Phylogenetic and taxonomic status of the coral Goniopora stokesi and related species (Scleractinia: Poritidae) in Japan based on molecular and morphological data

Background: Goniopora stokesi is an uncommon species mainly found in tropical and subtropical regions but was also reported in temperate regions of Japan. This species has two unique characteristics. First, it does not typically attach to hard substrates, i.e., it is free-living. Although non-free-living colonies were reported, it is unclear whether their morphologies exhibit intraspecific or interspecific differences. Second, they can asexually form daughter colonies, which are secondary colonies that grow on the surface (coenosac) of a parent colony and subsequently detach. To date, this specific characteristic has not been reported in Japan, and it is not clear whether this species occurs in Japan. Results: To clarify the taxonomic status of this species, we investigated its reproductive mechanism in Japan and morphologically and genetically analyzed specimens collected from both subtropical and temperate regions. We found that this species forms daughter colonies in Okinawa, Japan and that free-living colonies in the temperate region, which were formerly recognized as G. stokesi, likely constitute a morphological variation of a separate species. In addition, all non-free-living colonies with G. stokesi-like morphologies were also morphological variations of other species. Conclusions: Overall, free-living colonies with large, deep calices, thin walls, and well-developed epitheca on the underside were G. stokesi, whereas other free-living and all non-free-living G. stokesi-like colonies belonged to other species, such as Goniopora djiboutiensis.