ACANTHOCHROMIS POLYACANTHUS, A FISH LACKING LARVAL DISPERSAL, HAS GENETICALLY DIFFERENTIATED POPULATIONS AT LOCAL AND REGIONAL SCALES ON THE GREAT-BARRIER-REEF

Acanthochromis Gill is a monotypic genus within the damselfish family Pomacentridae, erected for an unusual species [A. polyacanthus (Bleeke r)] that uniquely lacks larval dispersal. Instead, offspring are reare d in the parental territory, in the manner of cichlids, and fledged in to the surrounding habitat. Phenotypic and genotypic variation was sur veyed on the basis of body colouration and 7 polymorphic loci in 19 po pulations from 5 regions of the central and southern Great Barrier Ree f (GBR). Variation in both characters was found at regional and local scales. Two colour morphs were recognised: a bicoloured morph from the three northern regions and a uniform dark morph from the two southern regions. Isozyme analysis showed a similar pattern with greatest vari ation between the different morphs, but also with significant variatio n at both regional and local scales within morphotypes. Heterozygosity was maximal in the central populations, which, together with other me asures of variability, suggests a mixing of separate gene pools in thi s region and denies species status to the two morphotypes despite nume rous fixed differences in allele frequencies between the most distant populations. The presence of fixed differences in multiple alleles bet ween populations separated by 1000 km indicates negligible gene flow o ver such distances and long isolation of these gene pools. These patte rns may reflect recolonisation of the GBR after the last sea-level ris e by fish from two stocks. Founder effects and random drift in small p opulations after colonisation are probably the major sources of the lo cal and regional variations observed at smaller spatial scales. This d iversity has been maintained among populations at all scales by the ve ry low levels of gene flow possible without an effective strategy for larval dispersal between coral reefs.