What Happens When a Hermit Crab Outgrows Its Shell?

Most crustaceans are covered head-to-tail with a hard, durable exoskeleton, but not the hermit crab. This scavenging crab has no protection for its soft abdomen, and they must find discarded shells from whelks or other mollusks in order to protect themselves from predators and to stay hydrated when the blistering sun is at its hottest. As a hermit crab grows, it needs a larger shell -- and that’s when life gets difficult. A good shell can be hard to find, and there are many other crabs also looking to upgrade. In an amazing display of cooperation, hermit crabs have been observed lining up in size order to swap for a new shell. A hermit crab in need of a larger home will try on a hand-me-down shell discarded by a bigger, newly-outfitted crab, and, in turn, leave its old shell for a smaller crab to inhabit.

A different kind of shell game:

  • Hermit crabs cannot grow very well if they are living in a shell that is too small. They are also liable to be eaten, since they cannot retract completely into their shells.
  • To get into a shell, a hermit crab squeezes in backwards, anchoring with its four back legs. Four other legs are used for walking, and the front two legs with pincers, known as the chelipeds, are used to grab prey and fend off foes.
  • If a suitable shell can’t be found, a hermit crab might don other objects, such as hulls from large tree nuts, or even pieces of beach litter. On beaches where people collect shells, finding new digs can be a challenge for a growing hermit crab.
More Info: Discover magazine

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