Spatial genetic structure and phenotypes of Galaxea corals in the Ryukyu Archipelago

Galaxea is a genus of hermaphroditic corals that are mainly distributed in the tropical coastal areas of the Indo-West Pacific region. It is possible to increase the number of clones of these corals by asexual reproduction. Galaxea corals in the Ryukyu Archipelago are classified into at least three types, based on their mitochondrial DNA (mtDNA); however, the mechanism of population maintenance in these corals is not clear as yet. In this study, we elucidated as to how Galaxea corals maintain their local populations, focusing on inter- and intraspecific genotypes, morphs, germ cells, and sex. We collected branches from 289 colonies of Galaxea corals at four sites (Amami, Okinawa, Miyako, and Iriomote) in the Ryukyu Archipelago. We recorded the location of these colonies with respect to their GPS coordinates and took a photograph for each colony. Histological examinations were done on branches collected from Okinawa. After the extraction of DNA from the branches, fragment analysis was performed using 15 microsatellite and one mtDNA (nad2–cytb) loci. The branches used for histological examination were fixed and decalcified using Bouin’s solution and were stained with hematoxylin and eosin; the sex was determined by observation of the germ cells in the gonad using a microscope. At all sites, except for Amami, we found clonal colonies, and the genotype distribution was locally biased by fragmentation. All clonal colonies with the same genotype showed the same colored morph. In addition, the histological observations showed a biased sex ratio between the different mtDNA types. All clonemates had the same sex, and therefore, the sex could be determined by the genotype. In case there are fewer colonies from sexual reproduction and the sex ratio is biased, the released larvae are extremely fewer in number and might not contribute to the larval supply to other reef sites.