VARIATION IN THE GENETIC COMPOSITION OF CORAL (POCILLOPORA-DAMICORNISAND ACROPORA-PALIFERA) POPULATIONS FROM DIFFERENT REEF HABITATS

The genetic structure of populations of the corals Pocillopora damicor nis and Acroporapalifera was examined in three habitats at One Tree Is land during March and April 1993, using electrophoretically detectable variation at six allozyme loci. There were significant genetic differ ences among populations of P. damicornis within each of the reef crest , lagoon and microatoll habitats. The level of differentiation among p opulations was similar in each of the habitats. Differences between po pulations of P. damicornis from lagoon and microatolls were no greater than that within habitats, but genetic differentiation of these from crest populations was much higher. There was no difference in the gene tic composition of A. palifera populations within or between the lagoo n and microatolls, the only habitats where this species was found. Bot h coral species had observed:expected (G(O):G(E)) genotypic diversity ratios >0.80, indicating predominantly sexual reproduction. These data , the high genotype diversity and general conformance of genotype freq uencies to those expected under conditions of Hardy-Weinberg, suggeste d panmixis at each site. The high degree of sexual reproduction in the P. damicornis populations is unusual for a species where asexual repr oduction has been the dominant mode of reproduction reported to date. Gene flow in both species was considerable between the lagoon and the closed microatolls. The genetic differences between populations of P. damicornis in these habitats and the reef crest may reflect the relati ve isolation of all populations within the closed One Tree Lagoon from those outside. However, local currents appear to offer effective mean s of dispersal between the habitats, suggesting that the genetic diffe rences result from natural selection in the different environments wit hin One Tree Lagoon and the reef crest.