Cytherissa sinistra Mazepova, 1984

Cytherissa sinistra Mazepova, 1984

Figs. 34–37 View FIGURE 34 View FIGURE 35 View FIGURE 36 View FIGURE 37 ; 38J View FIGURE 38 ; 39J View FIGURE 39 ; 40D, 40O View FIGURE 40 .

Type locality. According to the label, specimens were collected from three different locations of Lake Baikal: 1) the underwater Academichesky Ridge (approximate coordinates 53º37’10.42” N, 108º10’11.69” E) GoogleMaps ; 2) southern basin, near Bugul’deika Village (approximate coordinates 52º30’47.88” N, 106º04’03.66” E) GoogleMaps ; 3) between Eligey and Gorevoi Utes Cape (approximate coordinates 53º22’56.97” N, 108º21’44.49’’ E) GoogleMaps .

Type material. Lectotype No. O17 (female) and paralectotype No. 1 (male): dwm No. O16-060872. Paralectotypes Nos. 2–9 ( 8 females): dwm No. O17-060872. Paralectotypes Nos. 10–16 ( 7 males): dwm No. O18- 060872. Paralectotypes Nos. 17–24 ( 4 females; 4 males): swm No. 33. Paralectotype No. 25 (female): swm No. 33 (valves) and wm No. O19-060872 (limbs). Paralectotype No. 26 (male): swm No. 33 (valves) and wm No. O20- 060872 (limbs). Paralectotype No. 27 (male): swm No. 33 (valves) and wm No. O21-060872 (limbs) .

According to the label, specimens were collected in three different locations of Lake Baikal: 1) 11 females and 9 males: the underwater Academichesky Ridge , August 6, 1972, depth 230 m, sample No. 337 from the collection of taxocoenoses of G.F. Mazepova; 2) 14 females: southern basin, near Bugul’deika Village , September 13, 1991, 360 m depth, brown silt, sampled using a research deep-submergence vehicle “Pisces”; 3) 3 males: between Eligey River and Gorevoi Utes Cape, August 9, 1972, 520 m depth. Since all specimens were in one tube with a common label, we did not determine where each particular specimen was collected from .

Description. Female. Carapace ( Figs. 34A–34D View FIGURE 34 ; 35A–35F View FIGURE 35 ; 39J View FIGURE 39 ) laterally oval-ovate: L=730–780 µm (mean 755 µm, n=10), largest H=495–520 µm (mean 510 µm, n=10), located on posterior border of anterior 1/ 3 L. Dorsal margins of valves slightly arcuate at center.Anterior end of carapace broadly rounded, posterior end almost straight, narrowly rounded. Ventral margins of both valves equally concave on inner side and arcuate on outer side; ventral margin protrudes more strongly on RV than on LV. RV overlaps LV throughout. Inner lamella broad. Marginal pore canals may be short and not reach margin of inner lamella or occupy 100% of width of inner lamella; some marginal pore canals branching (in anterior part of valves). Inner lamella of each valve with 26–33 marginal pore canals (8–12 short, 18–21 long). Pore canals of outer lamella sieve-type, branching, grouped (up to five canals in group), single near margin of valves; each canal with one sensillum ( Fig. 37B, 37D, 37E View FIGURE 37 ). Single pore canals near valve margin may be elongate ( Fig. 37D View FIGURE 37 ) or round ( Fig. 37E View FIGURE 37 ). Outer lamella of each valve with at least 125–155 pore canals. Inner side of each valve with approximately 40 canal apertures. Hinge teeth on LV, crenulated, posterior tooth slightly extending beyond margin of valve; bar on RV, almost smooth. Microrelief of valves surface almost homogeneous, consisting of large pits, diameter almost equal on entire surface of valves ( Fig. 37A View FIGURE 37 ). Dorsal margin almost smooth, with highly smoothed microrelief. Ventral margin with highly elongated pits; row of very small pits along valve margin ( Fig. 37C View FIGURE 37 ). Site of greatest carapace width on ventral and dorsal views in central part of L ( Fig. 35C, 35D View FIGURE 35 ).

A1, A2, Md, Mxl, L5–L7 as in female C. heckyi .

Male. Carapace ( Figs. 34E–34H View FIGURE 34 ; 36A–36F View FIGURE 36 ; 39J View FIGURE 39 ) slightly longer and taller than in female: L=805–860 µm (mean 835 µm, n=10), H=495–520 µm (mean 515 µm, n=10). In dorsal and ventral views, males ( Fig. 36C, 36D View FIGURE 36 ) significantly narrower than females ( Fig. 35C, 35D View FIGURE 35 ). Microrelief on dorsal margin more pronounced ( Fig. 36D View FIGURE 36 ) than in females ( Fig. 35D View FIGURE 35 ). Inner lamella of each valve with 29–31 marginal pore canals. Geniculate (transformed) legs on left side of body. Other morphological features of carapace structure, microrelief, A1, A2, Md, Mxl, right (non-geniculate) legs L5 ( Fig. 40D View FIGURE 40 ) and L6 as in female. Both legs L7 and geniculate L5 as in male C. heckyi .

L6. Left leg. Protopod with four setae and several groups of pseudochaetae. Endopod one-segmented ( Fig. 40O View FIGURE 40 ), most probably without seta (or very small), distal part with gently pointed outgrowth.

Hemipenis ( Fig. 38J View FIGURE 38 ) small, elongate-triangular; outer appendage lanceolate, with slightly curved main axis. Length of hemipenis in quiescent state 370 µm (n=1), greatest width 265 µm. Diameter of copulatory process 105–125 µm.

Brush organ. Both rami with 25 apical setae, rows of small pseudochaetae in medial, subapical and apical parts.

Differential diagnosis. The hinge teeth on the left valve and cellular microrelief of the valves of C. sinistra are identical with the individuals of the recently redescribed Cytherissa latirecta Mazepova, 1985 ( Alekseeva et al. 2025c). The species have well-distinguished valve outlines (in lateral view) and carapace shape in dorsal and ventral views. The outlines of C. sinistra shells are generally similar to C. dextima individuals, but the first species has hinge teeth on the left valve and the second, on the right valve.

Notes. The list of type specimens ( Mazepova 2001: pp. 551, 552) includes 20 paratypes ( 11 females and 9 males). The handwritten version of G.F. Mazepova’s catalog lists many more— 37 type specimens ( 25 females and 12 males); among them 11 females and 9 males were collected at the underwater Akademicheskiy Ridge, 14 females opposite Bugul’deyka Village and 3 males between Eligey River and Gorevoy Utes Cape. The label on the tube shows 27 types specimens ( 15 females and 12 males), indicating the same three sampling sites mentioned above. Paratype designation is not valid without a holotype, thus all the specimens are referred to as syntypes. Inside of the tube we found and analyzed 28 type specimens ( 14 females and 14 males).

Geographic distribution. Endemic to Lake Baikal, found in the northern and southern basins, in the Maloye More Strait, at depths from 50 to 900 m (for details see Mazepova 1990: p. 386). Inhabits silty sand, silt and clay.