Genetic structure and cryptic speciation in the threatened reef-building coral Heliopora coerulea along Kuroshio Current

The blue coral Heliopora coerulea (Pallas, 1766), a reef-building coral, is the sole member of the alcyonarian order Coenothecalia. Recently, local and global anthropogenic stresses on its habitat have led to this species being listed as threatened. Strong genetic differentiation has been observed between populations of H. coerulea that have been found only 500 m apart. Despite the wide geographical distribution of H. coerulea in the Indo-Pacific oceans, such localized genetic structuring has led to speculation about whether H. coerulea populations are completely isolated over different geographic scales (including speciation). To investigate gene flow in H. coerulea, we genotyped seven microsatellite loci in 611 H. coerulea colonies collected from along the Kuroshio Current. Genetic admixture analysis and principal coordinate analysis suggested the existence of two cryptic species in the studied area. In addition, distinct typical gross morphologies were shown by both these cryptic species. Patterns of isolation by distance were more obvious and significant when each genetic clade was analyzed separately than when the populations were analyzed together. In addition, we found that some populations had extremely low genotypic diversity. These findings indicate that these populations may be more threatened than previously believed and emphasize the threatened status of H. coerulea.