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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鈥攖hey
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. 鈥淚f one species can
cause eggs of another species to die without being fertilised, that takes a
possible competitor for space out of the game,鈥� says Lambert.

This may partly explain why the males produce so many sperm. 鈥淥ne function of
the excess sperm may be to make some of the other species鈥� eggs infertile,鈥�
Lambert says. He believes that germ-cell warfare between species may be more
common than anyone realised. 鈥淔ew people have looked to see if sperm that cannot
fertilise an egg can still render that egg incapable of fertilisation,鈥� he says.