2021 年 68 巻 2 号 p. 117-128
Six specimens collected from Kochi, Nagasaki, Kagoshima, and Okinawa Prefectures, on the Pacific Coast and the East China Sea, and seven museum specimens, were identified as the purple ray Myliobatis hamlyni Ogilby, 1911, based on a combination of specific morphological characteristics. Initially considered an Australian endemic, subsequent studies have shown M. hamlyni to have a patchy distribution from the Australian coast to Japan, the latter record being based on a single specimen collected from Okinawa Prefecture. However, the current study has revealed the widespread distribution of M. hamlyni in waters from the surface to 500 m depth off southwestern and western Japan, a specimen from Kanagawa Prefecture representing the northernmost record of the species. Because the distribution of M. hamlyni may broadly overlaps that of the Japanese species M. tobijei, and morphometric characters and depth preference separating them suggested in the previous study were unclear, detailed comparisons were made so as to establish a basis for distinguishing between the two species in the present study. Myliobatis hamlyni differs most clearly from M. tobijei as follows: greatest span of pelvic fins 20.9–24.7% (mean 23.5%) of disc width [vs. 27.1–36.3% (30.7%) in M. tobijei], greatest span of pelvic fins 44.5–53.6% (50.4%) of pectoral-fin posterior margins [vs. 58.8–78.5% (67.4%)], and distance from edge of disc to first gill slit 51.0–68.3% (57.6%) of distance from pectoral-fin insertion to dorsal-fin origin (horizontal) [vs. 32.4–44.7% (41.3%)]. The new standard Japanese name “Sumire-tobiei” is proposed for M. hamlyni.