Grabbing Real Estate: In Need of Big Homes Hermit Crabs Yank Out Others Shells

Social behavior serves many purposes and is exhibited by an extraordinary wide variety of animals, including invertebrates, fish, birds, and mammals. Some animals are successful in getting food if they search as a group. Manimals live in social groups partly for protection.

But a peculiar purpose of social interaction among the terrestrial hermit crab is both scary and practical. When they congregate, their self serving social agenda is to grab another crab's shell so as to get a larger home.

A dozen or so species of land based hermit crabs differ from the rest of the hermit crabs that abandon snail shells for their homes. What is unique about them is that they hollow out and remodel their shells, at times doubling the internal volume. This process provides more room to grow more eggs (a thousand or more eggs) and a lighter home to drag around as they hunt.

According to Mark Laidre, a UC Berkeley Mille Post-Doctoral Fellow, empty snail shells are rare on land and the crabs next best hope of moving to a new home is to kick others out of their remodeled shells.

According to the report, when three or more terrestrial hermit crabs congregate, they quickly attract dozens of crabs. 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," said Laidre, who is in 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."

Laidre says, "the crabs' unusual behavior is a rare example of how evolving to take advantage of a specialized niche -- in this case, land versus ocean -- led to an unexpected byproduct: socialization in a typically solitary animal."

"No matter how exactly the hermit tenants modify their shelters, they exemplify an important, if obvious, evolutionary truth: living things have been altering and remodeling their surroundings throughout the history of life," wrote UC Davis evolutionary biologist Geerat J. Vermeij in a commentary in the same journal.

For decades, Vermeij has studied how animals' behavior affects their own evolution -- what biologists term "niche construction" -- as opposed to the well-known Darwinian idea that the environment affects evolution through natural selection.

"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," Vermeij concluded.

This study was conducted on the Pacific shore of Costa Rica, as this shore is a habitat for millions of hermit carb Coenobita compressus. The researcher tethered individual crabs, the largest about three inches long, to a post. He later monitored the free-for-all that typically appeared within 10-15 minutes.

"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," Laidre said.

The importance of remodeled shells became evident after an experiment in which he pulled crabs from their homes and instead offered them newly vacated snail shells. None survived.

Apparently, he said, "only the smallest hermit crabs take advantage of new shells, since only the small hermit crabs can fit inside the unremodeled shells. Even if a crab can fit inside the shell, it still must expend time and energy to hollow it out, and this is something hermit crabs of all sizes would prefer to avoid if possible."