Diversity and distribution of air-breathing sea slug genus Fleming, 1822 (Gastropoda: Onchidiidae) in southern Japanese waters.

Species of the genus Fleming, 1822, are air-breathing onchidiid sea slugs that inhabit intertidal reef flats of temperate to tropical zones. In the Ryukyu Islands of southern subtropical Japan, species are a traditional food source for local people. To date, there have been three species recorded around Okinawajima Island; and , along with recently described , which was described as possibly endemic to Okinawajima Island. This study aimed to map the distribution ranges of these three species within the Ryukyu Islands using molecular analyses in order to understand the specific distribution of each species. Since species are generally indistinguishable by gross external morphology, a DNA barcoding approach was employed to identify specimens. The molecular data showed that there are four species present in the Ryukyu Islands. (unit #1 Dayrat et al., 2020) was dominant at almost all locations, while was present in much lower numbers than , but found across a relatively wide range in the Ryukyu Islands. We newly record and from Amami Oshima Island and from several places around Okinawajima Island, and also identified high levels of genetic variation within . and have been thought to be endemic to Okinawajima Island and to Honshu, mainland Japan, respectively. However, as both species were observed around Okinawajima and Amami Oshima islands, other islands of the Ryukyus are also likely to harbor these species, and their distribution ranges are wider than previously thought. Based on the results from molecular analyses, we provide general descriptions of each species. Sizes of specimens were consistently smaller for and relatively larger for specimens. On the other hand, and had similar size ranges, but had comparatively much more distinct papillae. This study revealed that the Ryukyu Islands are the only region currently known with four sympatric species, and this work provides a basis for future research on these species throughout the northwest Pacific Ocean, representing the first step in more effective management of the local fisheries in the Ryukyu Islands.