Hermit Crabs Socialize to Trade Up for Larger Shells

Posted on October 26, 2012

Hermit Crabs Congregate to Trade Shells


University of California, Berkeley, biologist Mark Laidre has discovered that terrestrial hermit crabs socialize to trade up for larger shells. The photograph above shows hermit crabs congregating, with all crabs intent on displacing someone else to get a larger shell.

Hermit crabs use abandon snail shells for homes, which they remodel by hollowing them out to make them bigger. Laidre says these 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. Laidre says that when three or more terrestrial hermit crabs congregate they quickly attract dozens of other crabs eager to trade. The hermit crabs then form a conga line from smallest to largest, with each holding onto the crab in front of it. Once a crab is wrenched from its shell, they all simultaneously move into larger shells. The unfortunate crab yanked out of its shell tends to wind up with the smallest shell.

Laidre says in a release, "The one that gets yanked out of its shell is often left with the smallest shell, which it can't really protect itself with. Then it's liable to be eaten by anything. For hermit crabs, it's really their sociality that drives predation."

Coenobita compressus Hermit Crab


Laidre studied the hermit crab Coenobita compressus (pictured above) on the Pacific shore of Costa Rica. In his study, he tethered individual crabs to a post and watched as congregations of hermit crabs ready to trade shells appeared with 10-15 minutes. His paper was published in the journal Current Biology.

Photos: Mark Laidre, UC Berkeley