Candona concava Alekseeva & Krivorotkin sp. nov.

Figs. 49–52; 53A; 54; 55; 56C; 74A; 75C, 75G

Type locality. Lake Baikal, coastal zone opposite the Zhilishche Valley, opposite the LIN SB RAS station (approximate coordinates: 51°53′55 N, 105°03′49 E).

Type material. Holotype No. O17 (female) and allotype No. O17.1 (male): dwm No. O1-170869. Paratypes Nos. 1–3 (2 females, male): dwm No. O2-170869. Paratypes Nos. 4–12 (5 females, 4 males): swm No. 43. Paratype No. 13 (female): swm No. 43 (valves) and wm No. O3-170869 (limbs). Paratype No. 14 (female): swm No. 43 (valves) and wm No. O4-170869 (limbs). Paratype No. 15 (female): swm No. 43 (right valve) and wm No. O5- 170869 (limbs). Paratype No. 16 (male): swm No. 43 (valves) and wm No. O6–170869 (limbs). Paratype No. 17 (male): swm No. 43 (valves) and wm No. O7-170869 (limbs). Paratype No. 18 (male): swm No. 43 (left valve) and wm No. O8-170869 (limbs).

All specimens were collected in Lake Baikal, southern basin, in the coastal zone opposite Zhilishche Valley, opposite the LIN SB RAS station, August 17, 1969, depth 114 m, conglomerates, sample No. 204 from the collection of G.F. Mazepova.

Etymology. The name of the species comes from the word “concavus” (Latin) – concave, curved and refers to the morphology of the ventral margin of the valves.

Description. Female. Carapace (Figs. 49A–49D; 50A–50F; 56C; 74A) laterally crescent-shaped; L = 1170– 1195 µm (average 1185 µm, n = 6), greatest H = 495–520 µm (average 505 µm, n = 6) located slightly anteriorly to center of L. Dorsal margin smoothly arcuate.Anterior margin broadly rounded near ventral margin, posterior margin pointed near ventral margin. Ventral margin distinctly concave on 2/3 of L. LV barely overlaps RV throughout. Inner lamella relatively broad, especially in anterior and posterior parts of valves. Marginal pore canals in anterior and posterior parts of valves occupy about 15–70% of width of inner lamella (Fig. 52D, 52E). Inner lamella of each valve of same individual with at least 100–105 marginal pore canals. Sensillae of outer lamella relatively sparse, but numerous in anterior and posterior parts of ventral margin (Fig. 52A, 52C), emerging from unbordered pores (Fig. 52B). Valve surface smooth and homogenous. Site of greatest width (both ventrally and dorsally) weakly defined and located in anterior 1/3 of L.

A1 (Fig. 54A) eight-segmented. First segment with one seta. Second segment with three setae. Third and fourth segments with one seta each. Fifth segment with one long seta and one short seta. Sixth segment with one long seta. Seventh segment with five setae (two long, two medium and one short). Eighth segment with three setae (two long, one half as long) and stick-shaped aesthetasc (ya).

A2 (Fig. 54B, 54C) five-segmented. Coxal segment with three smooth setae, one of them on this segment proper, two located between coxal and basal segments. Basal segment with one ventro-distal seta. Exopod with three smooth setae. Endopod three-segmented. First endopodal segment with posteromedial club-shaped aesthetasc (Y), two posterodistal setae of different lengths, and row of pseudochaetae. Second endopodal segment with one ventromedial seta, dorsal-medial aesthetasc (y1), two t-setae (t1, t2); three z-setae (z1–z3) of almost equal length; three G-claws (G2 small and six times shorter than G1 and G3) and distal aesthetasc (y2). Terminal segment with two G-claws (GM slightly longer than Gm; Gm a thin seta) and two setae, one of them basally fused with stick-shaped aesthetasc (y3).

Md (Fig. 53A). Coxa with one subapical seta and one seta on apical inner angle. Apparently, no setae between teeth of endite. Branchial plate with seven plumose setae and one smooth seta. Palp four-segmented. First segment with four setae: S1 (long, plumose), S2 (relatively long, pappose), α (thin and relatively long) and proximal seta. Second segment with six posterior setae (β-seta relatively long) and two anterior setae. Third segment with four posterodistal setae (γ-seta small, smooth) and three anterior setae. Distal segment with two claws of equal length and two setae.

Mxl. Protopod without setae. First endite with 14 setae, two at base of endite, 12 at apex. Second endite with nine setae. Third endite with eight setae and two claws. Palp two-segmented; first segment with four setae. Distal segment with two claws and seta apically and three setae medially. Branchial plate with 23 setae and row of long pseudochaetae.

L5 (Fig. 55A). Protopod with three setae (a, b, d). Endite with 14 setae, arranged in two groups of four and ten setae, respectively. Palp with three setae. Branchial plate represented by two setae of different lengths.

L6 (Fig. 55D) five-segmented. First through third segments with distal seta (d1, e, f) each; first segment with rows of pseudochaetae. Fourth segment with two small distal g-setae. Fifth segment with long claw (h2) and two small setae (h1 and h3) of different lengths.

L7 (Fig. 55E) five-segmented. First segment with anterior d1-seta, posterior dp-seta and rows of small pseudochaetae; d2 seta absent. Second and third segments without pseudochaetae. Fourth segment with posterodistal g-seta. Fifth segment with three serrate setae (h1–h3).

UR (Fig. 55F) symmetrical, with slightly curved main axis. Each ramus with one posterior seta distal to middle of ramus, one short anterodistal seta and two slender distal claws of nearly equal length.

Male. Carapace (Figs. 49E–49H; 51A–51F; 74A) laterally crescent-shaped, shorter and lower than in female; L = 1140–1170 µm (average 1150 µm, n = 6), greatest H = 480–500 µm (average 490 µm, n = 6). Center of ventral concavity with slight convexity on both valves (Fig. 51B, 51C, 51E) compared to that of female (Fig. 50B, 50C, 50E). Other morphological features of carapace, A1, A2 (protopod, exopod and first endopodal segment), Md, Mxl, L5 protopod, L6, L7, as in female.

A2 (Fig.54D–54G) six-segmented(four-segmented endopod).Second endopodal segment with one ventromedial seta, dorsal-medial aesthetasc (y1), three t-setae (t2 and t3 modified into sensory setae with oval ends). Third endopodal segment with three z-setae (z1–z3) of equal length, three G-claws (G2 2.5 times longer than G1 and G3) and distal aesthetasc (y2). Terminal segment with long Gm-claw, shorter GM-claw in form of seta, and two setae, one of them basally fused with stick-shaped aesthetasc (y3).

Prehensile palps of L5 (Fig. 55B, 55C) asymmetrical. Both palps wide and thick proximally to site on insertion of two setae, sharply curved distally: left palp crescent-curved towards body, and right palp curved in opposite direction. Distal tip of both palps with short and thick seta.

Hemipenis (Fig. 75C) large, rectangular, 305 µm long and 120 µm wide; lobes oval, weakly protruding.

Zenker organ (Fig. 75G) 310 µm long, with 5+2 rings of spines 90 µm in diameter, diameter of central tube 30 µm, vesicle large, 85 µm in diameter.

UR (Fig. 55G) as in female, but main axis straighter, posterior claw slightly shorter.

Comparisons. The carapace of the individuals of the new species is structurally similar to Candona memoranda Mazepova, 1990 (Fig. 56A); both species have similar valve outlines and shapes of the ventral margin. C. memoranda shells are remarkably lower (mean H of females 415 µm, males – 400 µm) than the shell height of the new species (mean H of females 505 µm, males – 490 µm), and the shell L/H ratio of C. memoranda (females – 3.05, males – 2.96) is higher than in C. concava sp. nov. (females and males – 2.35). The valves of the species compared differ in the structure of the inner lamella, particularly the posterior part. The male individuals show differences in the structure of the L5 prehensile palps and hemipenes.

Notes. Valve outlines of the new species are very close to those of Candona godlewskii Mazepova, 1984 sensu Karanovic & Sitnikova (2017) (Fig. 56D). It seems likely that they belong to related species. In the authors’ view, C. godlewskii sensu Karanovic & Sitnikova (2017) (Fig. 56D) and C. godlewskii sensu Mazepova (1984) (Fig. 56B) are different species. For reliable comparisons, we present SEM images of the shell of C. godlewskii sensu Mazepova (1984) syntype (Fig. 56B). It is evident that these individuals have different valve shapes (Fig. 56B, 56D).According to the valve images and description, the microrelief of C. godlewskii sensu Karanovic & Sitnikova (2017) is pitted in the center of the valves (Karanovic & Sitnikova 2017: p. 29: Fig. 4C). After examining SEM images of the lateral view of the valves (Karanovic & Sitnikova 2017: p. 29: Fig. 4A, 4B) it appears that their microrelief is not pitted. Judging by the microphotographs of an enlarged valve (Karanovic & Sitnikova 2017: p. 29: Fig. 4C), it may be suggested that these microphotos show Candona rupestris dissona Mazepova, 1990 (= Baicalocandona rupestris disona (Mazepova, 1990) Karanovic & Sitnikova 2017) from the same work (Karanovic & Sitnikova 2017: p. 39: Fig. 12E). However, it is still possible that individuals of C. godlewskii sensu Karanovic & Sitnikova (2017) studied actually had a microrelief, since it is mentioned in the valve description. In such a case, the microrelief is a significant distinctive feature of C. godlewskii sensu Karanovic & Sitnikova (2017), and different from the new species.

Geographic distribution. Endemic to Lake Baikal, found in the coastal zone opposite the Zhilishche Valley, opposite the LIN SB RAS station (depth 114 m). Lives on conglomerates.