We all know that hermit crabs have an unusual way of finding shelter: the resourceful crustaceans seek out abandoned snail shells to call home, remodeling them as needed, and moving up when growth demands it.

But for terrestrial crabs, empty shells are hard to come by. What to do? Find a suitable shell, evict its inhabitant and make the home their own, according to Mark Laidre, a UC Berkeley Miller post-doctoral fellow who reported the behavior in a recent issue of the journal Current Biology.

Rather than a basic crab-vs.-crab takeover, the cunning crabs have a more complicated way of doing business. When three or more crabs gather together, they quickly draw a crowd of other crabs in need of new digs. They form a conga line of sorts, smallest to largest, each grasping the crab in front of it. Once one of the crabs is forced from its shell, each crab, one by one, takes over the newly vacated shell in front of it.

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

Also of interest is that once the crabs have their new shells, they remodel the structures by working to carve out more room. By increasing the volume, they have more room to grow and more space for their eggs (up to 1,000 more) as well as making the structure lighter and easier to cart around.

Laidre notes that this unusual behavior is an uncommon example of how evolving in a unique niche, like land versus ocean, has led to a surprising outcome: socialization in a typically loner animal.

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