Rissoella golikovi

Rissoella golikovi (Gulbin, 1979)

(Figs 3 E–F, 5A–G)

Jeffreysina golikovi Gulbin, 1979: 89, figs. 1–2 (holotype ZIN 41388/1; Vanino Bay, Sea of Japan, Russia); Kantor & Sysoev, 2006: 248, pl. 123, fig. E (paratype); Hasegawa, 2017: 398, 1063, pl. 355, fig. 7. Type material not available for analyses.

Material examined. Fourteen mature specimens (ICHUM RT3001, RT3002, RT3003, RO3001, RO3002, RO3003, RK3001, RM3001, RM3002, RM3003, ROM3001, RA3001, RA3002, RA3003). For information on specimens collection locality and GenBank accession numbers see Table 1.

Description. Shell minute, smaller (296–450 µm) in comparison to other rissoellids, thin, extremely fragile, translucent or whitish opaque, skeneiform (width about 150% of length), with deep, widely perforate umbilicus (Fig. 5A). Protoconch smooth, of about one whorl (Figs. 5B, C). Teleoconch smooth with distinct growth lines, slightly deep suture, of about three convex whorls; aperture simple, entire, nearly circular but with margin adjacent to previous whorl flattened. Operculum typical of family (Fig. 5D). Head–foot brown or dark grey with colorless sole; oral lobes short; cephalic tentacles slightly longer than oral lobes; oral lobes and cephalic tentacles proximally having similar coloration to head, gradually becoming transparent in distal portion. Mantle dark brown or black pigmented, with black or darker brown patch on center of dorsal portion of body whorl; another smaller dark patch placed on left of neck (Figs. 3E, F). Radular formula 12–13 × 1.R.1 (Fig. 5E). Central tooth higher than wide (width about 52% of length), with medial narrow ridge, cutting edge with one small central sharp cusp flanked by several larger cusps (Figs. 5F, G). Lateral teeth triangular (width about 41% of length), each with median ridge; cutting edge with larger median cusp, flanked by 4–5 sharp cusps, consecutively decreasing in size (Figs. 5E, F).

Distribution and microhabitat. Known from Vanino Bay, as well as middle Kurile Islands, Russia. Material in this study was collected from Hokkaido, Japan: Otaru and Kamoenai (Sea of Japan), near Omu (Sea of Okhotsk), and Akkeshi and Muroran (Pacific). It was found on various algae including the coralline algae Corallina spp.

Remarks. The type material of Rissoella golikovi was not examined, due to restrictions on shipping biological material. However, our newly sampled material agrees with its description. The present morphospecies is nearly identical in radula morphology to Rissoella globularis (Forbes & Hanley), which has been reported from France to northern Norway, as illustrated by Sars (1878). Nevertheless, these two species can clearly be distinguished by shell morphology. In R. globularis, the shell is depressed conical, while it is skeneiform in R. golikovi. The skeneiform shell of this species makes it easily distinguishable from other species in the family where shells are either ovate or elongate. Rissoella golikovi was first described by Gulbin (1979) from Vanino Bay, Russia, and was subsequently recorded from the eastern part of Hokkaido (Hasegawa 2017).