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Change for a Sand Dollar?

13 March 2008 (All day)
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Dawn Vaughn; (inset) Kevin Schafer/Corbis

Breaking up.
A sand dollar larva in the act of splitting off a smaller clone. (Inset) Adult sand dollar.

When predators are nearby, sand dollar larvae split--literally. The young marine invertebrates divide in half to become too small for hungry fish to detect. Researchers say this is the first time the strategy, known as cloning, has been documented as a form of defense.

Scientists have long known that adult starfish are capable of cloning: They reproduce by breaking off a piece of themselves, avoiding the vulnerable larval stage altogether. But it was only 5 years ago that researchers discovered that the larvae of other members of the spiny-skinned, radially shaped echinoderm phylum, such as sea urchins, sea cucumbers, and sand dollars, could perform the same trick. If larvae of these species encounter temperatures that are conducive to growth, or if food is abundant, they will clone themselves, creating a horde of new identical twins that can take advantage of the favorable conditions. Now, marine ecologists Dawn Vaughn and Richard Strathmann of the University of Washington's Friday Harbor Laboratories have found that sand dollar larvae also employ this strategy when they detect fish nearby.

Vaughn and Strathmann exposed sand dollar larvae to fish mucus in the laboratory. When they checked back 24 hours later, the larvae had cloned themselves. This behavior makes sense, the researchers speculate, because the immature sand dollars are just big enough for predatory fish to see. When the larvae clone themselves, they halve their size, effectively becoming invisible to their predators. Although cloning can take hours and has yet to be observed in the wild, it could be an effective strategy because the larvae begin to split as soon as they detect fish mucus in the water. That could give them enough lead time before the fish attack, the team reports in the 14 March issue of Science.

The cloning may come with a cost, however. Smaller size makes larvae vulnerable to other enemies on the seabed, such as predatory crustaceans whose gape prevents them from handling larger prey, says marine biologist Jonathan Allen of Bowdoin College in Brunswick, Maine. "The rules of the game change." However, he notes that sand dollars tend to stick together in dense beds where smaller sand dollars may be protected by larger ones. So going small may be the safest strategy for juvenile sand dollars after all, Allen says.

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