New records of associations between ectoparasitic snails of the genus <i>Mucronalia</i> (Caenogastropoda: Eulimidae) and their ophiuroid hosts from Japan and New Caledonia, with description of a new species

Abstract

Gastropods of the genus Mucronalia (Eulimidae) are known ectoparasites of brittle stars (Ophiuroidea). However, the identities of host species are only known for three of some ten described species in the genus. Here, we report associations of Mucronalia snails, including one new species, with ophiuroid hosts from Japan and New Caledonia: M. spp. aff. alba with Ophiomastix annulosa, Ophiomastix mixta, and Ophiocoma cynthiae; M. sp. cf. exquisita with Amphioplus (Amphioplus) sp.; Mucronalia ryukyuensis n. sp. with Ophionereis porrecta; and Mucronalia sp. with Amphiura velox. Mucronalia ryukyuensis n. sp. is described from Okinawa, southwestern Japan. The new species is characterized by its slender shell with a large body whorl and aperture compared with other species of the genus. Among Japanese congeners, the shell of the new species is most similar to that of M. exilis, but differs in having a teleoconch with less-convex whorls, regularly increasing in diameter. The present study corroborates Warénʼs (1983) general rule that eulimids in a single genus parasitize hosts of the same echinoderm class, and our records suggest that closely related species of Mucronalia tend to exploit hosts of a more restricted lineage. We also briefly discuss the divergence processes of Mucronalia snails, which likely involve host shifts and geographic isolation.