Low genetic diversity of symbiotic dinoflagellates (Symbiodinium) in scleractinian corals from tropical reefs in southern Hainan Island, China

Endosymbiotic dinoflagellates in the genus Symbiodinium are among the most abundant and important group of photosynthetic protists found in coral reef ecosystems. In order to further characterize this diversity and compare with other regions of the Pacific, samples from 44 species of scleractinian corals representing 20 genera and 9 families, were collected from tropical reefs in southern Hainan Island, China. Denaturing gradient gel electrophoresis fingerprinting of the ribosomal internal transcribed spacer 2 identified 11 genetically distinct Symbiodinium types that have been reported previously. The majority of reef-building coral species (88.6%) harbored only one subcladal type of symbiont, dominated by host-generalist C1 and C3, and was influenced little by the host's apparent mode of symbiont acquisition. Some species harbored more than one clade of Symbiodinium (clades C, D) concurrently. Although geographically isolated from the rest of the Pacific, the symbiont diversity in southern Hainan Island was relatively low and similar to both the Great Barrier Reef and Hawaii symbiont assemblages (dominated by clade C Symbiodinium). These results indicate that a specialist symbiont is not a prerequisite for existence in remote and isolated areas, but additional work in other geographic regions is necessary to test this idea.