GENETIC DIFFERENTIATION IN RELATION TO RESTRICTED LARVAL DISPERSAL OF THE CONVICT SURGEONFISH ACANTHURUS-TRIOSTEGUS IN FRENCH-POLYNESIA

Genetic markers were used to study larval dispersal in coral reef fishes in order to investigate the impact of migration during the pelagic phase on island recruitment. Samples of Acanthurus triostegus were collected from 11 sites in French Polynesia in order to examine 3 different spatial scales (Island, Archipelago and Polynesia). To examine the effects of gene flow on population heterogeneity and population substructuring in A. triostegus, starch gel electrophoresis on 1 0 polymorphic loci encoding 10 enzymes was used [heterozygoty (H) = 0.325 +/- 0.045; polymorphism level (P0.95) = 0.3231. Nei's genetic distances calculated on the 10 polymorphic loci were very high (between 0.004 and 0.194 with an average of 0.058) and G-tests made on a single locus between pairwise samples showed 5 groups to be significantly different: 1 group comprised samples from Tiahura, Tubuai, Maiao, Tetiaroa, Pt. Paroa and Tahiti; 1 group comprised samples from Muroroa and Managreva and 3 other groups comprised the isolated samples from Takapoto, Bora-Bora and Nuku-Hiva respectively. Fixation index (F(st)) values indicate significant differentiation between the 11 samples (F(st) = 0.0886 on 11 populations), even between Bora-Bora and Moorea which are separated by 250 km. Estimates of gene flow (N(e)m: absolute numer of migrants at equilibrium), assuming equilibrium between the samples, were less-than-or-equal-to 10 individuals per generation, which is sufficient to ensure that the same alleles will be shared over long periods, but not sufficient to maintain identical allelic frequencies between populations. A hypothesis is proposed to illustrate the genetic structure of A. triostegus observed in French Polynesia based on ocean currents and the behaviour of the larval oceanic phase.