Comparison of phylotypic composition of endosymbiotic dinoflagellates of a coral: Acropora digitifera among coral reefs with different seawater temperatures at Okinawa Island, Japan

To determine whether symbiotic dinoflagellates (Symbiodiniaceae) are associated with high-temperature adaptation of corals, we studied Acropora digitifera, one of the most common Acropora species along the west coast of Okinawa Island in the Ryukyu Archipelago of Japan. The study site included two adjacent areas with very different seawater temperatures. A high-throughput sequencing approach with metabarcoding analysis was employed to examine genotypes of symbiotic algae. Genotyping of algal symbionts in A. digitifera colonies in each area showed that these corals host predominantly Cladocopium. In the low seawater temperature area, adult colonies hosted mainly subtypes C50a and C50c, which have been reported previously in A. digitifera, whereas in the high-temperature area, we detected Cladocopium goreaui (C1) and Durusdinium glynii (D1 and D4). This study is the first to show that Durusdinium may be involved in adaptation of adult Acropora colonies to high seawater temperatures in the Ryukyu Archipelago of Japan.