Isolation and gene flow in Indo-Pacific species with pelagic larvae

Larval dispersal in marine species allows gene flow across broad expanses of pelagic ocean, and can potentially provide genetic cohesion within widespread species. However, genetic connectivity across discontinuous ranges spanning thousands of kilometers must also rely on the availability of intermediate habitat. Here, I use comparative phylogeography and coalescent modeling to explore the interplay between habitat requirements and gene flow in eight pelagically dispersing species.;First, I test the hypothesis that periods of lowered sea levels during glacial maxima promoted allopatric differentiation across the Indonesian-Australian Archipelago by comparing phylogeographic patterns in six species co-distributed across this region. Mitochondrial COI datasets from all six species are discordant in the degree and location of genetic structure across the archipelago, ranging from near reciprocal monophyly to admixtures between divergent clades, to a nearly complete absence of structure. However, all six species show strong departures from the neutral Wright-Fisher model, and a coalescent model of demography suggests that each species has expanded its range in response to sea-level rise and restoration of habitat on the Sunda and Sahul shelves at the end of the Pleistocene. Discordant phylogeographic patterns among species may arise from different habitat requirements, which could determine the degree to which local populations were impacted by sea level fluctuations.;I then examine genetic structure across the South Pacific in two species of amphidromous freshwater Neritid gastropods that have retained marine pelagically dispersing larvae. Results show surprisingly low levels of genetic structure among Western Pacific archipelagos, despite the rarity of their freshwater habitat.;Finally, I compare gene flow across the South Pacific in marine and amphidromous Neritid gastropods to test the hypothesis that intermediate atoll stepping-stones facilitate genetic connectivity in the marine species. Under a model of the structured coalescent, all four species have high levels of gene flow in the Western Pacific. In the Central Pacific, where a biophysical model of larval dispersal predicts connectivity through atoll stepping-stones, gene flow was significantly lower in the marine species, and negligible in the amphidromous species.