Source: The Courier Mail

09/14/06 

CORAL reef fish like hanging out with their mates so much they often change their sex according to who they grow up with.

If sex is on the menu...
A NEW breed . . . some bluehead wrasse coral reef fish have been proved to turn from sexless juveniles into males to increase their chances of breeding in larger schools.

The unusual strategy has developed so each fish can increase its chances of breeding, according to a team of Australian and American scientists.

Philip Munday of the Centre of Excellence for Coral Reef Studies and the School of Marine and Tropical Biology at Queensland's James Cook University has found that bluehead wrasse all start life as larvae with neither male nor female characteristics.

As they develop into juveniles they become one or the other.

Dr Munday and colleagues from the University of California Santa Barbara reported in leading journal Proceedings of the Royal Society B: Biological Sciences that, as adults, there were two types of males – large and colourful, and smaller and drab. Smaller males appeared to have been male all their lives.

By raising young fish, either alone or in groups of three, researchers found major differences in the number of males developing.

When raised alone, a few fish would turn into males. When raised in groups, one would usually turn into a male.

Tests also included young wrasse collected from populations that originally had many males but the results were still the same.

"This shows that sex is not genetically predetermined as it is in mammals and birds," Dr Munday said.

When there are only a few other fish on the reef, turning male is rare at a young stage because they are less likely to breed than females, due to larger males that monopolise the breeding pool.

But when the crowd is big it gets harder for the large males to control so many females, giving smaller males a chance.