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Mollusca
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Molluscs[Spelled mollusk in the USA; the spelling "mollusc" is preferred by some authors, see the reasons given by .] are animals belonging to the phylum Mollusca. There are around 93,000 recognized extant species, making it the largest marine phylum with about 23% of all named marine organisms. Representatives of the phylum live in a wide range of environments including marine, freshwater, and terrestrial environments.
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Molluscs[Spelled mollusk in the USA; the spelling "mollusc" is preferred by some authors, see the reasons given by .] are animals belonging to the phylum Mollusca. There are around 93,000 recognized extant species, making it the largest marine phylum with about 23% of all named marine organisms. Representatives of the phylum live in a wide range of environments including marine, freshwater, and terrestrial environments. Molluscs are a highly diverse group, in size, in anatomical structure, in behaviour and in habitat. The phylum is typically divided into nine or ten taxonomic classes, of which two are entirely extinct. Cephalopod molluscs such as squid, cuttlefish and octopus are among the most neurologically advanced of all invertebrates. Either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total number of classified molluscan species.
Molluscs have such a varied range of body structures that it is difficult to find defining characteristics that apply to all modern groups. The two most universal features are a mantle with a significant cavity used for breathing and excretion, and the structure of the nervous system. As a result of this wide diversity, many textbooks base their descriptions on a hypothetical "generalized mollusc". This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle that covers the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom is very small, and the main body cavity is a hemocoel through which blood circulates molluscs' circulatory systems are mainly open. The "generalized" mollusc feeding system consists of a rasping "tongue" called a radula and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The "generalized mollusc" has two paired nerve cords, or three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors that detect chemicals, vibrations and touch. The simplest type of molluscan reproductive system relies on external fertilization, but there are more complex variations. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. A striking feature of molluscs is the use of the same organ for multiple functions. For example: the heart and nephridia ("kidneys") are important parts of the reproductive system as well as the circulatory and excretory systems; in bivalves, the gills both "breathe" and produce a water current in the mantle cavity which is important for excretion and reproduction.
There is good evidence for the appearance of gastropods, cephalopods and bivalves in the Cambrian period . However the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists.
Molluscs have been and still are an important food source for anatomically modern humans. However there is a risk of food-poisoning from toxins that accumulate in molluscs under certain conditions, and many countries have regulations that aim to minimize this risk. Molluscs have for centuries also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. Their shells have also been used as a money in some pre-industrial societies.
Mollusc species can also represent hazards or pests for human activities. The bite of the blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last for over a month. Stings from a few species of large tropical cone shells can also kill, but their sophisticated though easily-produced venoms have become important tools in neurological research. Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is transmitted to humans via water snail hosts, and affects about 200 million people. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some ecosystems.
Diversity
There are about 93,000 named mollusc species, which include 23% of all named marine organisms. Molluscs are second only to arthropods in numbers of living animal species far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000. It has been estimated that there are about 200,000 living species in total, and 70,000 extinct species.
Molluscs have more varied forms than any other animal phylum snails and other gastropods, clams and other bivalves, squids and other cephalopods, and other less well-known but similarly distinctive sub-groups. The majority of species still live in the oceans, from the seashores to the abyssal zone, but some are significant members of freshwater and terrestrial ecosystems. They are extremely diverse in tropical and temperate regions but can be found at all latitudes. About 80% of all known mollusc species are gastropods. Cephalopoda such as squid, cuttlefish and octopus are among the most neurologically-advanced of all invertebrates. The giant squid, which until recently had not been observed alive in its adult form, is one of the largest invertebrates. However a recently-caught specimen of the colossal squid, long and weighing , may have overtaken it.
General description
Definition
The word mollusc is derived from the French mollusque, which originated from the Latin molluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's t? ľa????a, "the soft things", which he applied to cuttlefish. The scientific study of molluscs is known as malacology.
Molluscs have developed such a varied range of body structures that it is difficult to find synapomorphies (defining characteristics) that apply to all modern groups. The following are present in all modern molluscs:
- The dorsal part of the body wall is a mantle which secretes calcareous spicules, plates or shells. It overlaps the body with enough spare room to form a mantle cavity.
- The anus and genitals open into the mantle cavity.
- There are at least two pairs of main nerve cords (three in bivalves)
Other characteristics that commonly appear in textbooks have significant exceptions:
| | Class | | Characteristic | Aplacophora | Polyplacophora | Monoplacophora | Gastropoda | Cephalopoda | Bivalvia | Scaphopoda |
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| Radula, a rasping "tongue" with chitinous teeth | Absent in 20% of Neomeniomorpha | Yes | Yes | Yes | Yes | No | Internal, cannot extend beyond body |
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| Broad, muscular foot | Reduced or absent | Yes | Yes | Yes | Modified into arms | Yes | Small, only at "front" end |
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| Dorsal concentration of internal organs (visceral mass) | Not obvious | Yes | Yes | Yes | Yes | Yes | Yes |
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| Large digestive ceca | No ceca in some aplacophora | Yes | Yes | Yes | Yes | Yes | No |
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| Large complex metanephridia ("kidneys") | None | Yes | Yes | Yes | Yes | Yes | Small, simple |
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A "generalized mollusc"
Because of the enormous variations between groups of molluscs, many text books start the subject by describing a "generalized mollusc", which some suggest may resemble very early molluscs and which is rather similar to modern monoplacophorans.
The mantle secretes a shell that is mainly chitin and conchiolin (a protein) hardened with calcium carbonate, except that the outermost layer is all conchiolin. Molluscs never use phosphate to construct their hard parts, with the questionable exception of Cobcrephora. The mantle cavity is a fold in the mantle that encloses a significant amount of space, and was probably at the rear in the earliest molluscs but its position now varies from group to group. In the "generalized mollusc" the anus, a pair of osphradia (chemical sensors), the hindmost pair of gills and the exit openings of the nephridia ("kidneys") and gonads (reproductive organs) are in the mantle cavity.
The underside of the body generally consists of a muscular foot, which has been adapted for different purposes in different classes. In gastropods, it secretes mucus as a lubricant to aid movement. In forms that have only a top shell, such as limpets, the foot acts a sucker attaching to the animal to a hard surface, and the vertical muscles clamp the shell down over it; in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell. In bivalves, the foot is adapted for burrowing into the sediment; in cephalopods it is used for jet propulsion, and the tentacles and arms are derived from the foot.
Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a hemocoel through which blood circulates and which encloses most of the other internal organs. The blood contains the respiratory pigment hemocyanin as an oxygen-carrier. The heart consists of one or more pairs of atria (auricles) which receive oxygenated blood from the gills and pump it to the ventricle, which pumps it into the aorta (main artery), which is fairly short and opens into the hemocoel.
The atria of the heart also function as part of the excretory system by filtering waste products out of the blood and dumping it into the coleom as urine. A pair of nephridia ("little kidneys") to the rear of and connected to the coelom extracts any re-usable materials from the urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity.
Most molluscs have only one pair of gills, or even only one gill. Generally the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so that water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly sediment entering the mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one connected to the heart.
Most molluscs have muscular mouths with radulae, "tongues" bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. This is primarily designed to scrape bacteria and algae off rocks. Their mouths also contain glands that secrete slimy mucus, to which the food sticks. Beating cilia (tiny "hairs") drive the mucus towards the stomach, so that the mucus forms a long string. At the tapered rear end of the stomach and projecting slightly into the hindgut is the prostyle, a backward-pointing cone of feces and mucus, which is rotated by further cilia so that it acts as a bobbin, winding the mucus string onto itself. Before the mucus string reaches the prostyle the acidity of the stomach makes the mucus less sticky and frees particles from it. The particles are sorted by yet another group of cilia, which send the smaller particles, mainly minerals, to the prostyle so that eventually they are excreted, while the larger ones, mainly food, are sent to the stomach's cecum (a pouch with no other exit) to be digested. The sorting process is by no means perfect. Periodically circular muscles at the entrance to the hindgut pinch off a piece of the prostyle so that it is excreted, preventing the prostyle from growing too large. The anus is in the part of the mantle cavity that is swept by the outgoing "lane" of the current created by the gills. Carnivorous molluscs usually have simpler digestive systems.
In general, molluscs have two pairs of main nerve cords, the visceral cords serving the internal organs and the pedal ones serving the foot. Both pairs run below the level of the gut, and include ganglia as local control centers in important parts of the body. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). The only ganglia above the gut are the cerebral ganglia, which sit above the esophagus (gullet) and handle "messages" from and to the eyes. The pedal ganglia, which control the foot, are just below the esophagus and their commissure and connections to the cerebral ganglia encircle the esophagus in a nerve ring.
A typical mollusc has: a pair of tentacles on the head, containing chemical and mechanical sensors; a pair of eyes on the head, a pair of statocysts in the foot which act as balance sensors; and a pair of osphradia, chemical sensors, in the incoming "lane" of the mantle cavity.
In the simplest molluscan reproductive systems, two gonads sit next to the coelom that surrounds the heart and shed ova or sperm into the coleom, from which the nephridia extract them and emit them into the mantle cavity. Molluscs that use such a system remain of one sex all their lives and rely on external fertilization. Some molluscs use internal fertilization and / or are hermaphrodites, where an individual can function as both sexes; both of these methods require more complex reproductive systems.
The most basic molluscan larva is a trochophore which is planktonic and feeds on floating food particles by using the two bands of cilia round its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus. New tissue grows in the bands of mesoderm in the interior, so that the apical tuft and anus are pushed further apart as the animal grows. The trochophore stage is often succeeded by a veliger stage in which the prototroch, the "equatorial" band of cilia nearest the apical tuft, develops into the velum ("veil"), a pair of cilia-bearing lobes with which the larva swims. Eventually the larva sinks to the seafloor and metamorphoses into the adult form. In some species the newborn larvae are already veligers, and other species have direct development, in which a miniature adult emerges from the egg.
Classification
Opinions vary about the number of classes of molluscs for example the table below shows eight living classes, and two extinct ones. However some authors combine the Caudofoveata and solenogasters into one class, the Aplacophora. Two of the commonly-recognized classes are known only from fossils
| Class | Major organisms | Described living species | Distribution | | Caudofoveata | worm-like organisms | 120 | | | Aplacophora | solenogasters, worm-like organisms | 200 | | | Polyplacophora | chitons | 1,000 | rocky tidal zone and seabed | | Monoplacophora | limpet-like organisms | 25 | | | Gastropoda | abalone, limpets, conch, nudibranchs, sea hares, sea butterfly, snails, slugs | 70,000 | marine, freshwater, land | | Cephalopoda | squid, octopus, cuttlefish, nautilus | 900 | marine | | Bivalvia | clams, oysters, scallops, mussels | 20,000 | marine, freshwater | | Scaphopoda | tusk shells | 500 | | | Rostroconchia | fossils; probable ancestors of bivalves | extinct | marine | | Helcionelloida | fossils; snail-like organisms such as Latouchella | extinct | marine | |
Evolution
Fossil record
There is debate about whether some Ediacaran and Early Cambrian fossils really are molluscs. Kimberella, from about , has been described as "mollusc-like", but others are unwilling to go further than "probable bilaterian".[It is possible that Kimberella is more closely related to, for instance, the brachiopods than the molluscs] There is an even sharper debate about whether Wiwaxia, from about was a mollusc, and much of this centers on whether its feeding apparatus was a type of radula or more similar to that of some polychaete worms. Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from are fragments of a genuinely mollusc-like radula.
However the Helcionellids, which first appear over in the Early Cambrian, are thought to be early molluscs with rather snail-like shells, and possibly the ancestors of the modern conchiferans, a group that includes all the well-known modern families gastropods, cephalopods and bivalves. As such, shelled molluscs predate the earliest trilobites. Although most Helcionellid fossils are only a few millimeters long, the discovery of larger specimens in 2008 has led to suggestions that the tiny specimens were juveniles and that adults were a few centimeters long, like most modern snails. Fossil gastropods, with their characteristic twisted shells, have been reported from "Latest Early Cambrian" rocks in Canada unfortunately it is impossible to give a numerical date for these rocks.
The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into a cuticular layer over the generations. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus (in the form of gills). Eventually, this cuticle would have become mineralised; this mineralisation may have happened one or many times, but uses the same genetic machinery (engrailed) as most other bilaterian skeletons. The first mollusc shell almost certainly utilised the mineral calcite.
There is a degree of uncertainty as to whether the ancestral mollusc was composed of repeating units or not - this has dramatic implications for the origin of the group, because if true it would suggest an origin from an annelid-like worm. The sum of all currently available data suggests that the ancestral mollusc was indeed metameric - in addition, it had a foot used for creeping, and its 'shell' was mineralised.
For a long time it was thought that Volborthella, some fossils of which pre-date , was a cephalopod. However discoveries of more detailed fossils showed that Volborthellas shell was not secreted but built from grains of the mineral silicon dioxide (silica), and that it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. Volborthellas classification is uncertain. The Late Cambrian fossil Plectronoceras is now thought to be the earliest clearly cephalod fossil, as its shell had septa and a siphuncle, a strand of tissue that Nautilus uses to remove water from compartments that it has vacated as it grows, and which is also visible in fossil ammonite shells. However Plectronoceras and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a "ballast" of stony deposits on what is thought to be the underside and had stripes and blotches on what is thought to be the upper surface. All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period . However the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today.
The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves. "Modern-looking" bivalves appeared in the Ordovician period, . One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous-Tertiary extinction. However, bivalves are now abundant and diverse.
Phylogeny
The phylogeny (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whether Kimberella and any of the "halwaxiids" were molluscs or closely related to molluscs, there are debates about the relationships between the classes of living molluscs.
Molluscs are members of the Lophotrochozoa, a group defined by having trochophore larvae and, in the case of living brachiopods, a feeding structure called a lophophore. The other members of the Lophotrochozoa are the annelid worms and seven marine phyla.
The diagram on the right summarizes a phylogeny presented in 2007. Two other widely-supported reconstructions of the evolutionary relationships within the molluscs are:
Interactions with humans
Uses by humans
Molluscs, especially bivalves such as clams and mussels, have been an important food source for many different peoples around the world at least since the appearance of anatomically modern humans and this has often resulted in over-fishing. Other molluscs commonly eaten include octopuses and squids, whelks, oysters, and scallops. In 2005, China accounted for 80% of the global mollusc catch, netting almost 11 million tonnes. Within Europe, France remained the industry leader. However some countries have strict regulations about the importation and handling of molluscs and other seafood, mainly to minimize the risk that humans may be poisoned by toxins that have accumulated in the animals.
Most molluscs that have shells can produce pearls, but only the pearls of bivalves and some gastropods whose shells are lined with nacre are valuable. The best natural pearls are produced by the pearl oysters Pinctada margaritifera and Pinctada mertensi, which live in the tropical and sub-tropical waters of the Pacific Ocean. Natural pearls form when a small foreign object gets stuck between the mantle and shell. There are two methods of culturing pearls, by inserting either "seeds" or beads into oysters. The "seed" method uses grains of ground shell from freshwater mussels, and over-harvesting for this purpose has endangered several freshwater mussel species in the southeastern USA. The pearl industry is so important in some areas that significant sums of money are spent on monitoring the health of farmed molluscs.
Other luxury and high-status products have been made from molluscs. Tyrian purple, made from the ink glands of murex shells, "... fetched its weight in silver" in the fourth-century BC, according to Theopompus. The discovery of large numbers of Murex shells on Crete suggests that the Minoans may have pioneered the extraction of "Imperial purple" during the Middle Minoan period in the 20th–18th century BC, centuries before the Tyrians. Sea silk is a fine, rare and valuable fabric produced from the long silky threads (byssus) secreted by several bivalve molluscs, particularly Pinna nobilis, to attach themselves to the sea bed. Procopius, writing on the Persian wars circa 550 CE, "stated that the five hereditary satraps (governors) of Armenia who received their insignia from the Roman Emperor were given chlamys (or cloaks) made from lana pinna (Pinna "wool," or byssus). Apparently only the ruling classes were allowed to wear these chlamys."
Mollusc shells, including those of cowries, were used as a kind of money in several pre-industrial societies. However these "currencies" generally differed in important ways from the standardized government-backed and -controlled money familiar to industrial societies. Some shell "currencies" were not used for commercial transactions but mainly as social status displays at important occasions such as weddings. When used for commercial transactions they functioned as commodity money, in other words as a tradable commodity whose value differed from place to place, often as a result of difficulties in transport, and which was vulnerable to incurable inflation if more efficient transport or "goldrush" behavior appeared.
Threats to humans
Stings and bites
When handled alive, a few species of molluscs in the wild can sting or bite, and in the case of an even lesser number of species, this can present a serious risk to the human who is handling the animal. To put this into the correct perspective however, deaths from mollusc venoms are less than 10% of the number of deaths from jellyfish stings.
All octopuses are venomous but only a few species pose a significant threat to humans. The blue-ringed octopuses in the genus Hapalochlaena, which live around Australia and New Guineau, bite humans only if severely provoked, but their venom kills 25% of human victims. Another tropical species, Octopus apollyon, causes severe inflammation that can last for over a month even if treated correctly.
Cone shells, carnivorous gastropods that feed on marine invertebrates and fish, produce a huge array of toxins, some fast-acting and others slower but deadlier they can afford to do this because their toxins are relatively cheap to make compared with those of snakes or spiders. Many painful stings have been reported and a few fatalities, although some of the reported fatalities may be exaggerations. Only the few species that can kill fish are likely to be seriously dangerous to humans. The effects of individual cone shell toxins on victims' nervous systems are so precise that they are useful tools for research in neurology, and the small size of their molecules makes it easy to synthesize them.
The traditional belief that a giant clam can trap the leg of a person between its valves, thus drowning them, is a myth.
Pests
Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is "second only to malaria as the most devastating parasitic disease in tropical countries. An estimated 200 million people in 74 countries are infected with the disease 100 million in Africa alone." The parasite has 13 known species, of which two infect humans. The parasite itself is not a mollusc, but all the species have freshwater snails as intermediate hosts.
Some species of molluscs, particularly certain snails and slugs, can be serious crop pests, and snails or slugs introduced into new environments can unbalance local ecosystems. One such pest, the giant African snail Achatina fulica, has been introduced to many parts of Asia, as well as to many islands in the Indian Ocean and Pacific Ocean. In the 1990s this species reached the West Indies. Attempts to control it by introducing the predatory snail Euglandina rosea proved disastrous, as the predator ignored Achatina fulica and went on to extirpate several native snail species instead.
Despite its name, Molluscum contagiosum is a viral disease, and is unrelated to molluscs.
Further reading
- Nunn, J.D., Smith, S.M., Picton, B.E. and McGrath, D. 2002. Checklist, atlas of distribution and bibliography for the marine mollusca of Ireland. in. Marine Biodiversity in Ireland and Adjacent Waters. Ulster Museum. publication no. 8.
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