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Crab mentality

By Massie Santos Ballon
Philippine Daily Inquirer
First Posted 22:49:00 05/14/2010

Filed Under: Health, Science (general)

THE TERM ?CRAB MENTALITY? IS OFten used to describe the behavior of crabs in a basket, pulling each other down from the walls in order to climb over the basket rim and escape from potentially becoming someone?s next meal.

In the May/June issue of the journal Behavioral Ecology, however, Massachusetts-based researchers from Harvard University, the company Pfizer and Tufts University described a different kind of crab mentality in which hermit crabs work together to find new homes.

Hermit crabs are easy to spot on the beach because they look like moving shells with claws. The 10-legged crabs use discarded shells to shield their soft underbellies from predators. Two pairs of legs are used to hold the shell armor in place, while two more pairs of legs allow the crustacean to move. The last pair of legs is used for feeding and defending itself.

Unfortunately for a hermit crab, it outgrows its shell and must find a new protective outer layer, invoking a house-hunting process can be done alone or with a group. The team headed by Randi Rotjan, a researcher at the New England Aquarium who started the project as a graduate student at Tufts University, referred to these methods as synchronous or asynchronous vacancy chains.

Vacancy chain

The term ?vacancy chain? was originally used to describe a chain of events by which a change to one person?s life, such as through a promotion or move to a new apartment, could benefit several people as these resources are freed up. To better understand each of these techniques in the context of animal behavior, the team studied hermit crabs in South America, running 24-hour experiments at times to track the crabs? social movements.

?Hermit crabs are really picky about real estate because they?re constantly getting thrown back into the housing market,? said study first author Rotjan in a statement.

When these crustaceans encounter an unused shell, they?ll check it out for size and potential fit, but without necessarily giving up their current shell. Sometimes the hermit crabs will choose an asynchronous vacancy chain, in which one crab sees a larger, vacant shell and trades up quickly to before continuing on its way. But Rotjan and her colleagues found that hermit crabs prefer a more social moving party.

When hermit crabs encounter an empty shell, the researchers wrote in their study, they?re more likely to hang out near the potential home and wait for more hermit crabs to come by. Over time, a small group of hermit crabs gather and they line up in order of size, which the largest next to the empty shell that will become its new home, and the smaller crabs right behind it waiting for a chance to move into the soon-to-be vacated shells.

At some unspoken signal, the largest crab drops its current shell and moves into the vacant empty shell. The next largest crab then grabs the newly vacant shell and moves in, and so on down the line.

In the paper, Rotjan and her colleagues observed that as many as 10 hermit crabs gained roomier shells in some of the synchronous vacancy chains they monitored. Not all vacancy chains were marked by smooth transfers though. In some cases, the researchers noted that several lines of crabs would form around a single empty shell, causing hermit crabs of the same size to engage in a tug-of-war when their turn came.

Turn to move

In these cases, the team noted, the rest of the hermit crabs in line would wait for several hours, though occasionally switching from one queue to another until it was their turn to move.

So the next time you see a hermit crab next to an empty shell by the beach, leave both of them alone. The crab might be waiting for a new home to come along, and the resulting shell swap might prove beneficial to more than just one tiny crab.

E-mail the author at massie@massie.com.

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