Crabs Aren’t Hermits When It Comes to House Hunting

Posted October 29th, 2012 at 4:59 pm (UTC+0)
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The land-based hermit crab Coenobita compressus lives inside a discarded snail shell and forages for food along the Pacific coast from Mexico to Peru. (Photo: Mark Laidre, UC Berkeley)

The land-based hermit crab Coenobita compressus lives inside a discarded snail shell and forages for food along the Pacific coast from Mexico to Peru. (Photo: Mark Laidre, UC Berkeley)

Despite its name, the hermit crab can be a very social creature.  In fact, when searching for a new home, the crustacean gets by with a little help from its friends, according to a new study in Current Biology.

The hermit crab, which is vulnerable to predators, protects itself by making its home in abandoned marine snail shells.

To fit its ever-growing body, the hermit crab will often hollow out and customize their appropriated shells, sometimes doubling its internal size.  But, after a while, the crab is forced to look for a bigger place to live.

There are about 800 species of hermit crabs and most live in the ocean, where empty snail shells are abundant due to the prevalence of predators with an appetite for marine snails. However, competition for shells among land-based crabs can be intense.

California researchers found that once three or more of these crabs congregate, dozens of others eager to find nicer homes also show up.

House hunting, hermit crab style. Whenever three or more of these crustaceans show gather, others are bound to join in. A free-for-all takes then place with all crabs intent on displacing someone else to get into a bigger shell. (Photo: Mark Laidre, UC Berkeley)

House hunting, hermit crab style. Whenever three or more of these crustaceans show gather, others are bound to join in. A free-for-all takes then place with all crabs intent on displacing someone else to get into a bigger shell. (Photo: Mark Laidre, UC Berkeley)

The crabs form a sort of conga line, from smallest to largest, each holding onto the crab in front of it.Once the largest of the group finds his dream home, he kicks out its current occupant and moves into the new shell. That frees up his old shell for the crab behind him.

This process continues until the smallest of the group moves into a new space.  As a result, like the children’s game musical chairs, the smallest crab is left without a shell of its own, making it susceptible to any predators in the mood for a little crab dinner.

“The one that gets yanked out of its shell is often left with the smallest shell, which it can’t really protect itself with,” says the study’s author, Mark Laidre, from the University of California, Berkeley. “Then it’s liable to be eaten by anything. For hermit crabs, it’s really their sociality that drives predation.”

Laidre conducted his research along the Pacific shore of Costa Rica, where millions of the hermit crab species,Coenobita compressus, can be found along the tropical beaches.

A marine snail shell newly vacated by its original owner (left) and a shell that has been remodeled and hollowed out by a hermit crab (right). (Photo: Mark Laidre, UC Berkeley)

A marine snail shell newly vacated by its original owner (left) and a shell that has been remodeled and hollowed out by a hermit crab (right). (Photo: Mark Laidre, UC Berkeley)

For his study, Laidre  tethered individual crabs to a post, the largest being about three inches long, and then observed the brouhaha which usually took place within 10 to 15 minutes of the crab being  tethered.

After moving into its new home, the crustacean usually gets to work immediately, remodeling and hollowing out the shell.  Laidre discovered how critical those remodeled shells are to the hermit crab after pulling them from their homes and offering newly vacated snail shells instead. He found that none of those crabs survived.

Apparently, Laidre says, only the smallest hermit crabs are able to take advantage of brand new shells, since only the smallest hermit crabs can actually fit inside shells that haven’t been hollowed.  Even if a crab does find a new shell it can fit in, hermit crabs of all sizes would rather not spend all of the time and energy to hollow it out itself.

The hermit crab’s unusual conduct, according to Laidre, serves as a rare example of how evolving to take advantage of a specialized niche – in this case, the hermit crab living on land versus in the ocean – led to an unexpected byproduct: socialization in what is typically solitary animal.

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