Inferring the distribution of octocorals Nanipora and Heliopora along the coast of the Kerama Islands, Okinawa, Japan, by eDNA metabarcoding

Two zooxanthellate octocorals, Heliopora coerulea and Nanipora kamurai (family Helioporidae, order Scleralcyonacea, class Octocorallia), produce aragonite calcium carbonate skeletons similar to those of scleractinian hexacorals and contribute to the formation of complex coral reef ecosystems. H. coerulea is known to occur along the coasts of the Kerama Islands, Okinawa, Japan, and N. kamurai was originally described from the Kerama Islands. An environmental DNA metabarcoding (eDNA-M) method has recently been introduced for comprehensive surveys of zooxanthellate scleractinian corals, supplementing conventional visual census by SCUBA diving or snorkeling. In particular, our recently established Scl-eDNA-M (Scleractinia environmental DNA Metabarcoding) system, which targets mitochondrial 12S rDNA for eDNA amplification, has proven to be an efficient tool for taxonomically identifying nearly all scleractinian genera reported in Japan. Moreover, we have recently shown that Heliopora can also be detected using Scl-eDNA-M. This study is the first to examine whether the Scl-eDNA-M system is also applicable for the eDNA-based identification of Nanipora. Analyses of primer composition for PCR amplification and metabarcoding of eDNA from surface seawater from the Kerama Islands confirmed that Nanipora can be detected by this method. Importantly, the results of this study demonstrated that the overall distribution of Nanipora differs from that of Heliopora. Whereas Nanipora was distributed along the coasts of the inland sea of the Keramas, Heliopora occurred mainly along the exposed outer coasts, with only one site hosting both species. Although such differential distributions need to be confirmed by further visual census surveys, this study highlights the efficiency of the Scl-eDNA-M system for broad surveys of zooxanthellate Helioporidae.