Hermit crabs differ from other crabs in that they have soft abdomens. They’re as slow as any other crab so they have to find an empty shell to use as a home. They’re not super choosy, however, and will sometimes use a hollow piece of wood or stone instead.
On my recent trip to Aitutaki, we held a hermit crab race right after lunch. One of the guides had collected a bucket full of crabs, and drew concentric circles in the sand. We each got to choose our champion crab, and then counted down to their release in the centre of the circle. The crabs quickly moved away from each other, and the crab that got to the outer ring first was declared the winner.
Contrary to their name, and their desire in the race to run away from each other, hermit crabs are actually quite social creatures. As they grow out of their scavenged shells, they meet up around a new, bigger shell. When the largest crab moves into the new shell and out of its old shell, the next largest moves into the newly-vacated shell. This forms a chain of hermit crabs trading up to the new shell. A kind of crustacean hand-me-down system.