Pentamera misakiensis Yamana & Kohtsuka 2018, sp. nov.

Pentamera misakiensis sp. nov.

[New Japanese name: Subesube-gokaku-namako]

(Figs 1F, 2F, 8A–H)

Material examined. Holotype, WMNH-INV- 2015-34 (St. 1, 26 August 2014, length 17 mm, width 8 mm). Paratypes: WMNH-INV- 2015-35 (St. 1, 26 August 2014, length 12 mm, width 5 mm); WMNH-INV- 2015-36 (St. 1, 26 August 2014, length 13 mm, width 6 mm).

Description. Body fusiform to barrel-shaped, with posterior end turned slightly upwards (Fig. 1F); body wall thin, hardened. Body, pedicels, and introvert pink-white or yellowish white (living and preserved animals). Tentacles 10, in single circle, two ventral tentacles smaller. One row of villi-like short projections surrounding oral opening. Color of tentacles pale brown (in preserved specimens). Pedicels thin, forming two longitudinal rows along each radius, apparently non-retractile, lacking on introvert, each row with approximately 24–35 pedicels in ventral three radii, 15–22 pedicels in dorsal two radii. Pedicels becoming gradually smaller anteriorly and posteriorly. Five anal papillae and five anal teeth in radii.

Calcareous ring elongated, with posterior bifurcations (prolongations), however, plates not fragmented, radial and interradial plates joined, sutures clearly visible. (Fig. 2F). Each radial element long, with long paired subdivided posterior prolongations and an anterior notch/bifurcation. Interradial elements also long, with anterior, sometimes bifid/notched prolongation but no posterior prolongations. Polian vesicle and stone canal single. Gonad in mid-body, in two clusters, one on each side of dorsal mesentery, occasionally tubules with one or two branches.

Body wall ossicles comprise plates and tables. Plates abundant, elongated, irregular (Fig. 8F, G, Table 6), centrally thickened, multilocular. Tables with irregular disc (Fig. 8F, G, Table 6), disc thick, large, multilocular, with high to moderate-sized spire of two pillars and one or two cross-beams, terminating in a reduced crown bearing a few teeth. Smaller tables occasionally present (Fig. 8F, G, Table 6), mostly with eight disc perforations and low spire formed by fusion of two pillars. Anal papilla with supporting tables and plates (or supporting tables with rudimentary spire) (Fig. 8H, Table 6). Skin around anus with usually smaller tables, spire occasionally with one cross-beam, and 4–12 minute processes/teeth distally.

Tentacle ossicles comprise flattened narrow plates and rod-like plates, varying in size (Fig. 8A, Table 6), mostly with single or double series of central perforations, and two or more rows of distal perforations, occasionally without central perforations. Peri-oral skin and pharyngeal villi with flattened, rosette-like plates (Fig. 8B, C, Table 6), with slightly knobbed margins and centrally with one or two large perforations, and distally with many smaller/minute perforations, frequently occluded. Introvert with thick narrow plates, with two larger central perforations and other smaller perforations in single or double series (Fig. 8D, Table 6).

Pedicels with small, circular endplate and comparatively large supporting tables (Fig. 8E, Table 6). Endplate thin, weak, with coarse and irregular perforations, occasionally with approximately three different forms of perforations arranged in three concentric series, with large perforations marginally and smaller perforations medially. Supporting tables with narrow arched disc, with 2–4 central perforations and 1–3 distal ones; spire low, with two pillars and one cross-beam, with 2–3 pairs of minute processes on top of spire (Fig. 8E). Supporting tables, frequently with reduced spire, resulting in ossicles in the form of narrow plates/rods. Gonad lacking ossicles.

Remarks. This species has 10 tentacles, an elongated, complex, calcareous ring, and plate and table ossicles in the body-wall. This combination of features is typical of the genus Pentamera Ayres, 1852, currently represented by two nominal species from Japan (Ohshima 1915 and 1918a, b): Pentamera constricta (Ohshima, 1915), from Genkai sea, north of Kyushu, and off Goto Islands, west of Kyushu; and P. calcigera (Stimpson, 1851), from off Goto Islands, west of Kyushu, and off Karafuto (Sakhalin) Island. The present species is easily distinguished from P. constricta in which the introvert possesses tables with a large disc; however in the new species the introvert possesses only plates. The body wall ossicles of P. calcigera is similar to that of the present species (see Deichmann 1930; Panning 1949), but the pedicels and tentacles with red and purple color, respectively, and the pedicels with four rows in middle body (Stimpson 1851), are different from those of the present species.

Among the present nominal species of Pentamera, some species have a peculiar supporting tables, with tall spire which ends in flattened lobes and/or many teeth (e.g., Pentamera beebei Deichmann, 1938), which is thus easily distinguished from the present species. Although, other species have supporting tables with low spire, or lack detailed descriptions of the ossicle morphology, the present species is also distinguishable from all of them by following features: from P. chierchiae (Ludwig, 1887) and P. chiloensis (Ludwig, 1887) which have peculiar tables in body wall (Ludwig 1887); P. citrea (Semper, 1868) has peculiar supporting tables without a cross-beam (Semper 1868); P. lissoplaca (Clark, 1924) and P. montereyensis Deichmann, 1938 have button ossicle in body wall (Clark 1924; Deichmann 1938); P. populifera (Stimpson, 1864) and P. rigida Lambert, 1998 have thick plates in body wall, with 25–40 holes (Stimpson 1864; Lambert 1998); P. dubia Cherbonnier, 1951 and P. obscura Cherbonnier, 1951 have short calcareous ring without posterior prolongation (Cherbonnier 1951); P. paraibanensis Prata & Cristofferson, 2016 and P. pulcherrima Ayres, 1852 have brownish body color and bluish body color, respectively (Ayres 1852; Prata & Cristofferson 2016).

Distribution. So far known only from the type locality, off Jyogashima Island, south coast of Kanagawa Prefecture, sand bottom, 87–88 m, Pacific Ocean, middle Japan.

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