Mollusca

Molluscs Temporal range: Ediacaran or Cambrian - Recent
Caribbean Reef Squid, Sepioteuthis sepioidea
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca Linnaeus, 1758
Classes
Aplacophora † ?Bellerophontida Bivalvia Caudofoveata Cephalopoda Gastropoda † Helcionelloida Monoplacophora Polyplacophora † Rostroconchia Scaphopoda † Tentaculita

Molluscs (British spelling) or mollusks (American spelling) are animals belonging to phylum Mollusca. The word mollusc comes 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 112,000 species within the phylum, ranging 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. Class Cephalopoda molluscs such as squid, cuttlefish and octopus are among the most neurologically-advanced of all invertebrates.

The majority of mollusc species live in marine environments, and many of them are found intertidally, in the shallow subtidal and on the continental shelf. Pelagic species of octopus and squid live throughout the water column of the ocean, and some species of clam and limpet live in the abyssal depths of the oceans around hot hydrothermal vents. However, not all molluscs are marine: the bivalve and gastropod classes also contain freshwater species, and the gastropods additionally have representatives that live on land, the land snails and slugs.

A number of species of molluscs are valued by humans as seafood, or for their decorative shells. Edible species include various clams, snails, squid and octopuses.

Anatomy

The shell of the tiger top snail, Calliostoma tigris, from New Zealand.

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.

The giant squid, which until recently had not been observed alive in its adult form, 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.

Class	Major organisms	Extant species	Distribution
Caudofoveata	worm-like organisms	70	deep ocean
Aplacophora	solenogasters, worm-like organisms	250	deep ocean
Polyplacophora	chitons	600	rocky marine shorelines
Monoplacophora	limpet-like organisms	11	deep ocean
Gastropoda	abalone, limpets, conch, nudibranchs, sea hares, sea butterfly, snails, slugs	40,000 - 150,000	marine, freshwater, land
Cephalopoda	squid, octopus, cuttlefish, nautilus	786	marine
Bivalvia	clams, oysters, scallops, mussels	8000	marine
Scaphopoda	tusk shells	350	marine
Rostroconchia †	fossils; probable ancestors of bivalves	extinct	—
Helcionelloida †	fossils; snail-like organisms such as Latouchella	extinct	—

Evolution

Main article: Evolution of Mollusca

					Caudofoveata (?)
					Aplacophora
hypothetical					Polyplacophora
ancestral					Monoplacophora
mollusc					Gastropoda
					Cephalopoda
					Bivalvia
					Scaphopoda

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.

Dangerous molluscs

Common Octopus (Octopus vulgaris)

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

• Important publications on molluscs

References

General references

• 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. 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.

External links

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