Population differentiation in the barnacle Chthamalus malayensis: Postglacial colonization and recent connectivity across the Pacific and Indian Oceans

Chthamalus malayensis is a common intertidal acorn barnacle widely distributed in the Indo-West Pacific. Analysis of the mitochondrial cytochrome c oxidase subunit I (COI) sequences of samples from its distribution range revealed 3 genetically differentiated clades: the South China Sea, Indo-Malay and Taiwan clades. The clades have an allopatric distribution and differed by ˊ7.5 to 14\% in COI. They also differed in zonation patterns and abundances within vertical ranges. The genetic and ecological differences suggest that the 3 clades probably represent distinct species. There were signatures of postglacial demographic expansion, yet the timing of expansion varied among clades, which is attributable to the differences in their geographical distributions. The Indian Ocean population of the Indo-Malay clade apparently attained its present range by postglacial re-colonization from the Pacific and, as a result, genetic differentiation among populations in the 2 oceans is low (ΦCT = –0.01, p = 0.49). There were differences in cohort structure between populations in India and the Malay Peninsula. Together, this suggests a considerable level of contemporary gene flow over an evolutionary, but relatively restricted dispersal on an ecological time scale. Cohort structure also varied among Malaysia and Singapore, the South China Sea and Taiwan waters, indicating distinct larval supplies among the 3 clades, possibly determined by different ocean current systems. This physical dispersal of larvae interacts with local biological factors in determining the on-shore distribution and genetic structure of the barnacle populations. Our findings highlight the importance of combining ecological and genetic data to understand factors that mould biodiversity patterns.