When Does a Hermit Crab Change Its Shell?
A hermit crab changes its shell when it has grown large enough to need a bigger home. This type of crab has a soft abdomen or “tail”, which is folded up under the body, but it is not protected by a shell of its own as area most other crabs.
Instead, the hermit uses empty sea snail shells (although the shells of bivalves and scaphopods and even hollow pieces of wood and stone are used by some species) as portable shelters, often having to fight with another crab for possession of an attractive home. Sometimes the home seeker pulls out the original occupier, eats it and then takes over the shell.
One of the claws of the hermit is larger than the other. The crab uses this to stop up the entrance after withdrawing into the shell. The last two legs on its abdomen have roughened pads which grip the inside of the shell and hold the body in position. The crab has a spiral-shaped abdomen and moves in and out of its shell with a spiral movement.
One kind of hermit occupies a sponge which conveniently grows at the same pace as the crab. Sometimes sea-anemones enter into partnership with hermit crabs and take up residence on top of the shells.
The crab provides the sea-anemone with transport and in return, receives an extra shield against attack. The sea anemone benefits too, as it is in position to consume fragments of the hermit crabs meals. Other very close symbiotic relationships are known from encrusting bryozoans and hermit crabs forming bryoliths.
Several hermit crab species, both terrestrial and marine, have been observed forming a vacancy chain to exchange shells. When an individual crab finds a new empty shell it will leave its own shell and inspect the vacant shell for size. If the shell is found to be too large, the crab goes back to its own shell and then waits by the vacant shell for anything up to 8 hours.
As new crabs arrive they also inspect the shell and, if it is too big, wait with the others, forming a group of up to 20 individuals, holding onto each other in a line from the largest to the smallest crab.
As soon as a crab arrives that is the right size for the vacant shell and claims it, leaving its old shell vacant, then all the crabs in the queue swiftly exchange shells in sequence, each one moving up to the next size.
Hermit crabs often “gang up” on one of their species with what they perceive to be a better shell, and pry its shell away from it before competing for it until one takes it over.