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Coral Reef Studies

ARC Centre of Excellence for Coral Reef Studies
James Cook University Townsville
Queensland 4811 Australia

Phone: 61 7 4781 4000
Email: info@coralcoe.org.au

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Crowds cause fishy sex changes

Sep 2006

Changing sex is common among coral reef fish – but the cause can depend on who’s around, according to a recent study by a team of Australian and American scientists.

Dr Philip Munday, of the ARC Centre of Excellence for Coral Reef Studies and the School of Marine and Tropical Biology at James Cook University, has found that juvenile bluehead wrasse choose their sex according to the crowd they grow up with.

chools of bluehead wrasse contain a mix of females and small males that look identical to females. The size of the social group influences the proportion of juveniles that develop into small males. More juveniles become male when there are many other fish around, because small males have more chances to breed when the population is large. Photo courtesy of Will White

Dr Munday and colleagues from the University of California Santa Barbara reported on this remarkable occurrence in a study published in a leading journal, Proceedings of the Royal Society B: Biological Sciences.

“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 Dr Munday. “These fish are very sensitive to their social surroundings which ultimately determine whether they will become male or female,” he says.

This unusual strategy has developed so that each young fish can increase its chances of breeding within a complex social structure.

These reef fish all 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. These smaller males appear to have been male all their life, and Dr. Munday and his team wanted to discover if these small males were born male, or if their gender choice was influenced by their surroundings.

“We wanted to know exactly how sex is determined in the early life stages of these fish,” says Dr. Munday.

By raising the young fish either alone or in groups of three Dr. 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.

Dr Munday’s 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.

“This shows that sex is not genetically predetermined, as it is in mammals and birds”, says Dr. Munday, “because the number of juveniles that became male when reared alone was much less than when juveniles were in large groups on the reef.”

The reason why a juvenile’s social surroundings influence which sex it becomes has to do with its chances of breeding. When there are only a few other fish on the reef, turning male is rare at this young stage because they are less likely to breed than females – who are almost certain to – due to larger males that monopolize the reef’s breeding pool. These larger males dominate a territory on the reef and all the females within it – making it almost impossible for smaller males to breed when the population is small.

But when the crowd is big it becomes harder for the large males to control so many females, giving smaller males more of a chance to sneak in and take one. Small males even look like females, to help disguise their identity from larger territorial males who would like to keep all the breeding to themselves.

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 Dr. Munday.

This research has answered one long-standing question in the complex and curious issue of gender selection in sex-changing fish – an important issue for understanding the complexity of life on coral reefs and managing them so as to preserve their richness and diversity.

More information:
Australia: Dr Philip Munday, Chief Investigator, CoECRS, +61 7 4781 5341, +61 0419 422 815
USA: Professor Robert Warner, Department of Ecology, Evolution and Marine Biology, UCSB, +1 805 8932941, +1 805 2844524
Jenny Lappin, CoECRS, +61 7 4781 4222
Jim O’Brien, James Cook University Media Office, +61 7 4781 4822


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Coral Reef Studies

ARC Centre of Excellence for Coral Reef Studies
James Cook University Townsville
Queensland 4811 Australia

Phone: 61 7 4781 4000
Email: info@coralcoe.org.au