mollusk n : invertebrate having a soft unsegmented body usually enclosed in a shell [syn: mollusc, shellfish]

English

Noun

1. alternative spelling of mollusc

References

• American Heritage 2000
• Dictionary.com
• WordNet 2003

Molluscs (British spelling) or mollusks (American spelling) are animals belonging to phylum Mollusca. The word mollusc is derived from the French mollusque, which originated from the Latin molluscus, meaning thin-shelled, from mollis, soft. The scientific study of molluscs is known as malacology.

There are around 100,000 extant species within the phylum with an estimated 70,000 extinct species. They range widely in size from micromolluskan snails and clams to larger organisms such as the Colossal Squid, believed to be the world's largest invertebrate. Molluscs are typically divided into ten taxonomic classes, of which two are entirely extinct is one of the largest invertebrates; however the colossal squid is even larger.

Classification

There are ten classes of molluscs; eight of the classes have living representatives, the other two classes are known only from fossils. More than 250,000 species of mollusc are recognized and named. Snails (Gastropoda) account for about 80% of living mollusc diversity.

Evolution

It is believed that the bivalves and scaphopods are sister groups, as are the gastropods and cephalopods, as indicated in the relationship diagram to the right.

In this phylum's level of organization, organ systems from all three primary germ layers can be found:

1. Nervous system (with brain)
2. Excretory system (nephridium or nephridia)
3. Circulatory system (open circulatory system - except cephalopods which have a closed system)
4. Respiratory system (gills or lungs)

All major molluscan groups possess a skeleton, though it has been lost through evolution 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 shell 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.

The first definitive evidence for molluscs comes from an early Cambrian radula, but the Ediacaran organism Kimberella is held by some to be an ancestral mollusc.

Dangerous molluscs

A very small minority of molluscs can represent a serious risk to humans under the wrong circumstances.

All octopuses are venomous but only a few species pose a significant threat to humans, such as octopuses in the genus Haplochlaena which have a very poisonous bite. A few of the larger tropical cone snail species have a very poisonous sting. These bites and stings can sometimes be fatal.

Some people are severely allergic to shellfish as a food item. However, even for people without these allergies, clams can sometimes be 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.

Despite its name, the disease molluscum contagiosum is caused by a virus, and is not connected with molluscs in any way.

See also

sisterlinks Mollusc

• Important publications on molluscs

References

General references

• Biology: The Unity and Diversity of Life
• Nunn, J.D., Smith, S.M., Picton, B.E. and McGrath, D. 202. 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.
• Ponder, Winston F. and Lindberg, David R. (Eds.) (2008) Phylogeny and Evolution of the Mollusca. Berkeley: University of California Press. 481 pp. ISBN 978-0520250925.

External links

• - 210,000 mollusca pictures.
• Hardy's Internet Guide to Marine Gastropods
• Molluscs in captivity