Size matters for hermit crabs, says study


A new finding from UC Berkeley has determined two things: 1) For hermit crabs, size matters; and 2) they’re kind of jerks about it.

While social animals congregate for protection, to capture prey or to bump uglies, the solitary hermit crab will break out of its seclusion to gang up on the guy with the largest shell. Once targeted, the crabs will oust the victim from its shell to move into the roomier abode.

The report comes from Mark Laidre, a UC Berkeley biologist who observed this unusual behavior while studying terrestrial hermit crabs in Costa Rica:

“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.”

Here’s how it works: three or more terrestrial hermit crabs will congregate, and quickly dozens of others will join the house swap. They typically form a conga line —I’m not making this up — from smallest to largest, with each crab holding the crab in front of it.

Once the biggest crab is evicted from its shell, each crab moves into the next one’s roomier shell.

The evolutionary behavior occurs because the market for roomy, renovated shells on land is as scarce as affordable housing in San Francisco.

While hermit crabs that live in the ocean have it good with an abundance of empty snail shells, land hermit crabs only come across shells that are washed along the shore.

Laidre’s full report appears in this month’s scientific journal Current Biology.

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