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.

Comparative morphology of the foot structure of four genera of Loxosomatidae (Entoprocta): Implications for foot functions and taxonomy

“…The accessory cells along the foot groove were often referred as accessory gland cells (Schmidt, 1876). However, Iseto et al (2008) suggested that these cells lack secretory granules in L. elegans and L. plakorticola and proposed to call them groove accessory cells (GACs). In this study, we confirmed that the accessory cells did not contain any secretory granules in L. stomatophora .…”

“…The foot groove is a longitudinal furrow along the underside of the foot, which meets the basal opening of the foot gland at the anterior end (Schmidt, 1876). The structure of the foot permits animals to glide over the substratum similar to slugs (Iseto, 2002) or grab the body surface of the host animal (Iseto et al, 2008). Nevertheless, in many Loxosomella species, the foot degenerates in the adult stage and the organisms permanently attach to the substratum (Nielsen, 1989).…”

“…The groove lumen is filled with large bulges from the cells in the upper wall of the groove, which was assumed to reflect apocline secretion of adhesive substances by Nielsen and Jespersen (1997). They assumed that these gland systems in the foot of L. elegans are “engaged in adhesion to and release from the substratum.” Another ultrastructural study revealed that the foot of a sponge‐inhabiting species, Loxosomella plakorticola, grabs the sponge surface by closing the groove filled with numerous microvilli (Iseto et al, 2008).…”

Reproduction and population dynamics of the solitary entoproct Loxosomella plakorticola inhabiting a demosponge, Plakortis sp.

“…Adults of many Loxosomella species are known to glide over the substratum using their basal foot, and this is another possible way of recruitment of L. plakorticola between the populations. However, the foot of the present species is specialized to attach to the sponge surface (Iseto et al 2008 and seems unsuitable for moving between sponges.…”

“…(Iseto et al 2008), where it often forms dense aggregates. This is the first quantitative survey on the seasonal fluctuation of reproduction of solitary entoprocts in natural conditions.…”