A school of bluehead wrasse containing a mix of females and 'sneaky' males that look identical to females.
Credit: James Cook University
SYDNEY, 11 September 2006: Sex changes are common among coral reef fish - but gender can depend on who's around, according to a recent study by a team of Australian and American scientists.
Philip Munday of James Cook University and colleagues from the University of California Santa Barbara have found that juvenile bluehead wrasse choose their sex according to the crowd they grow up with.
"It turns out that social effects are really important to whether a bluehead wrasse becomes a male or a female when it is young," says Munday. "These fish are very sensitive to their social surroundings which ultimately determine whether they will become male or female."
The researchers, who published their findings in the British journal Proceedings of the Royal Society B: Biological Sciences, argue that this unusual strategy has evolved so that each young fish can increase its chances of breeding within a complex social structure.
Bluehead wrasse begin life as tiny larvae which have neither male nor female characteristics. As they develop into juveniles they become one or the other and their bodies change as they grow into adults.
As adults, there are two types of males: large colourful males and smaller drab coloured males called 'sneaky primary males', named for their strategy of disguising themselves as females. Munday and his team wanted to discover whether these small males were born male, or whether their gender choice was influenced by their surroundings.
By raising the young fish either alone or in groups of three, Munday and his team found major differences in the number of males developing in the two tests. When raised alone only a tiny fraction of the fish would turn into males. When raised in groups, however, one of the fish would usually turn into a male.
The tests also included young wrasse collected from populations which originally had many males, but the results were still the same - the young fish weren't any more likely to turn into males when raised alone.
"The more individuals there are when they grow up, the more likely they are to become these little sneaky primary males," says Munday. "This shows that sex is not genetically predetermined, as it is in mammals and birds."
This strategy of social setting influencing sex has evolved to increase the chances of breeding for each young fish, according to the researchers. When there are only a few other fish on the reef, turning male at this young stage is rare because small males are less likely to breed than females - who are almost certain to - due to larger males that monopolise the reef's breeding pool.
But when the crowd is big it becomes harder for the large males to control so many females, giving smaller males - disguised as females - more of a chance to sneak in and seduce a mate.
Incredibly, most of these large and dominant males started life as a female and changed to males when they had grown big enough - making the picture of these sexually-versatile fish even more complicated. "When these fish alter their sex from adult female to adult male the change is very dramatic. They look completely different, their sex organs transform, their behaviour changes - their whole life story changes," says Munday.
While his study suggests that social setting is critically important to determining sex, Munday thinks genes also play a part. "It did seem that individuals varied in their propensity to become a male or not ... so what we have is a subtle interaction between genes and the environment. The environment's really, really important, but it's not everything."
With James Cook University
