Phylogeography of Southeast Asian seahorses in a conservation context

This thesis investigates the potential role of historical isolation of ocean basins in promoting diversification among marine organisms in Southeast Asia. It also questions the possible effects of Pleistocene exposure of the Sunda Shelf on present day marine distributions and genetic diversity. Four species of exploited seahorses (genus Hippocampus), with differing ecological parameters, are used to test historical hypotheses. The results (based on cytochrome b DNA sequencing) suggest that significant phylogeographic structure does exist among seahorse populations in Southeast Asia, but that the patterns are only partially concordant across species. Distinct phylogeographic breaks are seen in H. barbouri, H. kuda, and H. trimaculatus, whereas greater spatial overlap of haplotypes in H. spinosissimus indicates more extensive gene flow. The phylogeographic history of the two shallow water species (H. barbouri and H. kuda) appears to have been primarily shaped by fragmentation and/or long-distance colonisation events. Both species show patterns consistent with hypotheses of divergence mediated by ocean basins separations. The deeper water species (H. spinosissimus and H. trimaculatus) show more evidence of range expansion and isolation by distance. Hippocampus trimaculatus shows a deep east-west phylogeographic division at right angles to that predicted by the separation of the Indian versus Pacific Ocean basins and instead parallels the terrestrial division known as Wallace's Line. Different species have also responded differently to the reflooding of the Sunda Shelf at the end of the last Ice Age: the two deeper water species have colonised it extensively suggesting limited barriers to movement, whereas the shallow water species have not. It is possible that the populations of H. kuda now inhabiting the shelf may stem from populations that found refuge in brackish water lakes when the shelf was exposed to the air. All four species are heavily traded as traditional medicines, curiosities, and aquarium fish to levels that provoke conservation concerns. The results of this thesis, especially recognition of distinct genetic breaks, estimation of the spatial scale of dispersal for the different species, and congruence (or not) of patterns across species, can provide both data for species-specific conservation and management and the beginning of a framework for regional marine conservation planning in Southeast Asia. On a broader scale, this thesis reviews the current role of phylogeographic, and more generally biogeographic data in marine conservation planning.