Rissoella japonica Chira Siadén & Wakeman & Webb & Hasegawa & Kajihara 2019, n. sp.

Rissoella japonica Chira & Hasegawa, n. sp.

(Figs 3 G–I, 6A–G)

Rissoella sp.— Hasegawa, 2000: 700 -701, plate 349, fig. Rissoellidae-1; Hasegawa, 2017: 398, 1063, pl. 355, fig. 5.

Type material. Holotype: adult, 0.9 mm (ICHUM RK2001); Kamoenai, Hokkaido, Japan, 43°08′10.5″N 140°25′43.1″E, 6 November 2016. Paratypes: 3 specimens (ICHUM RK2002, RO2001, RO2002) from Oshoro Bay, Hokkaido, Japan; 1 specimen (RSH2001); Shakotan, Hokkaido, Japan. For information on specimens collection locality and GenBank accession numbers see Table 1.

ZooBank registration: urn:lsid:zoobank.org:act: E3102674-B307-40F6-A700-7696AE32FCA8

Etymology. The species name, Rissoella japonica, refers to the geographical distribution from where the species was found.

Diagnosis. Protoconch with rippled sculpture at suture. Radula, central tooth with 10-13 sharp cusps on cutting edge. Lateral teeth narrow, with median ridge becoming basal process, and outer lateral projection on base; cutting edge with major median cusp, flanked by 5-6 (along outer margin) or 7-9 (along inner margin) sharp cusps. Marginal teeth similar in shape to lateral one but smaller, cutting edge with median cusp, flanked by 3-5 smaller sharp cusps on each side.

Description. Shell minute (764–1091 µm), thin, fragile, translucent or whitish opaque, elongate (width about 67% of length), with narrow umbilicus, spire of about 30% of total length (Fig. 6A). Protoconch smooth, of about 1 whorl, with rippled sculpture along suture (Figs. 6B, C). Teleoconch smooth, with distinct growth lines, deep suture, about 2 ½ convex whorls; aperture simple, entire, semicircular, slightly longer than 50% of total length. Operculum typical of family (Fig. 6D). Head–foot dark brown with colorless sole; oral lobes and tentacles dark brown. Mantle dark brown or black pigmented, with black patch placed slightly to left on dorsal portion of body whorl (Figs. 3 G-I). Radular formula 11-13 × 1.1.R.1.1 (Fig. 6E). Central tooth higher than wide (width about 48% of length), cutting edge with 10–13 sharp cusps of different sizes (Figs. 6E, F). Lateral teeth narrow (width about 23% of length), with median ridge becoming basal process, outer lateral projection on base; cutting edge with larger median cusp, flanked by 5–6 (along outer margin) or 7–9 (along inner margin) sharp cusps (Figs. 6E, G). Marginal teeth with similar shape to lateral one but smaller (width about 33% of length); cutting edge with median cusp, flanked by 3–5 smaller sharp cusps on each side (Figs. 6E, G).

Distribution and microhabitat. In the Sea of Japan from Otaru to Setana, Japan. It was found in the intertidal zone on various algae including the coralline algae Corallina spp.

Remarks. Both R. japonica n. sp. and R. elatior occur sympatrically in some localities, and they might be confused. However, they can be distinguished by the head-foot coloration (being dark brown in R. japonica n. sp. and white in R. elatior) and the radula morphology, as well as by conchological characters such as spire/total length and aperture/total length ratios. Based on radula morphology, R. japonica n. sp. belongs to a group containing the type species of Rissoella s.s., R. diaphana illustrated by Thiele (1929 –1935; as R. glabra), in having a symmetrical configuration with five teeth per row. Rissoella japonica n. sp. can be distinguished from R. diaphana by the relatively narrower and smaller central tooth.