Why Do Hermit Crabs Live in Discarded Shells?
Unlike other crabs, the hermit crab has no hard outer shell to protect its soft body. So it “borrows” the shells of other animals, moving to bigger ones as it grows.
It may use the empty shell of a sea snail after the snail has died. But it may steal the shell even if the snail is still inside. It pulls out the live snail, getting a new house and a feast as well.
Sometimes one hermit pulls another hermit from the shell it wants. The crab then backs its own body into the empty shell until nothing can be seen but its big claws barring the doorway.
As the hermit crab grows in size it finds a larger shell and abandons 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.
When an individual crab finds a new shell it will leave its own shell and inspect the new 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 several hours.
As new crabs arrive they also inspect the shell and, if it is too big, they wait with the others, forming a group of several 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.