Most of the time, hermit crabs live up to their names, keeping to themselves and refusing to socialize with each other members of their species nearby. But sometimes hermit crabs hang out... and they're invariably total assholes to each other.
About a dozen hermit crab species live on land, and it's these terrestrial species that like to live inside old snail shells. What's more, the crabs hollow out the shells to allow more room for eggs and their own growth. It's a sweet life, but the trick is in finding a snail shell, which aren't really all that common in hermit crab habitats. So when a crab is in the market for a new shell, its best bet isn't to go searching for one — it's to trick another crab into giving up its own shell.
It's a remarkable process that pretty much confirms every preconception you might have about the natural world being nasty and brutish. To kick off the "trading" process, only three hermit crabs need to meet up, which in turn attracts dozens more crabs to the location. Then, as researchers at UC Berkeley have discovered, the crabs actually organize themselves from smallest to largest shell in a sort of kleptomaniac crab conga line. Or, as you can see in the photo on the left, it's a frenzied crustacean mosh pit. Either way, it sounds awesome, even if it's not entirely pleasant for the hermit crabs involved.
The crab with the second-largest shell seizes the even larger shell in front of it and make the largest shell its own. This process repeats all down the line until every crab has traded up — except for the crab who started the event with the best shell of the bunch, who now either has to climb into the smallest leftover shell or go without. While the former is the better option, it's not much better, as researcher Mark Laidre explains:
"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."
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It's a particularly intriguing phenomenon because these crabs are naturally solitary creatures — again, they're not called hermit crabs for nothing. But the evolutionary path that led them to hollow out and live inside snail shells also forced them to socialize on occasion, even if it's clear they're still a few eons away from evolving good manners.
Via Current Biology. Images by Mark Laidre, UC Berkeley.