Stenosemus undatopleuralis, Sirenko, 2023

Sirenko, B. I., 2023, Stenosemus undatopleuralis sp. nov. (Mollusca: Polyplacophora) from Northeast Pacific, Ruthenica, Russian Malacological Journal 33 (2), pp. 73-77 : 74-76

Stenosemus undatopleuralis sp. nov.

( Figs 1–5 )

Zoobank registration: urn:lsid:zoobank.org:act:88900946-3E28-4ABA-8155-D8EC77AD80B5

Type material. Holotype ( ZIN 2454 ), damaged, now disarticulated, consisting of mounts of shell, perinotum and radula.

Type locality. Northeast Pacific, Gulf of Alaska, 55˚23.7’N, 134˚12.0’W, 532 m ( R / V Vityaz, cruise 45, station 6126, «Ocean» grab).

Etymology. The specific name is two Latin objec- tives undatus (undulating) and pleuralis, reflecting pleural areas of intermediate valves are undulating.

Distribution. Only known from the type locality.

Diagnosis. Animal of small size, BL of the holotype 6.0 mm, oval, shell rather elevated, distinctly carinated, intermediate valves not beaked, pleural areas of intermediate valves with longitudinal undulating folds. Lateral areas weakly raised.Aesthete pores arranged in radiating rows on head valve, lateral areas of intermediate valves and postmucronal areas. Jugal plates absent. Slit formula 14/1–2/16. Girdle dorsally covered with juxtaposed, flattened, slightly bent, smooth spicules with pit on top. Central teeth of radula long, rectangular, major lateral teeth of radula with a bidentate head, main denticle long and sharply pointed, accompanied by a very small outer denticle.

Description. Holotype small, body length 6.0 mm, all valves broken. Valves moderately elevated, carinated, not beaked. Color of tegmentum in preservation white, slightly rose. Head valve semicircular, posterior margin widely V-shaped, clearly notched in the middle, sculptureless, except for some faint concentrical growth line. Intermediate valves broadly rectangular, front margin slightly convex, side margins and posterior margin straight, lateral areas weakly raised, pleural part of central areas with longitudinal undulating folds, lateral areas sculptured like head valve. Tail valve noticeably smaller than head valve, mucro anterior, little elevated, anterior and posterior slopes slightly concave, postmucronal area sculptured like head valve.

Articulamentum thin, translucent, apophyses short and wide, jugal plate absent, insertion plates short, slit formula 14/1–2/16, teeth sharp, slit rays weakly perceptible, eaves very narrow with rare, small pores.

Girdle relatively narrow, about 0.2 mm in width near valve V, covered dorsally with juxtaposed, flat- tened, slightly bent, smooth calcareous corpuscules with pit on top (110–200 x 53–75 μm). Marginal fringe composed of two kinds of spicules: small one (40 x 8 µm) on bristle (up to 60–80 µm) and much longer, smooth sharply pointed spicules (100–114 x 20 µm). Ventrally the girdle is covered with radiating rows of short, rectangular scales (70 x 26 μm).

Radula of the holotype is 1.9 mm long and com- prising 34 transverse rows of mature teeth. Central tooth somewhat longer than wide, rectangular with small blade, first lateral teeth wing-shaped, major lateral teeth of radula with a bidentate head, main denticle long and sharply pointed, accompanied by a very small outer denticle.

Holotype with twenty three gills on each side, extending from valve III to valve VIII.

Remarks. The new species differs from other species of the genus inhabiting in Northeast Pacific by absence of jugal plate, by having pleural areas of intermediate valves with longitudinal undulating folds, flattened and smooth dorsal calcareous corpus- cules, rectangular central teeth of radula.

Stenosemus undatopleuralis sp. nov. is most similar to S. stearnsi , differing from the latter in the absence of sculpture on areas of the tegmentum with the exception of pleural part of central area, and the different shape of dorsal calcareous corpuscles and ventral scales, as well as in the presence of marginal spicules.

The new species differs from S. albus , and S. sharpi by having rectangular central teeth of radula (vs. pyriform central tooth in both species), flattened and smooth dorsal calcareous corpuscules (vs. cylindrical and ribbed calcareous corpuscules in both species). Moreover new species has no jugal plate (vs. jugal plate present in both species)

Stenosemus undatopleuralis sp. nov. differs from S. moskalevi , S. kolesnikovi and S. golikovi by having bidentate head of major lateral tooth (vs. unidentate head in the three species), flattened and smooth dorsal calcareous corpuscules (vs. cylindrical in S. moskalevi and S. golikovi , and ribbed in S. kolesnikovi ). Moreover new species has no jugal plate (vs. jugal plate present in the three species).

Figures

FIG. 1. Stenosemus undatopleuralis sp. nov., holotype (ZIN 2454), Gulf of Alaska, northeast Pacific, BL–6.0 mm. A. Whole specimen, dorsal view. B. Whole specimen, lateral view.