A New Species of Hypermastus (Prosobranchia: Eulimidae) Associated with Echinodiscus tenuissimus (Echinoidea: Astriclypeidae) from off Okinawa, Japan

Abstract

Hypermastus ryukyuensis n. sp. is described based on specimens from the clypeasteroid echinoid Echinodiscus tenuissimus caught at 10 m in depth in Oura Bay, Okinawa Prefecture, Japan. Of 30 currently valid species in the genus, H. tenuissimae, H. serratus and H. auritae resemble the new species, which is, however, distinguished from these three species by having a mucronate protoconch of nearly 2.5 whorls, apical teleoconch whorls abruptly increasing in diameter, indistinctly convex teleoconch whorls, a smaller aperture and prosocline outer lip mar-gin, which is not sinuate below the suture. In addition to the new species, H. peronellicola and H. tokunagai are known to occur in Japanese waters, and H. tenuissimae has been reported from E. tenuissimus from Thailand.