Hermit crabs socialize to get bigger homes

30 Oct

A marine snail shell newly vacated by its gastropod owner (left) and a shell that has been remodeled by a hermit crab. (Photo courtesy of Mark Laidre, UC Berkeley)

Land hermit crabs are often desperate to “Keep up with the Jones,” and will congregate with other crabs for a solely selfish purpose–to move into a bigger shell–a recent study shows.

Mark Laidre, a UC Berkeley Miller Post-Doctoral Fellow, found that while hermit crabs in the ocean can often just find abandoned shells laying around, shells on land are few and far between. This is why certain species of land hermit crabs congregate to kick others out of their shells.

Laidre found that when three or more hermit crabs come together, others realize what is going on and join in. The crabs usually will line up from smallest to biggest, while each crab is holding on to the one in front of it. When one crab is “evicted” the others simultaneously start moving in to bigger homes.

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

These crabs move into bigger shells so they have more time to grow and to produce more eggs. A crab in a bigger shell can sometimes produce 1,000 more eggs.

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. (Photo courtesy of Mark Laidre, UC Berkeley)

Laidre explained that this rare behavior is an example of how animals evolving and moving to a specialized niche can prompt new behaviors, in this case turning a loner animal into a socialite.

UC Davis evolutionary biologist Geerat J. Vermeij said this study is a perfect example of evolution.

“No matter how exactly the hermit tenants modify their shellters, they exemplify an important, if obvious, evolutionary truth: living things have been altering and remodeling their surroundings throughout the history of life,” Vermeij wrote in a commentary in Current Biology, the same journal in which Laidre’s study was published.”Organisms are not just passive pawns subjected to the selective whims of enemies and allies, but active participants in creating and modifying their internal as well as their external conditions of life.”

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