Letting go

To escape a predator's grasp, some prey would rather give up a limb than give up on life. Through a process called autotomy, muscles can contract violently along the base of an appendage, breaking off the limb. Thus can sea stars cast off an arm, and various reptiles shed their tails.

Kerstin Wasson, a biologist at the Elkhorn Slough National Estuarine Research Reserve in Watsonville, California, and her colleagues decided to quantify the benefits of autotomy. They put Petrolisthes porcelain crabs in the company of larger, predatory crabs and waited until a predator grabbed a porcelain by the claw. Two-thirds of the porcelains jettisoned the claw and escaped alive as the predator munched the detached hors d'oeuvre. The other third fought back, but most of them ended up as a full entree.

Porcelains have large claws for their body size. Such claws may divert a predator's attention away from the main part of the porcelain's body, Wasson and her colleagues suggest, and keep the predator safely busy for a time. ("Hair-trigger autotomy in porcelain crabs is a highly effective escape strategy," Behavioral Ecology 13:481-86, July 2002)

Stephan Reebs is a professor of biology at the University of Moncton in New Brunswick, Canada, and the author of Fish Behavior in the Aquarium and in the Wild (Cornell University Press).

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