Pusillina (Vicinirissoa) tumidula G.O. Sars 1878

Pusillina (Vicinirissoa) tumidula (G.O. Sars, 1878)

Cingula tumidula G.O. Sars 1878: 174 –175, Tab 10, fig. 2.

Rissoa griegi Friele 1879: 274.

Cingula griegi— Warén 1973: 11, figs 18–19. Warén 1974: 125.

Punctullum minutum Golikov 1987: 82 –83, fig. 34.

Obtusella tumidula— Warén 1989: 11, figs 8 C–D, F– G. Warén 1996: 215, fig. 12 D. Sneli et al. 2005: 52. Kantor & Sysoev 2006: 71, fig. 34 G. Høisaeter 2009: 43 –44. Nekhaev 2014: 89.

Setia tumidula— Golikov et al. 2001: 105.

Type material. Cingula tumidula: holotype (Fig. 1 D), ZMO D659, single dried specimen damaged by acidic glass; Cingula griegi— lectotype (Fig. 1 G) designated by Warén (1973) and two paralectotypes, all with number ZMB 20857; Punctullum minutum— holotype (Fig. 1 H), ZIN 35666/1 and 81 specimens labeled as “paratypes” (1♀ dissected), ZIN 35667/2, however the paratypes are not mentioned in the original description and therefore these specimens cannot be included in the type series.

Type localities. Cingula tumidula: littoral zone at Vardø, Northern Norway; Rissoa griegi: Vöringen, st. 353 (77°58'N, 05°10'E, 2438 m); Punctullum minutum: White Sea, Kandalaksha Bay, 50 m.

Other examined material: Southwestern Barents Sea: 1♂ (dissected), Malaya Sharkovka Bay, 27 m, 69°12.57'N, 34°54.31'E, 20 Sept. 2012, MS VIKING-2, MMBI, no number; 1♂ (dissected), Bolshaya Sharkovka Bay, 25 m, 69°12.59'N, 34°56.45'E, 20 Sept. 2012, MS VIKING-2, MMBI, no number; 1 spc, Zavalishina Bay, 12 m, 69°11.38'N, 35°14.78'E, 10 Oct. 2010, MMBI, no number; 2 spc, Orlovka Bay, 23 m, 69°12.35'N, 35°16.00'E, 11 Oct. 2010, MMBI, no number; 1 spc, Korabelnaya Bay, 11 m, 69°10.22'N, 35°08.79'E, 10 Oct. 2010, MMBI, no number; 6 spcs (3♀ dissected), Yarnishnaya Inlet, 67 m, 69°08.71'N, 36°00.44'E, 2 June 2009, RV DALNIE ZELENTSY, MMBI, no number; 1 spc, 73 m, 69°07.64'N, 36°02.01'E, 19 Sept. 2012, MS VIKING-2; 9 spc (1♀ dissected), Dalne-Zelenetskaya Bay, 59 m, 69°07.92'N, 36°05.45'E, 4 June 2009, RV DALNIE ZELENTSY, MMBI, no number; Eastern Barents Sea (Pechora Sea): 2 spc, 17.5 m, 69°15.61'N, 57°22.76'E, 15 Nov. 2010, RV BUJNITSKIY, MMBI, no number; 1 spc, 20 m, 69°23.15'N, 57°44.62'E, 15 Nov. 2010, RV BUJNITSKIY, MMBI, no number; Franz Joseph Land: 13 spcs (2♀ and 1♂ dissected), 44 m, 79º53.4'N, 51º26.06'E, 24 Aug. 2006, RV DALNIE ZELENTSY, MMBI, no number; White Sea: 23 spcs. (1♂ and 1♀ dissected), 85 m, 66°20'N, 30°E, 8 June 1926 RV PERSEY, ZIN 35668/3; 2 spcs, NE Kara Sea, 1937 RV SEDOV, ZIN 33631/2.

Shell. (Fig 1 A–H) Ovate-conical, small, white, usually semitransparent, covered by thin transparent periostracum. Up to 3.5 total whorls, well rounded with deep sutures. Sculpture of adult shell consists of slightly prosocline growth lines and thin, numerous (30 or more on body whorl) spiral riblets. Some specimens form wide costae usually limited to the upper parts of whorls. Protoconch (Fig. 2 A–D) of 1–1.2 whorls, 420–570 µm in diameter, with nucleus of about 130– 200 µm; larval whorls with more frequent and delicate spiral striae than those on teleoconch. Aperture oval, slightly angulated in upper part. Umbilicus relatively wide, partly covered by edge of lip. Operculum concave, transparent, without peg.

Mean values of morphometric characters of 22 adult measured specimens: shell height = 2± 0.04mm, shell width = 1.43± 0.03mm, aperture height = 1.09± 0.05mm, aperture width = 0.74± 0.02mm, last whorl height = 1.37± 0.05mm, shell whorls number = 3.3. The largest measured specimen had shell height = 2.35mm, shell width = 1.58mm, aperture height = 0.98mm, aperture width = 0.80mm, last whorl height = 1.73mm, whorls number = 3.5.

Radula. (Fig 2 E–F) Rachidian tooth relatively wide with prominent central cusp and 5 lateral denticles; single pairs of both basal and lateral processes are present; U-shaped ventral extension well developed. Lateral teeth with one prominent cusp and 4 to 5 smaller cusps on each side. Both inner and outer marginal teeth elongated with relatively similar-sized denticles.

Animal. Mantle not pigmented. Cephalic tentacles slightly longer than bilobed snout in preserved state (Fig. 3 E). Small anterior pallial tentacle on the right side and single metapodial tentacle present. Ctenidium elongated, filaments absent osphradium shorter than ctenidium.

Male reproductive system. (Fig. 3 E) Penis (PE) flattened, almost parallel-sided, with acute distal end; duct (SD) submarginal. Prostate gland large, approximately twice as long as wide.

Female reproductive system. (Fig. 3 A–D) Ventral channel marked, vestibule not developed. Lower oviduct gland (LOG) longer than high. Bursa copulatrix (BC) relatively large, about half size of lower oviduct gland; seminal receptacle (SR) small, elongated, not found in all studied specimens. Upper oviduct gland (UOG) coiled, usually short lobes (L) visible (these probably involved in sperm storage).

Digestive tract: contained intact calcareous foraminiferans, detritus and sand.

Distribution. Pusillina tumidula has been reported from Iceland, and Faeroe, Norwegian, Barents, White and Kara Seas including the waters around Svalbard and Franz Joseph Land (Golikov 1987; Warén 1996; Golikov et al. 2001; Sneli et al. 2005; Nekhaev 2014). Study of ZIN collections shows that records of that species from the Bering Sea (Kantor & Sysoev 2006; Sirenko et al. 2013) are based on mis-identification of the another rissoid species.

Habitat. Found on substrates with sand, shell rock and stones, occasionally with silt and clay. Several samples of Pusillina tumidula were taken from water as shallow 11–12 m, however the holotype of Cingula tumidula is the only known specimen of the species collected in the intertidal zone. The lower reported distributional limit is 500 m (Warén 1996).

Remarks. Both the reproductive system and the radula of Cingula tumidula differ significantly from that of Obtusella intersecta (Wood, 1857), which is the type species of Obtusella: the females of O. intersecta have a welldeveloped vestibule modified for sperm storage, and the seminal receptacle is placed perpendicular to the bursa copulatrix (Ponder 1984; Nekhaev 2015). Males have a long penis which is oval in section and with a long, prominent filament. The central tooth of Obtusella intersecta has two pairs of basal denticles. The adult shell of Obtusella intersecta is similar to that of Cingula tumidula in shape and sculpture, but the protoconch of the former species is multiwhorled with very weak spiral striae (Ponder 1984; Warén 1989), not like that of Cingula tumidula.

Vicinirissoa Ponder, 1984, currently considered a subgenus of Pusillina Monterosato, 1884, was introduced for Pusillina harpa Verrill, 1880, which had been considered to be the only species in the subgenus until now. Pusillina harpa is characterized by a poorly developed vestibule, in having a coiled upper oviduct gland, and a penis with a pointed end and a submarginal duct (Ponder 1984). These features conform with the anatomy of Pusillina tumidula described here. Both Pusillina harpa and Pusillina tumidula are similar in having a paucispiral protoconch sculptured with dense, wavy spiral riblets, and the adult shells of both species have the same pattern of weak, dense spiral sculpture. Narrow axial costae are always present on the shells of Pusillina harpa, but vary with depth in their number and degree of development (Rex et al. 1988). The radula of Pusillina harpa does not differ from the radula of Pusillina tumidula. Both bursa copulatrix and seminal receptacle were lacking in Pusillina harpa, however their lack is not established with certainty (Ponder 1984). Based on these results, I place Cingula tumidula within Pusillina (Vicinirissoa) as the most suitable group based on current evidence in anatomy of Rissoidae.

Punctullum minutum Golikov & Fedjakov in Golikov, 1987 was described from the White Sea based on ribbed specimens of Pusillina tumidula. The present study has not detected differences between Pusillina tumidula and Punctulum minutum, either in shell morphology or in internal anatomy, and therefore confirms their conspecificity. Significant variation in the degree of development of the wavy axial costae is a characteristic feature of Pusillina and is also known in other rissoid genera including Rissoa Freminville in Desmarest, 1814, Benthonella Dall, 1889.