BIDIRECTIONAL SEX-CHANGE IN A CORAL-DWELLING GOBY

Bi-directional sex change has recently been reported among obligate co ral-dwelling gobies of the genus Gobiodon. However, neither the functi onal role of this pattern of sex change nor the frequency of sex chang e in either direction in natural populations is known. We investigated the social structure and pattern of sex change of Gobiodon histrio at Lizard Island on the Great Barrier Reef. The social structure of G. h istrio within coral colonies usually consisted of a single juvenile or a heterosexual adult pair. The size of adult social groups was not co nstrained by coral colony size. In contrast to expectations for pair-f orming species, G. histrio was primarily a protogynous hermaphrodite. All immature G. histrio were females and sex change from female to mal e occurred readily when two mature females were placed in a coral colo ny. In addition, male G. histrio were able to change back to females w hen two mature males were placed in a coral. Sex change from female to male, however, occurred with over twice the frequency of sex change f rom male to female. Where two males were placed in a coral colony, het erosexual pairs were most frequently re-established by immigration of females from outside the treatment population. This pattern might be p redicted if sex change from male to female is more expensive than sex change from female to male for G.. histrio. Where sex change is expens ive, movement may be favoured over sex change, particularly where cora l densities are high and movement among corals incurs little mortality risk.