A marine fish follows Wallace's Line: the phylogeography of the three‐spot seahorse (<i>Hippocampus trimaculatus</i>, Syngnathidae, Teleostei) in Southeast Asia

Abstract Aim To test the potential of two contrasting biogeographical hypotheses (‘Indian/Pacific Ocean Basin’ vs. ‘Wallace's Line’) to explain the distribution of genetic diversity among populations of a marine fish in Southeast Asia. Location The marine waters of Asia and Southeast Asia: from India to Japan, and east to the Indonesian islands of Sulawesi and Flores. Methods We sequenced a 696 base pair fragment of cytochrome b DNA of 100 individuals of Hippocampus trimaculatus Leach 1814 (three‐spot seahorse), obtained from across its range. We tested our hypotheses using phylogenetic reconstructions and analyses of molecular variance. Results Significant genetic divergence was observed among the specimens. Two distinct lineages emerged that diverged by an average of 2.9%. The genetic split was geographically associated, but surprisingly it indicated a major east–west division similar to the terrestrial Wallace's Line (Φ ST = 0.662, P &lt; 0.001) rather than one consistent with an Indian‐Pacific ocean basin separation hypothesis (Φ ST = 0.023, P = 0.153). Samples from east of Wallace's Line, when analysed separately, however, were consistent with an Indian/Pacific Ocean separation (Φ ST = 0.461, P = 0.005). The degree of genetic and geographical structure within each lineage also varied. Lineage A, to the west, was evolutionarily shallow (star‐like), and the haplotypes it contained often occurred over a wide area. Lineage B to the east had greater genetic structure, and there was also some evidence of geographical localization of sublineages within B. Main conclusions Our results indicate that the genetic diversity of marine organisms in Southeast Asia may reflect a more complex history than the simple division between two major ocean basins that has been proposed by previous authors. In particular, the east–west genetic division observed here is novel among marine organisms examined to date. The high haplotype, but low nucleotide diversity to the west of Wallace's Line is consistent with post‐glacial colonization of the Sunda Shelf. Additional data are needed to test the generality of these patterns.