First Japanese Record of Muraenichthys gymnopterus (Anguilliformes, Ophichthidae) from Ishigaki-jima Island, Ryukyu Archipelago

A single specimen of Muraenichthys gymnopterus (Bleeker, 1853) was collected from a sandy intertidal flat having rocks and dead corals in the estuarine area of Nagura Amparu, Ishigaki-jima Island, southern Japan, in October 2020. This specimen collection constitutes the first record of M. gymnopterus from Japanese waters. In this study, the diagnostic characters between M. gymnopterus and M. hattae Jordan and Snyder, 1901 are provided, based on our morphological observations of 37 specimens, including Okinawan specimen, and previous studies, as follows: head length [ M. gymnopterus 11.8– 15.0% of total length (TL) vs. M. hattae 9.4–11.0%], trunk length (24–25.4% of TL vs. 28–31%), the horizontal distance from the dorsal-fin origin to a vertical line through the anus 73–87% of head length vs. 13–49%), the number of vertebrae (total 129–130 vs. 148–155; predorsal 30 vs. 47–53; preanal 41–44 vs. 51–55), the number of the lateral-line pores before the anus (43–45 vs. 51–55). Additionally, the body depth at the gill opening in TL and the trunk length in TL can also be used to distinguish between these two species (2.8–3.7% of TL vs. 1.4–3.0%). Although M. gymnopterus has previously been reported from tropical to temperate regions, we suspect that the records from temperate regions are based on misidentification of M. hattae .