AskDefine | Define mollusk

Dictionary Definition

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

User Contributed Dictionary

English

Noun

  1. alternative spelling of mollusc

References

  • American Heritage 2000
  • Dictionary.com
  • WordNet 2003

Extensive Definition

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.

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

mollusk in Arabic: رخويات
mollusk in Min Nan: Nńg-thé tōng-bu̍t
mollusk in Bulgarian: Мекотели
mollusk in Catalan: Mol·lusc
mollusk in Czech: Měkkýši
mollusk in Welsh: Molwsg
mollusk in Danish: Bløddyr
mollusk in German: Weichtiere
mollusk in Estonian: Limused
mollusk in Modern Greek (1453-): Μαλάκια
mollusk in Spanish: Mollusca
mollusk in Esperanto: Molusko
mollusk in Persian: نرم‌تنان
mollusk in French: Mollusca
mollusk in Korean: 연체동물
mollusk in Croatian: Mekušci
mollusk in Ido: Molusko
mollusk in Indonesian: Mollusca
mollusk in Interlingua (International Auxiliary Language Association): Mollusco
mollusk in Icelandic: Lindýr
mollusk in Italian: Mollusca
mollusk in Hebrew: רכיכות
mollusk in Latin: Mollusca
mollusk in Latvian: Moluski
mollusk in Luxembourgish: Weechdéieren
mollusk in Lithuanian: Moliuskai
mollusk in Hungarian: Puhatestűek
mollusk in Macedonian: Мекотели
mollusk in Dutch: Weekdieren
mollusk in Japanese: 軟体動物
mollusk in Norwegian: Bløtdyr
mollusk in Norwegian Nynorsk: Blautdyr
mollusk in Occitan (post 1500): Mollusca
mollusk in Polish: Mięczaki
mollusk in Portuguese: Moluscos
mollusk in Romanian: Moluscă
mollusk in Quechua: Llamp'u uywa
mollusk in Russian: Моллюски
mollusk in Simple English: Mollusc
mollusk in Slovak: Mäkkýše
mollusk in Slovenian: Mehkužci
mollusk in Serbian: Мекушци
mollusk in Finnish: Nilviäiset
mollusk in Swedish: Blötdjur
mollusk in Telugu: మొలస్కా
mollusk in Thai: หอย
mollusk in Vietnamese: Động vật thân mềm
mollusk in Turkish: Yumuşakçalar
mollusk in Ukrainian: Молюски
mollusk in Chinese: 软体动物
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