Friday, May 27, 2011

Cocos (Keeling) Islands - Hermit Crab

Hermit Crab
Cocos (Keeling) Islands, Indian Ocean.

One of the four postcards sent by Kakak Norana from Cocos (Keeling) Islands.

This is from Wikipedia : Hermit crabs are decapod crustaceans of the superfamily Paguroidea.[1] Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell that is carried around by the hermit crab.

Most species have long, spirally curved abdomens, which are soft, unlike the hard, calcified abdomens seen in related crustaceans. The vulnerable abdomen is protected from predators by a salvaged empty seashell carried by the hermit crab, into which its whole body can retract. Most frequently hermit crabs use the shells of sea snails (although the shells of bivalves and scaphopods and even hollow pieces of wood and stone are used by some species). The tip of the hermit crab's abdomen is adapted to clasp strongly onto the columella of the snail shell.

As the hermit crab grows in size, it has to find a larger shell and abandon the previous one. This habit of living in a second hand shell gives rise to the popular name "hermit crab", by analogy to a hermit who lives alone. Several hermit crab species, both terrestrial and marine, use "vacancy chains" to find new shells: when a new, bigger shell becomes available, hermit crabs gather around it and form a kind of queue from largest to smallest. When the largest crab moves into the new shell, the second biggest crab moves into the newly vacated shell, thereby making its previous shell available to the third crab, and so on.

A gastropod shell that has been used by a hermit crab (or crabs) for a considerable period of time shows a notch in the columella where the abdomen was clamped on to the shell. Some shells that are occupied for very long periods of time are subject to so much external abrasion that they develop holes, which make them unsuitable for use.

Most species are aquatic and live in varying depths of saltwater, from shallow reefs and shorelines to deep sea bottoms. Tropical areas host some terrestrial species, though even those have aquatic larvae and therefore need access to water for reproduction.

A few species do not use a "mobile home" and inhabit immobile structures left by polychaete worms, vermetid gastropods, corals and sponges.

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