Sitnikovia ratschuli, Chertoprud & Palatov & Vinarski, 2020

Sitnikovia ratschuli View in CoL sp. nov.

( Figs 3b, 3d, 4b, 4d, 4f, 4h)

Holotype. Adult mollusc, Georgia, Racha-Lechkhumi , vicinity of Kveda Tlughi Village , Shaori Lake basin, subterranean river inside Sakishore Cave, 42°26′31.1″N 43°09′30.9″E, 1188 m a.s.l., 13.VIII. 2019, E. Chertoprud leg., No. 513/1 ( ZIN). GoogleMaps

Paratypes. 22 adult molluscs, same collecting data as for the holotype, three under No. 513/2 ( ZIN), three under No. Lc-40754 ( ZMMU) and the rest in collections of the authors GoogleMaps .

Shell dimensions of the holotype (mm). WN 4.9; SH 2.15; SW 1.2; BWH 1.35; BWW 1.05; AH 0.8; AW 0.75. For morphometric characteristics of the entire type series see Table 1.

Description. Shell small (SH <2.6 mm), high conical, relatively slender (SW/SH ratio 0.56– 0.68), whitish. Whorl number up to 5.10. Spire high, its height 0.46 SH or more, and can reach 0.9 mm. Body whorl high and rather convex, its width extends beyond width of penultimate one. Tangent-line slightly concave. Surface of teleoconch with rare growth lines. Whorls rounded and visibly convex, separated by deep oblique suture. Aperture ovate, with obtuse upper angle. Umbilicus well developed. Protoconch consists of 1.5 whorls (270 µm in diameter, WE 158 µm). Surface of protoconch with mesh-like microsculpture. Protoconch separated from teleoconch by distinct axial line. Operculum ovate, corneous, thin and flat.

Radula ( Fig. 4h). Central (rachidian) tooth has a pair of basal cusps, large median cusp and five lateral cusp on each side of it. Lateral tooth asymmetric, with eight cusps, three at inner side, one largest, four at outer side (formula: 4-1-3). Median cusp elongated, almost twice as long as adjacent ones, and overlaps adjacent inner marginal tooth. Inner marginal tooth with 24 cusps, outer marginal tooth with 16 cusps.

Penis ( Fig. 3d) massive and swollen in central part, with bulging dorso-lateral lobe slightly spaced from penis, and having miniature pointed tip. Distal part of penis abruptly tapers and forms short, hook-shaped tip, curved downwards.

Animal blind.

Differential diagnosis. This species is similar to S. megruli sp.nov., but is larger (SH up to 2.6 mm) and having a relatively wider shell (SW/SH ratio 0.56–0.64). Body whorl is higher and more inflated, noticeably wider than the preceding one. Aperture of similar proportions but larger. The protoconch of S. ratschuli sp. nov. is noticeably smaller than that of S. megruli sp. nov. ( Figs 4e, 4f). Surface of protoconch has a similar mesh-like sculpture although the pattern is slightly different with irregular axial elevations forming feebly defined ridges surrounding the depressions. The penial lobe in S. ratschuli sp. nov. is low and wide, whereas in S. megruli sp. nov. it is rather slender and pointed. There are no noticeable differences in radular morphology between S. megruli sp.nov. and S. ratschuli sp. nov.

Etymology. The name is derived from Racha (Georgian რაჭა), a historical highland region of Western Georgia, located in the upper Rioni River and at the foothills of the Greater Caucasus Range.

Distribution. Endemic to Western Georgia. Known only from the type locality.

Habitats and ecology. Inhabits subterranean river flowing in the Sakishore Cave ( Fig. 3f). Molluscs live on the massive stones immersed in water. S. ratschuli sp.nov. may be characterized as a rheophilic species that prefers submerged hard substrates. Molluscs prevail in a truly underground part of a watercourse, whereas in the part of the cave where the sunlight penetrates only empty shells were found.