A free-for-all takes place whenever three or more hermit crabs congregate, with all crabs intent on displacing someone else to get a larger shell.
Credit: Mark Laidre, UC Berkeley
University of California, Berkeley, biologist Mark Laidre has discovered that terrestrial hermit crabs socialize to trade up for larger shells. Empty snail shells are rare on land, so the best hope a hermit crab has of getting a new, larger shell as it grows is to kick another crab out of its shell. According to the researchers, when three or more hermit crabs congregate they attract dozens of other crabs eager to trade their small shells to larger ones. The hermit crabs then form a line from smallest to largest. Once a crab is wrenched from its shell, they all simultaneously move into larger shells. The smallest crab is left with the last, and often the tiniest, shell.