Habitat selection and aggression as determinants of spatial segregation among damselfish on a coral reef

Adults of many closely related coral reef fish species are segregated along gradients of depth or! habitat structure. Both habitat selection by new se ttlers and subsequent competitive interactions can potentially produce such patterns, but their relative importance is unclear. This study examines th e potential roles of habitat selection and aggression in determining the sp atial distribution of adults and juveniles of four highly aggressive damsel fishes at Lizard Island, northern Great Barrier Reef. Dischistodus perspici llatus, D. prosopotaenia, D. melanotus, and D. pseudochrysopoecilus maintai n almost non-overlapping distributions across reef zones, with adults of on e species dominating each reef zone. Juveniles exhibit slightly broader dis tributional patterns suggesting that subsequent interactions reduce overlap among species. Although habitat choice experiments in aquaria suggest, tha t associations between juveniles and substrata types in the field are partl y due to habitat selection, large overlaps in the use of substrata by the d ifferent species were also found, suggesting that substratum selection alon e is insufficient in explaining the discrete spatial distributions of adult s. The strength of aggressive interactions among all four species was teste d by a "bottle" experiment, in which an adult or juvenile of each species w as placed in the territories of adult fish on the reef. The greatest levels of interspecific aggression were directed against adults and juveniles of neighbouring species. The highest levels of aggression were associated with species exhibiting the greatest levels of overlap in resource use. Evident ly both habitat selection and interspecific aggression combine to determine the adult distributions of these species.