The distribution of reef-dwelling Waminoa flatworms in bays and on capes of Okinawa Island

Abstract

Waminoa spp. are acoel worms living on the surface of anthozoans. Despite being potential parasites, there is still not much information available regarding the genus’ distribution or host selectivity. The aim of this study was to evaluate the distribution and occurrence of Waminoa on coral reefs along the west coast of Okinawa Main Island (26.5° N, 127.9° E) in southern Japan, comparing sites located on capes with sites located in bays and identifying associated hosts. Scuba surveys were conducted along eight coral reef sites on the west coast of the island, between June and October 2016. Seven out of eight sites showed the presence of Waminoa. Occurrence and associated hosts varied considerably from site to site, and between sites located within bays and those at capes facing the open ocean. Waminoa individuals were significantly more abundant close to the sandy bottom at the lower end of the reef slopes and were always absent in the shallowest 5 m of every site. Waminoa coverage varied depending on the host anthozoan species. No significant differences in coverage were observed within the same host species at different sites. Thus, in Okinawa, the distribution of Waminoa on coral reefs is not random, but has patterns specific to certain anthozoan host species, with the highest occurrences within 10 m in depth from the sandy bottom areas of reefs.