Influence of the North Equatorial Current on the population genetic structure ofTridacna crocea(Mollusca: Tridacnidae) along the eastern Philippine seaboard

Tridacna croceapopulations were sampled from 15 locations throughout the eastern Philippine seaboard and screened for allozyme variation at 7 polymorphic loci in order to examine the influence of the North Equatorial Current (NEC) bifurcation on population genetic structure. Significant genetic differentiation among all populations was detected (F ST= 0.065). Ordination methods and cluster analysis revealed 2 regional groups and a north–south spatial genetic structure broadly concordant with the bifurcation of the NEC into the Kuroshio and Mindanao current branches. Analysis of molecular variance (AMOVA) indicated greater partitioning of genetic variance among groups (F CT= 0.049) than among populations within groups (F SC= 0.029). An isolation-by-distance signal across the entire Philippine seaboard and marked geographical variation of allele frequencies between Kuroshio and Mindanao Current regions suggested that genetic differentiation is likely due to limited larval exchange and genetic drift. The Mindanao current populations are characterized by greater genetic diversity and differentiation (observed heterozygosity,H O= 0.298;F ST= 0.056) than the Kuroshio populations (H O= 0.152;F ST= 0.025), attributable to variable atmospheric and hydrographic regional conditions. Weaker connectivity among Mindanao current populations are attributed to complex patterns of hydrographic circulation south of the NEC bifurcation, which may translate to greater entrainment potential, in turn influencing dispersal and recruitment of planktonic propagules at varying spatial and temporal scales. Fine-scale genetic differentiation was also detected within Kuroshio and Mindanao current populations, indicating the influence of small-scale temporal and spatial physical processes that affect larval dispersal and recruitment along the eastern Philippine seaboard.