Time to split

Natural History, June, 2008 by Stephan Reebs

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Sand dollars are a beachcomber's bread and butter. Their abundance may stem, at least in part, from a curious ability of their larvae: they can split themselves into two in an act of asexual cloning. The larvae tend to divide in times of plenty--extra food enables the small clones to quickly regain normal size and also, a new study shows, in times of danger.

At the University of Washington's laboratories in Friday Harbor, Dawn Vaughn and her graduate advisor Richard R. Strathmann exposed sand-dollar larvae to fish mucus, and found that as many as 40 percent went into cloning mode. By contrast, none of the larvae in an unexposed control group did.

Vaughn and Strathmann think the larvae clone themselves when they detect signs of predators, such as fish--not so much to increase their numbers as to reduce their size. Many fish are visual hunters and may overlook the tiny larval clones; after fission, they're barely one-fiftieth of an inch long, about half their original size.

If so, cloning 'is a remarkable way for sand-dollar larvae to achieve what many prey animals can only do by cowering: make themselves as small as possible. (Science)