Beware Of House-Jacking Hermit Crabs
October 27, 2012

Terrestrial Hermit Crabs Congregate To Claim Remodeled Shells

redOrbit Staff & Wire Reports - Your Universe Online

In their search for larger homes, land-based hermit crabs will socialize with their fellow decapods only to force them out of their shells and claim it for their own, researchers from the University of California-Berkeley have discovered.

While all types of hermit crabs will claim abandoned snail shells as their own, only the terrestrial types of these crustaceans hollow them out and remodel the shells. This allows them to increase the internal volume so that there is more space for growth, more room for eggs, and less weight to carry while searching for food.

However, the rarity of these empty snail shells means that land-based hermit crabs have a better chance of finding larger living areas by removing other hermit crabs from their renovated shells, Mark Laidre, a post-doctoral fellow with the university, claims in the latest edition of the journal Current Biology.

"When three or more terrestrial hermit crabs congregate, they quickly attract dozens of others eager to trade up," the university explained in a prepared statement. "They typically form a conga line, smallest to largest, each holding onto the crab in front of it, and, once a hapless crab is wrenched from its shell, simultaneously move into larger shells."

"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," added Laidre, who notes that this behavior led the species to evolve from a solitary creature to a socially-minded one.

The biologist conducted his research on the Pacific coast of Costa Rica, home to the hermit crab species Coenobita compressus. Laidre took individual subjects, none larger than three inches in length, and tethered them to a post. He then observed the crab as several others arrived within 10 to 15 minutes time, attempting to displace it from its home and claim its shell as their own.

"Most of the 800 or so species of hermit crab live in the ocean, where empty snail shells are common because of the prevalence of predators like shell-crushing crabs with wrench-like pincers, snail-eating puffer fish and stomatopods, which have the fastest and most destructive punch of any predator," the researcher reported. "On land, however, the only shells available come from marine snails tossed ashore by waves."

"Their rarity and the fact that few land predators can break open these shells to get at the hermit crab may have led the crabs to remodel the shells to make them lighter and more spacious," the university added. "The importance of remodeled shells became evident after an experiment in which [Laidre] pulled crabs from their homes and instead offered them newly vacated snail shells. None survived."

The reason for that, according to the Berkeley biologist, is that only the tiniest hermit crabs are able to take full advantage of the new shells that they claim, as they are the only ones that can fit inside of them before they are remodeled. Even if a crab is able to do so, Laidre continued, it would have to work long and hard in order to hollow it out -- something that he said that all hermit crabs, regardless of size, would rather avoid.

Image 2 (below): 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.