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.

Solitary entoprocts are sessile aquatic invertebrates inhabiting various non-living or living substrata. They proliferate through both sexual and asexual reproduction. However, fecundity and population dynamics of entoprocts are poorly understood, probably because the small size of the individuals (0.2 to 5 mm length) makes field studies difficult. To clarify the seasonal fluctuation of fecundity via sexual and asexual reproduction, we sampled the solitary entoproct Loxosomella plakorticola Iseto & Sugiyama, 2008 (Entoprocta: Loxosomatidae) monthly over 2 yr. L. plakorticola often forms dense aggregates on a coral-reef demosponge, Plakortis sp. Budding occurred all year round and was most active from spring to early summer. There was a significant negative relationship between bud production and water temperature 2 mo previously. In contrast, sexual reproduction rarely occurred during our monitoring. Monthly monitoring of 4 sponges for 13 to 23 mo revealed that the density and distribution pattern of L. plakorticola on the sponges fluctuated highly regardless of season. Large populations sometimes disappeared from the host sponge within short periods, while, in other cases, a small, sparse population grew to a large, dense population within a few months. DNA sequencing analysis demonstrated that individuals on the same sponge did not belong to a single clone. Thus, the entoprocts probably disperse from one sponge to others. Populations on sponges were mainly formed and maintained by budding; sexual reproduction may contribute to longdistance dispersal and maintenance of genetic heterogeneity. KEY WORDS: Budding · Reproductive strategy · Genetic heterogeneity · Coral reef · KamptozoaResale or republication not permitted without written consent of the publisher