As with many fish species, seahorses (genus Hippocampus) attain their highest species richness in Southeast Asia/Australasia. It is likely that the complex geological history of the region and the effects of Pleistocene sea-level changes have contributed to this
diversification. Since seahorses are presumed to be low dispersing, relative to many other marine species, they are ideal for testing hypotheses regarding the biogeographic effects of geological history. In this paper, spatial and temporal genetic patterns (based on mitochondrial cytochrome b sequence data) among populations of Hippocampus barbouri and its close relatives (H. comes, H. angustus, H. subelongatus) are investigated. Strong localisation of lineages was observed with no geographical overlap. One of the two majors lineages had a disjunct geographic distribution with respect to the other. The results suggest a role for two major mechanisms in determining present-day distributions: (1) island-hopping following the juxtaposition of the Australasian and Asian continental masses and/or emergence of the Philippine islands approximately 6-10 mya, and (2) Pleistocene isolation of ocean basins and subsequent post-glacial recolonisation. The patterns observed contribute to a growing
understanding of diversification of marine taxa across the Coral Triangle. Additional implications of the results include: (1) morphology appears to be a poor indicator of
phylogenetic relationships for this group, and (2) the existence of geographically localised monophyletic units supports fine taxonomic resolution. Small taxonomic
units, coupled with high levels of exploitation, raise conservation concerns for these seahorses. Phylogeographic patterns can be used to set regional conservation priorities.