Sea squirts in sperm warfare

March 21, 2000

Life under the waves can get pretty crowded when you are a sea squirt vying for space on the rocks. But these sea creatures have a novel weapon-they use their sperm to sabotage the eggs of other kinds of sea squirt. According to a marine biologist in California, this may be the first example of sperm competition between species.

Sea squirts spend most of their life clinging on to rocks, and many are broadcast spawners: females and males release eggs and sperm into the water, where they mix and fertilise. Any egg that is fertilised by more than one sperm will not develop properly, so, like other animals, sea squirts have evolved ways to prevent this happening. Once a sperm binds to an egg, the egg releases an enzyme that alters its surface, preventing other sperm attaching.

Now Charles Lambert of California State University in Fullerton has found that other species of sea squirts can subvert this mechanism. Lambert studied two species, Ascidia nigra and Ascidia sydneiensis, in the waters around Hawaii and Guam. Although the two cannot cross-fertilise, the sperm of one still triggers the defence mechanism in eggs of the other, preventing the species under attack from being fertilised. "If one species can cause eggs of another species to die without being fertilised, that takes a possible competitor for space out of the game," says Lambert.

This may partly explain why the males produce so many sperm. "One function of the excess sperm may be to make some of the other species' eggs infertile," Lambert says. He believes that germ-cell warfare between species may be more common than anyone realised. "Few people have looked to see if sperm that cannot fertilise an egg can still render that egg incapable of fertilisation," he says.
-end-
Reporter: Jon Copley

Source: The Biological Bulletin (vol 198, p 22)

New Scientist issue: 25th March 2000

PLEASE MENTION NEW SCIENTIST AS THE SOURCE OF THIS STORY AND, IF PUBLISHING ONLINE, PLEASE CARRY A HYPERLINK TO: http://www.newscientist.com

New Scientist

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