Typically, ‘social’ animals gather together for reasons to do with sex, food, or safety, but there are other reasons too, as some new research on hermit crabs has shown. In the new research, terrestrial hermit crabs have been observed grouping together to ‘evict’ a larger hermit crab from its shell, and then each trade up with each other to each get a larger shell.
All known species of hermit crabs rely on abandoned snail shells to function as their ‘homes’. But land-based hermit crabs take this even further, often hollowing out and remodeling the appropriated shells. Sometimes this is done until the internal volume has been more than doubled. All of this extra space has clear advantages; more space to grow, more space for eggs, and an easier to carry, lighter home.
Empty and unused snail shells are rare on land though, “so the best hope of moving to a new home is to kick others out of their remodeled shells,” said Mark Laidre, a UC Berkeley Miller Post-Doctoral Fellow.
“When three or more terrestrial hermit crabs congregate, they quickly attract dozens of others eager to trade up. 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.”
“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.
Vermeij has been studying the effects that animals can have on their own evolution for the past few decades now. The most common term for it is “niche construction,” in opposition to the commonly known idea that ‘evolution’ is primarily driven by environmental 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.
The study was conducted on the Pacific shore of Costa Rica — multiple hermit crab species are endemic to the area. The research was done by tethering crabs “to a post and monitoring the free-for-all that typically appeared within 10-15 minutes.”
“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 UC–Berkeley news release on this new research notes.
“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.”
An earlier experiment by the researchers clarified the importance of the time-intensive remodeling of shells. In the experiment, some crabs were taken from their homes and were offered only newly vacated snail shells. All of the crabs died shortly thereafter.
“Apparently, 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.”
The new research was just published in the journal Current Biology.
Some interesting information on the crabs from Wikipedia:
“As hermit crabs grow they require larger shells. Since suitable intact gastropod shells are sometimes a limited resource, there is often vigorous competition among hermit crabs for shells. The availability of empty shells at any given place depends on the relative abundance of gastropods and hermit crabs, matched for size. An equally important issue is the population of organisms that prey upon gastropods and leave the shells intact. Hermit crabs that are kept together may fight or kill a competitor to gain access to the shell they favour. However, if the crabs vary significantly in size, the occurrence of fights over empty shells will decrease or remain non-existent.
“For some larger marine species, supporting one or more sea anemones on the shell can scare away predators. The sea anemone benefits, because it is in position to consume fragments of the hermit crab’s meals.”