Pseudocolochirus misakiensis Yamana & Kohtsuka 2018, sp. nov.

Pseudocolochirus misakiensis sp. nov.

[New Japanese name: Misaki-gumimodoki]

(Figs 1C, 2C, 5A–H)

Material examined. Holotype, WMNH-INV-2015-350 (St. 3, 24 June 2015, length 18 mm, width 6 mm). Paratypes, WMNH-INV-2015-348 (St. 3, 24 June 2015, length 19 mm, width 8 mm); WMNH-INV-2015-349 (St. 3, 24 June 2015, length 10 mm, width 4 mm); WMNH-INV-2015-351 (St. 3, 24 June 2015, length 23 mm, width 7 mm).

Other material: 25 specimens, WMNH-INV-2015-321–345 (St. 2, 25 June 2015, length 10–24 mm, width 4–6 mm); 22 specimen, WMNH-INV-2015-352–373 (St. 3, 24 June 2015, length 6–13 mm, width 3–6 mm).

Description. Body barrel-shaped, straight or slightly curved, with posterior end slightly tapered (Fig. 1C); body wall thin, soft. Body, tentacles, pedicels, and introvert uniformly yellowish white or cream, color not seriously affected by fixation/preservation. Some swollen specimens translucent. Tentacles 10, in a single circle, two mid-ventral tentacles smaller (approximately half length of larger ones). Introvert short, narrow, tentacles and introvert well extended. Pedicels in two rows along each radius throughout body length, up to 30 pedicels per row, decreasing in size anteriorly and posteriorly; fewer and smaller in the dorsal radii. Anal papillae five, in radii, color same as that of the adjacent body wall. Anal teeth also five, in radii, not visible externally.

Calcareous ring low (Fig. 2C), simple; radial and interradial elements solid, not fragmented, not fused; each radial and interradial plate with anterior projection and a concave posterior depression or notch; radial plates also with a slight anterior notch. Polian vesicle and stone canal single, long, each reaching 6 mm in length in mature specimens. Oral opening lacking villi-like projections. Gonad in two clusters in mid-body, one on each side of dorsal mesentery; tubules unbranched. Mature specimens with mid-dorsal genital papilla close to tentacle bases, papilla reaching 1.3 mm in length.

Ossicles of body wall comprise abundant, slightly-knobbed, thick, small plates (Fig. 5F, G, Table 3), with usually few small perforations; rarely thick large plates (up to 0.1 mm in length) with up to 13 perforations and a blunt small projection (Fig. 5F, G) also present. Anal papillae with small and irregular supporting plates (Fig. 5 H, Table 3). Peri-oral body wall and pharynx tissue with narrow, elongate plates or rods, distally with smaller perforations and centrally with larger perforations (Fig. 5B, C, Table 3). Ossicles of the peri-oral region larger, those of the pharynx frequently rod-shaped (Fig. 5C, Table 3); margins of ossicles of both regions spinous (Fig. 5B, C).

Pedicels with irregular endplates and narrow supporting plates (Fig. 5E, Table 3); endplate sometimes composed of several small pieces. Perforations usually lacking in central part of supporting plate. Supporting plates rod-shaped with a single series of perforations, two or more series rare. Introvert rarely with long, thick plates (Fig. 5D, Table 3). In tentacles perforated plates of various shapes and sizes, usually with spinous margins.(Fig. 5A, Table 3). Gonad lacking ossicles.

Remarks. This species has 10 bushy tentacles, a simple and short calcareous ring, and lacks baskets or Xshaped ossicles (x-bodies) in the body wall. This combination of features is scattered in the genera Cucumaria de Blainville, 1830, Pseudocnus Panning, 1949, and Pseudocolochirus Pearson, 1910 amongst the Cucumariinae. However, Cucumaria has spinous plates in the body; Pseudocnus has heavily knobbed plates while Pseudocolochirus (as amended by Thandar & Samyn 2004) is the only genus with simple plates in the body wall. However, the present specimens differ from both the nominal species of these genera (P. violaceus Théel, 1886, the type species, and P. unicus Cherbonnier, 1988) in possessing pedicels restricted to the radii and the two ventral tentacles reduced. Perhaps the current material does not strictly belong in Pseudocolochirus and deserves a new genus but this step was not taken in order to restrict the number of monotypic genera.

Distribution. So far known only from the type locality: off Jyogashima Island, south coast of Kanagawa Prefecture, sand bottom, 101–111 m, Pacific Ocean, middle Japan.

Etymology. The specific name misakiensis was derived from the name of peninsula Misaki, near the type locality.