Rissoella elatior ( Golikov, Gulbin & Sirenko, 1987 )

Chira Siadén, Luis E., Wakeman, Kevin C., Webb, Stephen C., Hasegawa, Kazunori & Kajihara, Hiroshi, 2019, Morphological and molecular diversity of rissoellids (Mollusca, Gastropoda, Heterobranchia) from the Northwest Pacific island of Hokkaido, Japan, Zootaxa 4551 (4), pp. 415-431 : 418-419

Rissoella elatior ( Golikov, Gulbin & Sirenko, 1987)

( Figs 2 A–D, 3A–H)

Jeffreysina elatior Golikov, Gulbin & Sirenko, 1987: 35 , pl. 3, fig. 6; Kantor & Sysoev, 2006: 248, pl. 123, fig. D (holotype 36525/1; Moneron Island, Russia). Type material not available for analyses. Jeffreysiella elatior — Hasegawa, 2017: 398, 1063, pl. 355, fig. 6.

Material examined. Thirteen mature specimens (ICHUM RT1001, RT1002, RO1001, RO1002, RO1003, RSH1001, RSH1002, RK1001, RK1002, RM1001, RSU1001, RSE1001, and RSE1002). For information on specimens collection locality and GenBank accession numbers see Table 1.

Description. Shell minute (800–1270 µm) but relatively larger if compared to other species described here, thin, fragile, translucent or whitish opaque, elongate (width about 63% of length), with narrow umbilicus, spire of about 25% of total length ( Fig. 4A ). Protoconch smooth, slightly pointed, of approximately one whorl, without sculpture at suture ( Figs. 4B, C ). Teleoconch smooth except for faint markings of growth lines; with deep suture; up to 3 ½ convex whorls; aperture simple, entire, semicircular, slightly shorter or almost 50% of total length. Operculum typical of family ( Fig. 4D ). Head–foot opaque white, with slender oral lobes and longer cephalic tentacles. Mantle brown or black pigmented, with black patch centrally placed on dorsal portion of body whorl. Black patch hardly recognized in specimens with black mantle ( Figs. 3 B–D). Radular formula 15–16 × 2.1.R.1.2 ( Fig. 4E ). Central tooth wide (width about 61% of length), with 7–8 sharp cusps, latter gradually increasing in size from left to right until 6th (or in some cases 7th); right-most cusp slightly smaller than left ones. Group of 10–13 minute secondary cusps encircling upper margin of last right cusp ( Figs. 4E, F ). Lateral teeth elongate-triangular (width about 78% of length), each with large, sharp, smooth median cusp, and 8–12 smaller cusps along inner and outer margins ( Figs. 4E, G ). Inner marginal teeth represented by small, curved plates (width about 93% of length), each with large, sharp, smooth median cusp, flanked by 4–5 (along inner margin) or 5–7 (along outer margin) smaller cusps ( Figs. 4E, H ). Outer marginal teeth reduced, simple, plate-like (width almost 200% of length) ( Figs. 4E, H ).

Distribution and microhabitat. Originally reported from the northern part of the Sea of Japan (Moneron Island) ( Golikov et al. 1987), Russia, and subsequently reported to be widely distributed along the Japanese Archipelago from Hokkaido to Miyako Island, Okinawa ( Hasegawa 2017). Material in this study was collected from Rumoi to Setana on the Sea of Japan; and on the Pacific coast near Muroran, Japan. It was found on various algae including the coralline algae Corallina spp.

Remarks. Although the type specimens of Rissoella elatior have not been examined in this work, our newly sampled material agrees with the original description of this species ( Golikov et al. 1987), as well as the photograph of the holotype ( Kantor & Sysoev 2006: pl. 123, fig. D). In some specimens ( Fig. 3A ) the mantle coloration is brighter than others ( Figs. 3 B–D), the former being pale brown with yellowish white asymmetrical patches and a brown patch centrally placed on the dorsal portion of the body whorl.

Figures

FIGURE 2. Maximum-likelihood tree analysis of the COI sequences of Rissoella from Hokkaido, Japan. Scale bars represent raw percentage sequence divergence. MOTUs are indicated with letters in parenthesis.

FIGURE 3. Living specimens. (A–D) Rissoella elatior. (E–F) Rissoella golikovi. (G–I) Rissoella japonica n. sp. (J–K) Rissoella sp. 1.

FIGURE 4. SEM micrographs of Rissoella elatior. (A) Apertural view of the shell. (B) Apical view of the protoconch. (C) Detail of the protoconch showing the suture. (D) Inner face view of the operculum. (E) Radula. (F) Central teeth. (G) Lateral teeth. (H) Inner and outer marginal teeth.