Coalescent and biophysical models of stepping-stone gene flow in Neritid snails

Marine species in the Indo-Pacific have ranges that can span thousands of kilometers, yet studies increasingly suggest that mean larval dispersal distances are less than historically assumed. Gene flow across these ranges must therefore rely to some extent on larval dispersal among intermediate “stepping-stone” populations in combination with long-distance dispersal far beyond the mean of the dispersal kernel. We evaluate the strength of stepping-stone dynamics by employing a spatially explicit biophysical model of larval dispersal in the Tropical Pacific to construct hypotheses for dispersal pathways. We test these hypotheses with coalescent models of gene flow among high-island archipelagos in four Neritid gastropod species. Two of the species live in the marine intertidal, while the other two are amphidromous, living in freshwater but retaining pelagic dispersal. Dispersal pathways predicted by the biophysical model were strongly favored in 16 of 18 tests against alternate hypotheses...