A New Sponge-Inhabiting Loxosomella (Entoprocta: Loxosomatidae) from Okinawa Island, Japan, with Special Focus on Foot Structure

Abstract

A new solitary entoproct, Loxosomella plakorticola sp. nov., was found on a sponge, Plakortis sp., on a coral reef slope on the western coast of Okinawa Island, Ryukyu Archipelago, Japan. This species has a medium-sized body (up to about 1.2 mm), slender proportion (the stalk is 0.83–1.76 times longer than the calyx), a slug-like foot with a foot gland and foot groove, and 14 to 18 tentacles. Small black pigment granules are visible only in the living stage in the calyx, stalk, and buds. This is the first report of a commensal loxosomatid from the Ryukyu Archipelago and the second species inhabiting sponges reported from Japan. Detailed morphological observations indicate that this species attaches to sponges by narrowing the foot groove; the sponge surface is pinched in the deepest part of the groove, which is free of the cuticle layer but covered by microvilli of epidermal cells. The accessory cells lining the foot groove have long been believed to be gland cells, but they are not gland cells in ultrastructure, at least in this species.