Hermit Crabs at a Nude Beach

Hermit Crab in Skull by ~hwango

Hermit crabs are decapod crustaceans of the superfamily Paguroidea [1]. They are not closely related to true crabs. Hermit crabs are quite commonly seen in the intertidal zone, for example in tide pools.
Most species of hermit crabs have long soft abdomens which are protected from predators by the adaptation of carrying around a salvaged empty seashell, into which the whole crab's body can retract. Most frequently hermit crabs utilize the shells of sea snails, marine gastropod mollusks. 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 is what gave rise to the popular name "hermit crab", which is a reference to the idea of a hermit living alone in a small cave.

Shell theme inspired by Flowing Heat.



Shell competition
Hermit crabs fighting over a shell in a Central California tidepool
As hermit crabs grow they require larger shells. Since suitable intact gastropod shells are a limited resource, there is frequently strong competition among hermit crabs for the best available shells. The availability of empty snail shells at any given place depends primarily on the relative abundance of gastropods in the right range of sizes, compared to the demographics of the population of hermit crabs.
An equally important issue is the frequency of organisms which prey upon gastropods but leave the shells intact [2]

. A hermit crab with a shell that is too small cannot grow as fast as hermit crabs with well fitting shells, and is more likely to be eaten as it cannot withdraw completely into the shell [3].
For some larger marine hermit crab species, having one or more sea anemones growing on the shell can be very useful, because the anemones tend to scare away fish and other marine predators that might otherwise attack the crab. The sea anemone also benefits because it is well positioned to consume loose fragments of the hermit crab's meals. Furthering this mutualism, sea anemones can be transferred to a new shell when the hermit crab changes shells.

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