body plans. Coastal species need to be tough enough to withstand waves and swirling sediment particles, while oceanic species are so fragile that it is very difficult to capture them intact for study. In addition oceanic species do not preserve well, and are known mainly from photographs and from observers' notes. Hence most attention has until recently concentrated on three coastal
generaIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
—
Pleurobrachia,
Beroe and
Mnemiopsis. At least two textbooks base their descriptions of ctenophores on the cydippid
Pleurobrachia.
Since the body of many species is
almost radially symmetrical, the main axis is
oralThe mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva...
to
aboralIn biology, aboral surfaces are surfaces away from or opposite the mouth. The term is a compound of the Latin preposition ā, a, abs, meaning from or away from and the noun ōs, ōris n., meaning mouth....
(from the mouth to the opposite end). However since only two of the canals near the
statocystThe statocyst is a balance organ present in some aquatic invertebrates . It consists of a sac-like structure containing a mineralised mass and numerous innervated sensory hairs . The statolith possesses inertia, causing the mass to move when accelerated...
terminate in
analThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel 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, such as bones; food...
pores, ctenophores have no mirror-symmetery, although many have rotational symmetry, in other words if the animal rotates in a half-circle it looks the same as when it started.
Body layers
Like those of
cnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
ns, (
jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa Jellyfish (also known as jellies or sea jellies) are free-swimming members of...
,
sea anemoneSea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower...
s, etc.), ctenophores' bodies consist of a relatively thick, jelly-like
mesogleaMesoglea is the translucent, inert, jellylike substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phylum Cnidaria. It acts as the creatures' structural support in water, as they lack bones, cartilage or other more common means of support...
sandwiched between two
epitheliaIn biology and medicine, an epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body. Many glands are also formed from epithelial tissue...
, layers of
cellsThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
bound by inter-cell connections and by a fibrous
basement membraneThe basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
that they secrete. The epithelia of ctenophores have two layers of cells rather than one, and the cells in the upper layer generally have several cilia per cell.
The outer layer of the
epidermisEpidermis may refer to:* Epidermis , in plants, the outermost layer of cells covering the leaves and young parts of a plant* Epidermis , in vertebrates, the outermost layer of the skin...
(outer skin) consists of: sensory cells; cells that secrete
mucusIn vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. It is a viscous colloid containing antiseptic enzymes and immunoglobulins that serves to protect epithelial cells in the respiratory,...
, which protects the body; and interstitial cells, which can transform into other types of cell. In specialized parts of the body the outer layer also contains
colloblastColloblasts are a cell type of ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. On contact, vesicles containing a gluey substance rupture, and thus entangle the prey animal in the tentilla, the fine threads attached to the tentacle.The colloblasts are...
s, used in capturing prey, or cells bearing multiple large cilia, for locomotion. The inner layer of the epidermis contains a
nerve netA nerve net is a type of simple nervous system that is found in members of the cnidaria and echinodermata phyla . Nerve nets consist of interconnected neurons lacking a brain or any form of cephalization. This nervous system allows cnidarians to respond to physical contact. They may then detect...
, and myoepithelial cells that act as
muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s.
The internal cavity forms: a mouth that can usually be closed by muscles; a
pharynxThe pharynx is the part of the neck and throat situated immediately posterior to the mouth and nasal cavity, and cranial, or superior, to the esophagus, larynx, and trachea.-Functions:...
("throat"); a wider area in the center that acts as a
stomachIn most mammals, the stomach is a hollow, muscular organ of the gastrointestinal tract , between the esophagus and the small intestine. It is involved in the second phase of digestion, following mastication . The word stomach is derived from the Latin stomachus, which derives from the Greek word...
; and a system of internal canals. These branch through the mesoglea to the most active parts of the animal: the mouth and pharynx; the roots of the tentacles, if present; all along the underside of each comb row; and four branches round the sensory complex at the far end from the mouth — two of these four branches terminate in
analThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel 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, such as bones; food...
pores. The inner surface of the cavity is lined with an
epitheliumIn biology and medicine, an epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body. Many glands are also formed from epithelial tissue...
, the
gastrodermisThe cellular lining of the digestive cavity of certain invertebrates.--------------------------------------------------------------------------------The gastrodermis is the inner layer of cells that lines a gastrovascular cavity of Cnidarians....
. The mouth and pharynx have both cilia and well-developed muscles. In other parts of the canal system, the gastrodermis is different on the sides nearest to and furthest from the organ that it supplies. The nearer side is composed of tall nutritive cells that store nutrients in
vacuolethumb|400px|Plant cell structurethumb|400px|Animal cell structureA vacuole is a membrane organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells...
s (internal compartments),
germ cellGerm cells are progenitors of the gametes. These singled-out cells move through the gut to the developing gonads and undergo mitotic proliferation followed by meiosis and differentiation into either eggs or sperm . Plants do not have a germ line set aside in early development...
s that produce eggs or sperm, and
photocytesA Photocyte is a cell that specializes in catalyzing enzymes to produce light . Photocytes typically occur in select layers of epithelial tissue, functioning singly or in a group, or as part of a larger apparatus ....
that produce
bioluminescenceBioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
. The side furthest from the organ is covered with ciliated cells that circulate water through the canals, punctuated by ciliary rosettes, pores that are surrounded by double whorls of cilia and connect to the mesoglea.
Feeding, excretion and respiration
When prey is swallowed, it is liquidized in the
pharynxThe pharynx is the part of the neck and throat situated immediately posterior to the mouth and nasal cavity, and cranial, or superior, to the esophagus, larynx, and trachea.-Functions:...
by
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at...
s and by muscular contractions of the pharynx. The resulting slurry is wafted through the canal system by the beating of the cilia, and digested by the nutritive cells. The ciliary rosettes in the canals may help to transport nutrients to muscles in the mesoglea. The
analThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel 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, such as bones; food...
pores may eject unwanted small particles, but most unwanted matter is regurgitated via the mouth.
Little is known about how ctenophores get rid of waste products produced by the cells. The ciliary rosettes in the
gastrodermisThe cellular lining of the digestive cavity of certain invertebrates.--------------------------------------------------------------------------------The gastrodermis is the inner layer of cells that lines a gastrovascular cavity of Cnidarians....
may help to remove wastes from the mesoglea, and may also help to adjust the animal's
buoyancyIn physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter....
by pumping water into or out of the mesoglea.
Locomotion
The outer surface bears several rows of "combs", which are used for swimming. The rows run from near the mouth to the opposite end, and are spaced more or less evenly around the body, although spacing patterns vary by species. The "combs" run across across each row, and each consists of thousands of unusually long cilia, up to . These normally beat so that the propulsion stroke is away from the mouth, although they can also reverse direction. Hence ctenophores usually swim in the direction in which the mouth is pointing, unlike
jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa Jellyfish (also known as jellies or sea jellies) are free-swimming members of...
. When trying to escape predators, some species can accelerate to six times their normal speed.
It is uncertain how ctenophores control their buoyancy, but possibly they rely on
osmotic pressureOsmotic pressure is the pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane.Jacobus Henricus van 't Hoff first proposed a formula for calculating the osmotic pressure, but this was later improved upon by Harmon Northrop Morse.A related...
to adapt to water of different densities. Their body fluids are normally as
concentratedIn chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent.To...
as seawater. If they enter less dense brackish water, the ciliary rosettes in the body cavity may pump this into the
mesogleaMesoglea is the translucent, inert, jellylike substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phylum Cnidaria. It acts as the creatures' structural support in water, as they lack bones, cartilage or other more common means of support...
to increase its bulk and decrease its density, to avoid sinking. Conversely if they move from brackish to full-strength seawater, the rosettes may pump water out of the mesoglea to reduce its volume and increase its density.
Nervous system and senses
Ctenophores have no
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
or
central nervous systemThe central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of bilaterian animals—that is, all animals more advanced than sponges or jellyfish. In vertebrates, the central nervous system is enclosed in the meninges. It contains...
, but instead have a
nerve netA nerve net is a type of simple nervous system that is found in members of the cnidaria and echinodermata phyla . Nerve nets consist of interconnected neurons lacking a brain or any form of cephalization. This nervous system allows cnidarians to respond to physical contact. They may then detect...
(rather like a cobweb) that forms a ring round the mouth and is densest near structures such as the comb rows, pharynx, tentacles (if present) and the sensory complex furthest from the mouth.
The largest single sensory feature is the
aboralIn biology, aboral surfaces are surfaces away from or opposite the mouth. The term is a compound of the Latin preposition ā, a, abs, meaning from or away from and the noun ōs, ōris n., meaning mouth....
organ (at the opposite end from the mouth). Its main component is a
statocystThe statocyst is a balance organ present in some aquatic invertebrates . It consists of a sac-like structure containing a mineralised mass and numerous innervated sensory hairs . The statolith possesses inertia, causing the mass to move when accelerated...
, a balance sensor consisting of a statolith, a solid particle supported on four bundles of cilia, called "balancers", that sense its orientation. The statocyst is protected by a transparent dome made of long, immobile cilia. A ctenophore does not automatically try to keep the statolith resting equally on all the balancers. Instead its responses is determined by the animals' "mood", in other words the overall state of the nervous system. For example if a ctenophore with trailing tentacles captures prey, it will often put some comb rows into reverse, spinning the mouth towards the prey.
Cydippids
Cydippid ctenophores have bodies that are more or less rounded, sometimes nearly spherical and other times more cylindrical or egg-shaped; the common "sea gooseberry,"
Pleurobranchia, has an egg-shaped body with the mouth at the narrow end. From opposite sides of the body extends a pair of long, slender tentacles, each housed in a sheath into which it can be withdrawn. Some species of cydippids have bodies that are flattened to various extents, so that they are wider in the plane of the tentacles.
The tentacles of cydippid ctenophores may be fringed with tentilla ("little tentacles"), although some genera have simple tentacles without these sidebranches; the tentacles and tentilla are densely covered with microscopic
colloblastColloblasts are a cell type of ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. On contact, vesicles containing a gluey substance rupture, and thus entangle the prey animal in the tentilla, the fine threads attached to the tentacle.The colloblasts are...
s that capture prey by sticking to it. Colloblasts are specialized
mushroomA mushroom is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground on soil or on its food source. The standard for the name "mushroom" is the cultivated white button mushroom, Agaricus bisporus, hence the word mushroom is most often applied to those fungi that have...
-shaped cells in the outer layer of the epidermis, and have three main components: a domed head with
vesiclesA vesicle is a bubble of liquid within a cell. More technically, a vesicle is a small, intracellular, membrane-enclosed sac that stores or transports substances within a cell. Vesicles form naturally because of the properties of lipid membranes ...
(chambers) that contain adhesive; a stalk that anchors the cell in the lower layer of the epidermis or in the mesoglea; and a
spiralIn mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point.-Three-dimensional spirals:...
thread that coils round the stalk and is attached to the head and to the root of the stalk. The function of the spiral thread is uncertain, but it may absorb stress when prey tries to escape, and thus prevent the collobast from being torn apart. In addition to colloblasts, members of the genus
Haeckelia, which feed mainly on
jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa Jellyfish (also known as jellies or sea jellies) are free-swimming members of...
, incorporate their victims' stinging nematocytes into their own tentacles — some cnidaria-eating
nudibranchA nudibranch ) is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, shell-less marine opisthobranch gastropod mollusks, which are noted for their often extraordinary colors and striking forms...
s also incorporate nematocytes into their bodies for defense. The tentilla of
Euplokamis differ significantly from those of other cydippids: they contain
striated muscleStriated muscle is a form of fibers that are combined into parallel fibers. More specifically, it can refer to:* Skeletal muscle* Cardiac muscle~cardiac referring to the heart....
, a cell type otherwise unknown in the phylum Ctenophora; and they are coiled when relaxed, while those of all other known ctenophores elongate when relaxed. Euplokamis tentilla have three types of movement that are used in capturing prey: they may flick out very quickly (in 40 to 60 milliseconds); they can wriggle, which may lure prey by behaving like small planktonic worms; and they coil round prey. The unique flicking is an uncoiling movement powered by contraction of the
striated muscleStriated muscle is a form of fibers that are combined into parallel fibers. More specifically, it can refer to:* Skeletal muscle* Cardiac muscle~cardiac referring to the heart....
. The wriggling motion is produced by
smooth muscleSmooth muscle is an involuntary non-striated muscle, found within the tunica media layer of large and small arteries and veins, the bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, arrector pili of skin, the ciliary muscle, and iris of...
s, but of a highly specialized type. Coiling around prey is accomplished largely by the return of the tentilla to their inactive state, but the coils may be tightened by smooth muscle.
There are eight rows of combs that run from near the mouth to the opposite end, and are spaced evenly round the body. The "combs" beat in a
metachronal rhythmA metachronal rhythm or metachronal wave refers to wavy movements produced by the sequential action of structures such as cilia, segments of worms or legs. These movements produce the appearance of a travelling wave. A Mexican wave is an large scale example of a metachronal wave...
rather like that of a Mexican wave. From each balancer in the statocyst a ciliary groove runs out under the dome and then splits to connect with two adjacent comb rows, and in some species runs all the way along the comb rows. This forms a mechanical
system for transmitting the beat rhythm from the balancers to the combs, via water disturbances created by the cilia.
Lobates
The LobataLobata is an order of Ctenophora in the class Tentaculata with smaller tentacles than other ctenophores. It was first named by Johann Friedrich Eschscholtz in 1825. They are found in the Black Sea as well as the Caspian Sea. They are about 25 cm long....
have a pair of lobes, muscular extensions of the body that project ahead of the mouth. Their small tentacles run in convoluted grooves and spread out over the inner surface of the lobes. Between the lobes and surrounding the mouth, many species of lobates have four auricles, projections fringed with cilia that produce water currents that wash microscopic prey into the mouth. This combination of structures enables lobates to feed continuously on suspended prey such as planktonPlankton consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their phylogenetic or taxonomic classification...
.
There are eight comb-rows, two on each lobe and two on each side between the lobes. Most lobates are quite passive when moving through the water, using the cilia on their comb rows for propulsion, but members of the generaIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
Leucothea and OcyropsisOcyropsis is a genus of oceanic lobate ctenophores. Species include: O. crystallina, O. maculata, O. fusca and O. pteroessa....
can also simultaneously clap their large lobes for swimming. Unlike cydippids, the movements of lobates' combs are coordinated by nerves rather than by via water disturbances created by the cilia, and combs on the same row beat together rather than in Mexican wave style. This may have enabled lobates to grow larger than cydippids and to have shapes that are less egg-like. Members of the lobate generaIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
BathocyroeBathocyroe is a genus of ctenophores, the only genus in the family Bathocyroidae....
and OcyropsisOcyropsis is a genus of oceanic lobate ctenophores. Species include: O. crystallina, O. maculata, O. fusca and O. pteroessa....
can escape from danger by clapping their lobes, so that the jet of expelled water drives them backwards very quickly.
An unusual species first described in 2000, Lobatolampea tetragona, has been classified as a lobate, although the lobes are "primitive" and the body is medusaIn biology, a medusa is a form of cnidarian in which the body is shortened on its principal axis and broadened, sometimes greatly, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below...
-like when floating and disk-like when resting on the sea-bed.
Beroids
The Beroida, also known as NudaNuda is a class of Ctenophora known as comb jellies, and refers to the characteristic feature of representatives of this taxon, the lack of tentacles.-Anatomy:...
, have no feeding appendages, but their large pharynxThe pharynx is the part of the neck and throat situated immediately posterior to the mouth and nasal cavity, and cranial, or superior, to the esophagus, larynx, and trachea.-Functions:...
es, just inside their large mouths, bear "macrocilia", fused bundles of several thousand large cilia that bite off pieces of prey that is too large to swallow whole — almost always other ctenophores. Just behind the ring of macrocilia is an internal ridge that runs all round the mouth and "zips" it shut when the animal is not feeding, by forming intercellular connections with the opposite part of the ridge. This tight closure streamlinesA streamliner is any vehicle that incorporates streamlining to produce a shape that provides less resistance to air. The term is most often applied to certain high-speed railway trainsets of the 1930s to 1950s, and to their successor "bullet trains". Less commonly, the term is applied to fully...
the front of the animal when it is pursuing prey.
Other body forms
The Ganeshida have a pair of small oral lobes and a pair of tentacles. The body is circular rather than oval in cross-section, and the pharynx extends over the inner surfaces of the lobes.
The Thalassocalycida, only discovered in 1978 and known from only one species, are medusa-like, with bodies that are shortened in the oral-aboral direction, and short comb-rows on the surface furthest from the mouth. They capture prey by movements of the bell and possibly by using two short tentacles.
The Cestoida ("belt animals"), are ribbon-shaped planktonic animals, with the mouth and aboral organ in the middle of opposite edges of the ribbon. There is a pair of comb-rows along each aboral edge, and tentilla on the oral edge. Cestoids can swim by undulating their bodies as well as by means of their comb-rows. The two known species, Cestum veneris ("VenusVenus was a major Roman goddess principally associated with love, beauty and fertility, who played a key role in many Roman religious festivals and myths. From the third century BC, the increasing Hellenization of Roman upper classes identified her as the equivalent of the Greek goddess...
' girdle") and Velamen parallelum, are the largest ctenophores — up to and long respectively.
The Platyctenida have oval bodies that are flattened in the oral-aboral direction, with a pair of tentilla-bearing tentacles on the aboral surface. They cling to and creep on surfaces by everting the pharynx and using it as a muscular "foot". All but one of the known platyctenid species lack comb-rows.
Reproduction and development
Adults of most species can regenerate tissues that are damaged or removed, although only platyctenids reproduce by cloningCloning in biology is the process of producing populations of genetically-identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or organisms...
, splitting off from the edges of their flat bodies fragments that develop into new individuals.
Almost all species are hermaphrodites, in other words they function as both males and females at the same time — except that in two species of the genusIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
Ocryopsis individuals remain of the same single sex all 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 located in the parts of the internal canal network under the comb rows, and eggs and sperm are released via pores in the epidermis. Fertilization is externalExternal fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....
in most species, but platyctenids use internal fertilzation and keep the eggs in brood chambers until they hatch. Self-fertilization has occasionally been seen in species of the genus Mnemiopsis, and it is thought that most of the hermaphroditic species are self-fertile.
Development of the fertized eggs is direct, in other words there is no distinctive larval form, and juveniles of all groups generally resemble miniature cydippid adults. In the genus Beroe the juveniles, like the adults, lack tentacles and tentacle sheaths. In most species the juveniles gradually develop the body forms of their parents. In some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae, as they: live among the plankton and thus occupy a different 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 school if the members of these schools utilize significantly different...
from their parents; and attain the adult form by a more radical metamorphosisMetamorphosis 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...
, after dropping to the sea-floor.
Juvenile ctenophores appear capable of producing small quantities of eggs and sperm while they are well below adult size, and adults produce eggs and sperm for long as they have sufficient food. If they run short of food, they first stop producing eggs and sperm, and then shrink in size. When the food supply improves, they grow back to normal size and then resume reproduction. These features make ctenophores capable of increasing their populations very quickly.
Colors and bioluminescence
Most ctenophores that live near the surface are mostly colorless and almost transparent. However some deeper-living species are strongly pigmented, for example the species known as "Tortugas red" (see illustration here), which has not been formally described. Platyctenids generally live attached to other sea-bottom organisms, and often have similar colors to these organisms'. The gut of the deep-sea genus BathocyroeBathocyroe is a genus of ctenophores, the only genus in the family Bathocyroidae....
is red, which hides the bioluminescenceBioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
of copepodCopepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Many species are planktonic , but more are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s it has swallowed.
Many planktonic ctenophores produce a rainbow effect, which is not caused by bioluminescenceBioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
but by the scattering of lightDiffraction is normally taken to refer to various phenomena which occur when a wave encounters an obstacle. It is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings...
as the combs move. Most species are also bioluminescent, but the light is usually blue or green and can only been seen in darkness. However some significant groups, including all known platyctenids and the cydippid genusIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
Pleurobrachia, appear incapable of bioluminescence.
When some species, including Bathyctena chuni, Euplokamis stationis and Eurhamphaea vexilligera, are disturbed, they produce secretions that luminesce at much the same wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s as their bodies. Juveniles can luminesce more brightly in relation to their body size than adults, whose luminescence is diffused over their bodies. Detailed statistical investigation has not suggested the function of ctenophores' bioluminescence nor produced any correlationIn statistics, correlation indicates the strength and direction of a relationship between two random variables. The commonest use refers to a linear relationship. In general statistical usage, correlation or co-relation refers to the departure of two random variables from independence...
between its exact color and any aspect of the animals' environments, such as depth or whether they live in coastal or mid-ocean waters.
Distribution
Ctenophores are found in most marine environments: from polar waters to the tropics; near coasts and in mid-ocean; from the surface waters to the ocean depths. The best-understood are the generaIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
Pleurobrachia, Beroe and Mnemiopsis, as these planktonPlankton consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their phylogenetic or taxonomic classification...
ic coastal forms are easiest to study.
Prey and predators
Almost all ctenophores are predators — there are no vegetarians and only one genus that is partly parasitic. If food is plentiful, they can eat 10 times their own weight per day. While Beroe preys mainly on other ctenophores, other surface-water species prey on zooplanktonZooplankton are the heterotrophic type of plankton. Plankton are organisms drifting in the water column of oceans, seas, and bodies of fresh water. The name of zooplankton is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
(planktonic animals) ranging in size from the microscopic, including mollusc and fish larvae, to small adult crustaceans such as copepodCopepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Many species are planktonic , but more are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s, amphipods, and even krillKrill is a type of shrimp-like marine invertebrate animal. These small crustaceans are important organisms of the zooplankton, particularly as food for baleen whales, manta rays, whale sharks, crabeater seals, and other seals, and a few seabird species that feed almost exclusively on them. Another...
. Members of the genus Haeckelia prey on jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa Jellyfish (also known as jellies or sea jellies) are free-swimming members of...
and incorporate their prey's nematocysts (stinging cells) into their own tentacles instead of colloblastColloblasts are a cell type of ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. On contact, vesicles containing a gluey substance rupture, and thus entangle the prey animal in the tentilla, the fine threads attached to the tentacle.The colloblasts are...
s. Ctenophores have been compared to spiderSpiders are air-breathing chelicerate arthropods that have eight legs, and chelicerae modified into fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s in their wide range of techniques from capturing prey — some hang motionless in the water using their tentacles as "webs", some are ambush predators like Salticid jumping spiderThe jumping spider family contains more than 500 described genera and over 5,000 species, making it the largest family of spiders with about 13% of all species. Jumping spiders have good vision and use it for hunting and navigating. They are capable of jumping from place to place, secured by a...
s, and some dangle a sticky droplet at the end of a fine thread, as Bola spiders do. This variety explains the wide range of body forms in a phylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
with rather few species. The two-tentacled "cydippid" Lampea feeds exclusively on salpA salp is a barrel-shaped, free-floating tunicate. It moves by contracting, thus pumping water through its gelatinous body...
s, close relatives of sea-squirts that form large chain-like foating colonies, and juveniles of Lampea attach themselves like parasites to salps that are too large for them to swallow. Members of the cydippid genus Pleurobrachia and the lobate Bolinopsis often reach high population densities at the same place and time because they specialize in different types of prey: Pleurobrachias long tentacles mainly capture relatively strong swimmers such as adult copepods, while Bolinopsis generally feeds on smaller, weaker swimmers such as rotiferThe rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
s and mollusc and crustacean larvae.
Ctenophores used to be regarded as "dead ends" in marine food chains because it was thought their low ratio of organic matter to salt and water made them a poor diet for other animals. It is also often difficult to identify the remains of ctenophores in the guts of possible predators, although the combs sometimes remain intact long enough to provide a clue. Detailed investigation of chum salmonThe chum salmon, Oncorhynchus keta, is a species of anadromous fish in the salmon family. It is a Pacific salmon, and may also be known as dog salmon or Keta salmon, and is often marketed under the name Silverbrite salmon...
, Oncorhynchus keta, showed that these fish digest ctenophores 20 times as fast as an equal weight of shrimpShrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
s, and that ctenophores can provide a good diet if there enough of them around. Beroids prey mainly on other ctenophores. Some jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa Jellyfish (also known as jellies or sea jellies) are free-swimming members of...
and turtleTurtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
s eat large quantities of ctenophores, and jellyfish may temporarily wipe out ctenophore populations. Since ctenophores and jellyfish often have large seasonal variations in population, most fish that prey on them are generalists, and may have a greater effect on populations than the specialist jelly-eaters. The larvae of some sea anemoneSea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower...
s are parasites on ctenophores, as are the larvae of some flatwormThe flatworms, known in scientific literature as Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s that parasitize fish when they reach adulthood.
Ecological impacts
Ctenophores may balance marine ecoystems by preventing an over-abundance of copepods from eating all the phytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτον , or "plant", and πλαγκτος , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
(planktonic plants), which are the dominant marine producers of organic matter from non-organic ingredients.
On the other hand in the late 1980s the North Atlantic ctenophore Mnemiopsis leidyiThe warty comb jelly or sea walnut is a species of tentaculate ctenophore , originally native to the western Atlantic coastal waters....
was accidentally introducedAn introduced, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its native distributional range, which has arrived there by human activity, either deliberate or accidental...
into the Black Seaur a loser!The Black Sea is an inland sea bounded by Europe, Anatolia and the Caucasus and is ultimately connected to the Atlantic Ocean via the Mediterranean and Aegean Seas and various straits. The Bosporus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects it to...
and Sea of AzovThe Sea of Azov is the world's shallowest sea, linked by the Strait of Kerch to the Black Sea to the south. It is bounded on the north by Ukraine, on the east by Russia and on the west by the Crimean peninsula. The Don River flows into it.-Geology and bathymetry:The sea is long and wide and has...
via the ballast tanks of ships, and has been blamed for causing sharp drops in fish catches by eating both fish larvae and small crustaceans that would otherwise feed the adult fish. Mnemiopsis is well-equipped to invade new territories, as it can breed very rapidly and tolerate a wide range of water temperatures and salinitiesSalinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
. The impact was increased by overfishing, and by eutrophicationEutrophication is an increase in the concentration of chemical nutrients in an ecosystem to an extent that increases in the primary productivity of the ecosystem...
that gave the entire ecosystem a short-term boost, causing the Mnemiopsis population to increase even faster than normal — and above all by the absence of efficient predators on ctenophores. Mnemiopsis populations in those areas were eventually brought under control by the accidental introduction of the ctenophore-eating ctenophore Beroe, and by a cooling of the local climate from 1991 to 1993, which significantly slowed the animal's metabolism. However the abundance of plankton in the area seems unlikely to reach pre-Mnemiopsis levels.
In the late 1990s Mnemiopsis appeared in the Caspian SeaThe Caspian Sea is the largest enclosed body of water on Earth by area, variously classed as the world's largest lake or a full-fledged sea. It has a surface area of 371,000 square kilometres and a volume of 78,200 cubic kilometres...
. Beroe arrived shortly after, and is expected to reduce but not eliminate the impact of Mnemiopsis. Mnemiopsis also reached the eastern Mediterranean in the late 1990s and now appears to be thriving in the North SeaThe North Sea is a marginal, epeiric sea on the European continental shelf. The Dover Strait and the English Channel in the south and the Norwegian Sea in the north connect it to the Atlantic Ocean. It is more than long and wide, with an area of around...
and Baltic SeaThe Baltic Sea is a brackish inland sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and the...
.
Classification
The number of known living ctenophore species is uncertain, since many of those named and formally described have turned out to be identical to species known under other scientific names. Claudia Mills estimates that there about 100 to 150 valid species that are not duplicates, and that at least another 25, mostly deep-sea forms, have been correctly recognized as distinct but not yet analyzed in enough detail to support a formal description and naming.
The traditional classification divides ctenophores into two 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...
, those with tentacles (TentaculataTentaculata is a class of comb jellies. The common feature of this class is a pair of long, feathery, contractile tentacles, which can be retracted into specialised ciliated sheaths. In some species, the primary tentacles are reduced and they have smaller, secondary tentacles...
) and those without (NudaNuda is a class of Ctenophora known as comb jellies, and refers to the characteristic feature of representatives of this taxon, the lack of tentacles.-Anatomy:...
). The Nuda contains only one 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...
(Beroida) and familyIn biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus...
(Beroidae), and two generaIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
, Beroe (several species) and Neis (one species).
The TentaculataTentaculata is a class of comb jellies. The common feature of this class is a pair of long, feathery, contractile tentacles, which can be retracted into specialised ciliated sheaths. In some species, the primary tentacles are reduced and they have smaller, secondary tentacles...
are divided into the following eight 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...
:
- Cydippida, egg-shaped animals with long tentacles
- Lobata
Lobata is an order of Ctenophora in the class Tentaculata with smaller tentacles than other ctenophores. It was first named by Johann Friedrich Eschscholtz in 1825. They are found in the Black Sea as well as the Caspian Sea. They are about 25 cm long....
, with paired thick lobes
- Platyctenida, flattened animals that live on or near the sea-bed; most lack combs as adults, and use their pharynges
The pharynx is the part of the neck and throat situated immediately posterior to the mouth and nasal cavity, and cranial, or superior, to the esophagus, larynx, and trachea.-Functions:...
as suckers to attach themselves to surfaces
- Ganeshida, with a pair of small lobes round the mouth, but an extended pharynx
The pharynx is the part of the neck and throat situated immediately posterior to the mouth and nasal cavity, and cranial, or superior, to the esophagus, larynx, and trachea.-Functions:...
like that of platyctenids
- Cambojiida
- Cryptolobiferida
- Thalassocalycida, with short tentacles and a jellyfish-like "umbrella"
- Cestida
Cestida is an order of comb jellies with short tentacles. It has one family, Cestidae, with two genera: Cestum and Velamen, each containing one species....
, ribbon-shaped and the largest ctenophores
Fossil record
Because of their soft, gelatinous bodies, ctenophores are extremely rare as fossils, and fossils that have been interpreted as ctenophores have been found only in lagerstätteA Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness. Palaeontologists distinguish two kinds....
n, places where the environment was exceptionally suited to preservation of soft tissue. Until the mid-1990s only two specimens good enough for analysis were known, both from the early DevonianThe Devonian is a geologic period and system of the Paleozoic era spanning from . It is named after Devon, England, where rocks from this period were first studied....
period. Three additional putative species were then found in the Burgess ShaleThe Burgess Shale Formation — located in the Canadian Rockies of British Columbia — is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
and other Canadian rocks of similar age, about in the mid-CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
period. All three apparently lacked tentacles but had between 24 and 80 comb rows, far more than the 8 typical of living species. They also appear to have had internal organ-like structures unlike anything found in living ctenophores. One of the fossil species first reported in 1996 had a large mouth, apparently surrounded by a folded edge that may have been muscular. Evidence from China a year later suggests that such ctenophores were widespread in the Cambrian, but perhaps very different from modern species — for example one fossil's comb-rows were mounted on prominent vanes.
The early Cambrian 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 rock, or the hull of a ship in the case of barnacles. Corals lay down their own substrate....
frondThe term frond is used to refer to a large, divided leaf.In both common usage and botanical nomenclature, the leaves of ferns are referred to as fronds and some botanists restrict the term to this group...
-like fossil Stromatoveris, from China's Chengjiang lagerstätte and dated to about , is very similar to Vendobionta of the preceding EdiacaranThe Ediacaran Period is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
period. De-Gan Shu, Simon Conway MorrisSimon Conway Morris FRS is a British paleontologist. He was born on 6th November 1951 in Carshalton, Surrey and brought up in London, England. He made his reputation with a very detailed and careful study of the Burgess Shale fossils, an exploit celebrated in Stephen Jay Gould's Wonderful Life,...
et al. found on its branches what they considered rows of cilia, used for filter feeding. They suggested that Stromatoveris was an evolutionary "aunt" of ctenophores, and that ctenophores originated from sessile animals whose descendants became swimmers and changed the cilia from a feeding mechanism to a propulsion system.
Family tree
It is difficult to reconstruct the early stages in the evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
ary "family tree" of animals using only morphologyIn biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...
(their shapes and structures), because the large differences between Porifera (sponges), CnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
(jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa Jellyfish (also known as jellies or sea jellies) are free-swimming members of...
, sea anemoneSea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower...
s, etc.) plus Ctenophora, Placozoa and BilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum of animals, including the majority...
(all the more complex animals) make comparisons difficult. Hence reconstructions now rely largely or entirely on molecular phylogenetics, which groups organisms according to similarities and differences in their biochemistryBiochemistry is the study of the chemical processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
, usually in their DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
or RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
.
It is now generally thought that the Calcarea (sponges with calcium carbonateCalcium carbonate is a chemical compound with the chemical formula CaCO
3. It is a common substance found in rock in all parts of the world, and is the main component of shells of marine organisms, snails, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural...
spiculeSpicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs....
s) are more closely related to Cnidaria, Ctenophora (comb jellies) and BilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum of animals, including the majority...
(all the more complex animals) than they are to the other groups of sponges. In 1866 it was proposed that Cnidaria and Ctenophora were more closely related to each other than to Bilateria and formed a group called CoelenterataCoelenterata is an obsolete long term encompassing two animal phyla, the Ctenophora and the Cnidaria . The name comes from the Greek "koilos" , referring to the hollow body cavity common to these two phyla...
("hollow guts"), because Cnidaria and Ctenophora both rely on the flow of water in and out of a single cavity for feeding, excretion and respiration. In 1881 it was proposed that Ctenophora and Bilateria were more closely related to each other, since they shared features that Cnidaria lack, for example muscles in the middle layer (mesogleaMesoglea is the translucent, inert, jellylike substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phylum Cnidaria. It acts as the creatures' structural support in water, as they lack bones, cartilage or other more common means of support...
in Ctenophora, mesodermIn humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the...
in Bilateria). More recent analyses indicate that these similarities are rather vague, and the current view, based on molecular phylogenetics, is that Cnidaria and Bilateria are more closely related to each other than either is to Ctenophora. This grouping of Cnidaria and Bilateria has been labelled "Planulozoa" because it suggests that the earliest Bilateria were similar to the planulaA planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. In all cases, the planula forms directly from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...
larvae of Cnidaria.
In 2005 Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants of triploblastic animals, since ctenophores and the medusa stage of some cnidarians have striated muscleStriated muscle is a form of fibers that are combined into parallel fibers. More specifically, it can refer to:* Skeletal muscle* Cardiac muscle~cardiac referring to the heart....
, which in bilaterians arises from the mesodermIn humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the...
. They did not commit themselves on whether bilaterians evolved from early cnidarians or from the hypothesized triploblastic ancestors of cnidarians. A study in 2008 of 150 genes in 21 genera proposed a third hypothesis, that sponges and cnidarians are more closely related to each other than either is to ctenophores.
Since all modern ctenophores except the beroids have cydippid-like larvae, it has widely been assumed that their last common ancestor also resembled cydippids, having an egg-shaped body and a pair of retractable tentacles. Richard Harbison's purely morphological analysis in 1985 concluded that the cydippids are not monophyletic, in other words do not contain all and only the descendants of a single common ancestor that was itself a cydippid. Instead he found that various cydippid familiesIn biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus...
were more similar to members of other ctenophore 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...
than to other cydippids. He also suggested that the last common ancestor of modern ctenophores was either cydippid-like or beroid-like. A molecular phylogeny analysis in 2001, using 26 species including 4 recently-discovered ones, concluded that the cydippids are not monophyletic and that the last common ancestor of modern ctenophores was cydippid-like. It also found that the genetic differences between these species were very small — so small that the relationships between the Lobata, Cestida and Thalassocalycida remained uncertain. This suggests that the last common ancestor of modern ctenophores was relatively recent, and perhaps was lucky enough to survive the Cretaceous-Tertiary extinction while other lineages perished. When the analysis was broadened to include representatives of other phlyaIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
, it concluded that cnidarians are probably more closely related to bilaterians than either group is to ctenophores but that this diagnosis is uncertain.
Further reading
- R. S. K. Barnes, P. Calow, P. J. W. Olive, D. W. Golding, J. I. Spicer, The invertebrates — a synthesis, 3rd ed, Blackwell, 2001, ch. 3.4.3, p. 63, ISBN 0-632-04761-5
- R. C. Brusca, G. J. Brusca, Invertebrates, 2nd Ed, Sinauer Associates, 2003, ch. 9, p. 269, ISBN 0-87893-097-3
- J. Moore, An Introduction to the Invertebrates, Cambridge Univ. Press, 2001, ch. 5.4, p. 65, ISBN 0-521-77914-6
- W. Schäfer, Ctenophora, Rippenquallen, in W. Westheide and R. Rieger: Spezielle Zoologie Band 1, Gustav Fischer Verlag, Stuttgart 1996
- Bruno Wenzel, Glastiere des Meeres. Rippenquallen (Acnidaria), 1958, ISBN 3-7403-0189-9
- Mark Shasha, Night of the Moonjellies, 1992, Simon & Schuster, ISBN 0-6717-7565-0
External links