Caribbean Reef Squid, Sepioteuthis sepioidea
The molluscs (British spelling) or mollusks (American spelling) are members of the very large and diverse phylum of invertebrate animals known as Mollusca. There are some 112,000 species within this phylum. The scientific study of molluscs is known as malacology.
The vast majority of molluscs live in marine environments, and many of them are found intertidally, in the shallow subtidal and on the continental shelf. Species of octopus and squid live throughout the ocean depths and some species of clam and limpet live in the abyssal depths of the oceans around hot vents.
Not all molluscs are marine: two taxonomic groups or classes, the bivalves and the gastropods, also contain freshwater species. Only the gastropods have representatives that live on land: the land snails and slugs.
Molluscs are triploblastic protostomes and many demonstrate bilateral symmetry. The principal body cavity is a blood-filled hemocoel. They have a true coelom (eucoelom); any coelomic cavities have been reduced to vestiges around the hearts, gonads, and metanephridia (kidney-like organs). The body is often divided into a head, with eyes or tentacles, a muscular foot, and a visceral mass housing the organs.
Molluscs have a mantle, which is a fold of the outer skin lining the shell, and a muscular foot that in most species is used for locomotion. In most molluscs the mantle secretes a calcium carbonate external shell. In the majority of marine molluscs the gill or gills absorbs oxygen from the water.
All species of the phylum Molluscs have a complete digestive tract that starts from the mouth and runs to the anus. Many have a feeding structure, the radula, mostly composed of chitin. This radula is a feature only found in molluscs. Radulae are very diverse within the Mollusca, ranging from structures used to scrape algae off rocks, to the harpoon-like structures of cone snails. Cephalopods (squid, octopuses, cuttlefish) also possess a chitinous beak. Unlike the closely related annelids, molluscs lack body segmentation.
Development passes through one or two trochophore stages, one of which, (the veliger), is unique to the group. These larval stages suggest a close relationship between the molluscs and various other protostomes, notably the Annelids.
Molluscs, because of their shells, have left an excellent fossil record, and are found from the Cambrian onwards. The oldest fossil species seems to be Odontogriphus omalus, found in the Burgess Shale. It lived about 500 million years ago.
There are ten classes of molluscs, eight are still living, the others are known only from fossils. These classes make up the 250,000 and more species of mollusc:
- Class Caudofoveata (deep-sea wormlike creatures; 70 known species); now generally recognized as a subclass of Aplacophora.
- Class Aplacophora (solenogasters, deep-sea wormlike creatures; 250 species)
- Class Polyplacophora (chitons; 600 species, rocky marine shorelines)
- Class Monoplacophora (deep-sea limpet-like creatures; 11 living species)
- Class Bivalvia (also Pelecypoda) (clams, oysters, scallops, mussels; 8,000 species)
- Class Scaphopoda (tusk shells; 350 species, all marine)
- Class Gastropoda (sea snails with shells, such as abalone, limpets, conch, etc, and marine snails without a shell or with a reduced shell, such as nudibranchs, sea hares; sea angel, sea butterfly, sea lemon etc; land snails and slugs, freshwater snails, total estimated at 40,000 - 150,000 species)
- Class Cephalopoda (squid, octopodes, nautilus, cuttlefish; 786 species, all marine)
- Class † Rostroconchia (fossils; probably more than 1,000 species; probable ancestors of bivalves)
- Class † Helcionelloida (fossils; snail-like creatures such as Latouchella)
Brusca & Brusca (1990) suggest that the bivalves and scaphopods are sister groups, as are the gastropods and cephalopods, so indicated in the relationship diagram above.
In this phylum's level of organization, organ systems from all three primary germ layers can be found:
- Nervous System (with brain).
- Excretory System (nephridium or nephridia).
- Circulatory System (open circulatory system - except cephalopods which are closed).
- Respiratory System (gills or lungs).
All major molluscan groups possess a skeleton, though it has been lost evolutionarily in some members of the phylum. It is probable that the pre-Cambrian ancestor of the molluscs had calcium carbonate spicules embedded in its mantle and outer tissues, as is the case in some modern members. The skeleton, if present, is primarily external and composed of calcium carbonate (aragonite or calcite). The snail or gastropod shell is perhaps the best known molluscan shell, but many pulmonate and opisthobranch snails have secondarily reduced and internalized shells, or have lost the shell completely. The bivalve or clam shell consists of two pieces (valves), articulated by muscles and an elastic hinge. The cephalopod shell was ancestrally external and chambered, as exemplified by the ammonoids and nautiloids, and still possessed by Nautilus today. Other cephalopods, such as cuttlefish, have internalized the shell, the squid have mostly organic chitinous internal shells, and the octopods have lost the shell altogether.
A very small minority of molluscs can represent a serious risk to humans under the wrong circumstances. A few octopus species have a very poisonous bite, and a few of the larger tropical cone snail species have a very poisonous sting. Both of these type of assaults can sometimes be fatal to humans.
Some people are severely allergic to shellfish as a food item. However, even for people without these allergies, clams can sometimes be quite risky to eat. When there is a "red tide", or other blooms of noxious plankton, or when there are high concentrations of bacteria in the water from sewage run-off, bivalves such as clams and mussels can temporarily become very problematic as a food source. This is because bivalves are filter-feeders, and thus they can concentrate toxins from floating microorganisms within their tissues.
The traditional idea that the giant clam can trap the leg of a person between its valves, thus drowning them, has been shown to be a myth.
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- ↑ Feldkamp, S. (2002) Modern Biology. Holt, Rinehart, and Winston, USA. (pp. 725)
- ↑ Barnes, R. D. (1987) Invertebrate Zoology (Fifth Edition), Saunders College Publishing, Philadelphia, USA. (pg. 456)
- ↑ Kubodera, T. & Mori, K. (2005) PDF Proceedings of the Royal Society B: Biological Sciences 272 (1581), 2583-2586.
- Brusca & Brusca (1990). Invertebrates. Sunderland, Mass.: Sinauer Associates.
- Starr & Taggart (2002). Biology: The Unity and Diversity of Life. Pacific Grove, California: Thomson Learning.
- Nunn, J.D., Smith, S.M., Picton, B.E. and McGrath, D. 202. Checklst, atlas of distribution and bibliography for the marine mollusca of Ireland. in. Marine Biodiversity in Ireland and Adjacent Waters. Ulster Museum. publication no. 8.