Gracilideres mawatarii, a new genus and species of Franciscideridae (Allomalorhagida: Kinorhyncha) – A kinorhynch with thin body cuticle, adapted to the interstitial environment

A new genus and species of Kinorhyncha, Gracilideres mawatarii gen. et sp. nov. is described from coarse subtidal sand, west of Tanegashima Island, Japan. The genus and species is characterized by a cylindrical neck with sparsely arranged longitudinal crimps at surface; segments 1–10 each consisting of a single cylindrical cuticular plate; middorsal and ventromedial string-like glandular cells on segments 1–9; primary pectinate fringes with pectinate fringe teeth on segments 1–10; lateral accessory pores on segments 2–10; unpaired dorsal spines on segments 3–11 or 4–11, of which those on segments 3–9 are alternatingly laterally displaced left or right to paradorsal positions, and those on segments 10 and 11 in middorsal position; lateral accessory acicular spines on segment 5; lateroventral acicular spines on segments 4, 6–10; and tubes in lateroventral position on segments 2 and 5 and in ventrolateral position on segment 1. Together with the description of G. mawatarii gen. et sp. nov., emended diagnoses for the family Franciscideridae and the genus Franciscideres are provided. The introvert morphology of G. mawatarii gen. et sp. nov. is unique within Kinorhyncha, but showing some similarities to those of Franciscideres and Cateria, suggesting a close relationship between these three genera. The thin and flexible trunk cuticle in G. mawatarii gen. et sp. nov., Franciscideres, Neocentrophyidae, and some kentrorhagids would be related to the adaptation of interstitial environment. The thickening or thinning of cuticle has evolved independently at least twice in the kinorhynch lineages.