Cytherissa Sars, 1925

Alekseeva, Tatyana M., Krivorotkin, Roman S. & Timoshkin, Oleg A., 2025, Redescriptions of ten endemic Cytherissa species (Ostracoda, Podocopida, Cytherideidae) from the coastal and deep-water zones of Lake Baikal, Zootaxa 5673 (3), pp. 301-354 : 302-310

Genus Cytherissa Sars, 1925

Cytherissa glomerata Mazepova, 1990

Figs. 1–3 ; 7A–7C , 38A ; 39A ; 40A, 40G, 40Q .

Type locality. Lake Baikal , shallow water at the Selenga River estuary, north of Posol’skoye Village (approximate coordinates 52º02’39.13” N, 106º09’59.48” E; hereinafter approximate coordinates are given as the central point of the locality) or opposite Severnaya channel (approximate coordinates 52º25’28.52” N, 106º32’15.17” E) .

Type material. Lectotype No. O10 (female) and paralectotype No. 1 (male): dwm No. O1-080671. Paralectotypes Nos. 2–4 (3 females): dwm No. O2-080671. Paralectotypes Nos. 5–8 (4 males): dwm No. O3- 080671. Paralectotypes Nos. 9–16 (4 females; 4 males): swm No. 17. Paralectotype No. 17 (female): swm No. 17 (valves) and wm No. O4-080671 (limbs). Paralectotype No. 18 (female): swm No. 17 (valves) and wm No. O5- 080671 (limbs). Paralectotype No. 19 (male): swm No. 17 (valves) and wm No. O6-080671 (limbs). Paralectotype No. 20 (male): swm No. 17 (valves) and wm No. O7-080671 (limbs).

All ostracods were collected in Lake Baikal, in shallow water at the Selenga River estuary, in one of two locations: 1) opposite Severnaya channel, June 8, 1971, depth 55 m, gray silt; 2) north of Posol’skoye Village, July 12, 1974, depth 50 m, silt with sand, sample No. 695 from the collection of taxocoenoses of G.F. Mazepova. 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. 1A–1D ; 2A–2F ; 39A ) laterally rounded-ovate: L=1145–1220 µm (mean 1175 µm, n=10), largest H=860–930 µm (mean 895 µm, n=10), situated slightly posterior to anterior 1/3 of L. Dorsal margin slightly arcuate at center. Anterior end of carapace broadly rounded; posterior end more narrowly rounded. Central parts of ventral margin of both valves almost equally concave on inner side and arcuate on outer side. LV slightly overlaps RV in anterior and posterior parts of dorsal margin (in other parts valves are identical). Inner lamella broad. Marginal pore canals long, occupying 100% of width of inner lamella and ending in sensilla. Inner lamella of each valve with 24–25 marginal pore canals. Pore canals of outer lamella opening with branching group sieve pores (up to six canals in group), single near margin of valves; each canal ending in one sensillum. Outer lamella of each valve with 245–250 pore canals (here and in the following descriptions a minimal but accurately detected number of canals is given, their complete count is extremely difficult). Inner side of each valve with about 50–55 canal apertures. Hinge teeth on RV, relatively narrow and elongated, weakly crenulated, not extending beyond margin of valve; bar on LV, weakly crenulated. Microrelief of valve surface homogeneous, with numerous small rounded pits, differing in diameter by 2–3 times ( Fig. 7A–7C ). Site of greatest carapace width on ventral and dorsal views weakly pronounced and located in central part of carapace ( Fig. 2C, 2D ).

A1, A2, Md, Mxl, L5–L7 as in Cytherissa heckyi Alekseeva, Krivorotkin et Timoshkin, 2025 , but distal segments of L5 partially fused (almost as in Cytherissa pennata Mazepova, 1990 ).

Male. Carapace ( Figs. 1E–1H ; 3A–3F ; 39A ) slightly longer and lower than in female: L=1205–1250 µm (mean 1225 µm, n=10), greatest H=830–885 µm (mean 855 µm, n=10). In dorsal and ventral views, males ( Fig. 3C, 3D ) significantly narrower than females ( Fig. 2C, 2D ), site of greatest width slightly shifted toward posterior 1/ 3 L. Dorsal margin of male valves slightly straighter. Geniculate (transformed) legs on right side of body. Other morphological features of carapace structure, microrelief ( Fig. 7A ), A1, A2, Md, Mxl, left (nongeniculate) legs L5 and L6 as in female; distal segments of nongeniculate L5 partially fused ( Fig. 40A ). Both legs of L7 and right (geniculate) L5 as in male C. heckyi .

L6. Right leg ( Fig. 40G ). Protopod with four setae and several groups of pseudochaetae (at least four). Endopod two-segmented. First segment with very small seta. Distal part of second segment with small pointed outgrowth.

Hemipenis ( Fig. 38A ) large, elongate-triangular; outer appendage long, with smoothly curved main axis, almost saber-shaped. Length of hemipenis in quiescent state 620 µm (n=1), greatest width 395 µm. Diameter of copulatory process 180–200 µm.

Brush organ ( Fig. 40Q ). Both rami with 24 apical setae, row of small pseudochaetae in medial part, row of pseudochaetae in subapical part and barely visible row of pseudochaetae in apical part.

Differential diagnosis. Owing to the round shell outline in the lateral view, C. glomerata specimens are well distinguished from other Baikal species of the genus. They slightly resemble Cytherissa dextima Mazepova, 1990 , but have a quite different pattern of valve overlap (almost equal in C. glomerata , and in C. dextima the left valve is notably larger than the right one), microrelief ( C. glomerata has numerous fine pits, C. dextima —large cells) and an average carapace size ( C. glomerata : female: L=1175 µm; H=895 µm; male: L=1225 µm; H=855 µm; C. dextima : female: L=765 µm; H=495 µm; male: L=800 µm; H=490 µm).

Notes. Besides a list of type specimens published earlier ( Mazepova 2001), we have a handwritten catalog by G.F. Mazepova, on which her publication was based. Each tube with a species was supplied by a brief label showing the number of specimens and sample number from G.F. Mazepova’s collection of taxocoenoses. Unfortunately, in some cases all three versions (the published list, handwritten catalog and label) either differ from each other or have different numbers of specimens inside the tube. Therefore, we had to examine every case in detail.

For instance, the list of type specimens of C. glomerata ( Mazepova 2001: pp. 543) indicates that the collection contains 22 paratypes (11 females and 11 males). The list written by hand of Prof. G.F. Mazepova contains 21 type specimens (10 females and 11 males). The label on the tube also has 21 specimens (10 females and 11 males) that we studied. When describing most of endemic Baikalian ostracods, G.F. Mazepova did not indicate holotypes, and the paratypes ( Mazepova 2001) are not valid without a holotype indication. According to article 73.2 of the International Code of Zoological Nomenclature (1999)—hereinafter, ICZN, they should be regarded syntypes that was implemented in this work .

Geographic distribution. Endemic to Lake Baikal, found in the northern and southern basins at depths of 21–1300 m (for details see Mazepova 1990: p. 323). Inhabits silty sand and silt.

Cytherissa dubitabilis ( Bronstein, 1947) Mazepova 1990

Figs. 4–6 ; 7D–7F , 38B ; 39B ; 40C, 40K .

Type locality. Lake Baikal , shallow water at the Selenga River estuary, more precise location unknown (approximate coordinates 52º22’26.11” N, 106º22’9.74” E) .

Type material. Neotype No. O1 (female): dwm No. O37-300771.

Additional material. Specimen No. 1 (male): dwm No. O37-300771. Specimens Nos. 2–4 (3 females): dwm No. O38-300771. Specimen No. 5 (female): dwm No. O38-300771 (valves) and wm No. O40-300771 (limbs). Specimens Nos. 6–10 (5 males): dwm No. O39-300771. Specimen No. 11 (male): dwm No. O39-300771 (valves) and wm No. O41-300771 (limbs). Specimens Nos. 12–19 (4 females; 4 males): swm No. 12. Specimen No. 20 (female): swm No. 12 (valves) and wm No. O42-300771 (limbs). Specimen No. 21 (male): swm No. 12 (valves) and wm No. O43-300771 (limbs).

All ostracods were collected in shallow water at the Selenga River estuary, July 30, 1971, depth 8 m, silty sand with macrophytes, sample No. 320 from the collection of taxocoenoses of G.F. Mazepova.

Description. Female. Carapace ( Figs. 4A–4D ; 5A–5F ; 39B ) laterally elongate-ovate: L=1010–1060 µm (mean 1035 µm, n=10), largest H=600–645 µm (mean 625 µm, n=10), located on posterior border of anterior 1/ 3 L. Dorsal margins of valves slightly arched 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 arcuately curved on outer side. LV overlaps RV on dorsal margin. Inner lamella relatively narrow. Marginal pore canals may be long, occupy 100% of width of inner lamella, or open earlier. Inner lamella of each valve with 36–39 marginal pore canals (22 long and 15–17 short). Pore canals of outer lamella sieve-type, branching, grouped (up to six canals in group), near margin of valves single; each canal with one sensillum ( Fig. 7E ). Outer lamella of each valve with 250–290 pore canals. Inner side of each valve with 60–70 canal apertures. Hinge teeth on RV, weakly crenulated, not extending beyond margin of valve; bar on LV, weakly crenulated. Microrelief of valve surface heterogeneous, with different diameter round and oval shallow pits ( Fig. 7D, 7F ) covering whole carapace except smooth dorsal margin. Site of greatest carapace width on ventral and dorsal views is at posterior border 3/4 L ( Fig. 5C, 5D ).

A1, A2, Md, Mxl, L5–L7 as in female C. heckyi , but distal segments of L5 partially fused.

Male. Carapace ( Figs. 4E–4H ; 6A–6F ; 39B ) slightly longer and lower than in female: L=1045–1150 µm (mean 1120 µm, n=10), H=580–640 µm (mean 610 µm, n=10). In dorsal and ventral views, males ( Fig. 6C, 6D ) distinctly narrower than females ( Fig. 5C, 5D ), site of greatest width weaklier pronounced and located slightly posterior to 3/ 4 L. Posterior margin of carapace rounded. Geniculate (transformed) legs on right side of body. Other morphological features of carapace structure, microrelief, A1, A2, Md, Mxl, left (non-geniculate) legs L5 ( Fig. 40C ) and L6 as in female. Both legs L7 and right (geniculate) leg L5 as in male C. heckyi .

L6. Right leg. Protopod with four setae and several groups of pseudochaetae. Endopod two-segmented ( Fig. 40K ); first segment with small seta; distal part of second segment with pointed outgrowth.

Hemipenis ( Fig. 38B ) large, triangular; outer appendage in form of broad robust triangle with slightly curved tip. Length of hemipenis in quiescent state 495 µm (n=1), greatest width 375 µm. Diameter of copulatory process 235 µm.

Brush organ. Both rami with 13 apical setae, row of small pseudochaetae in medial part, row of pseudochaetae in subapical part and barely visible row of pseudochaetae in apical part.

Differential diagnosis. The shells of C. dubitabilis specimens on the lateral and dorsal views slightly resemble Cytherissa elongata elongata Bronstein, 1947 . The species are distinguished by the shell outlines in lateral view (notably more assymentrical in C. dubitabilis ), location of wedge-shaped constriction in dorsal view in females ( C. dubitabilis in posterior part, C. elongata elongata in anterior part of the body), size ( C. dubitabilis , female: L=1135 µm; H=625 µm; male: L=1120 µm; H=610 µm; C. elongata elongata , female: L=850 µm; H=450 µm; male: L=955 µm; H=460 µm).

Notes. The species was initially described by Z.S. Bronstein as Cytherissa lacustris var. dubitabilis Bronstein, 1947 . In 1990, G.F. Mazepova pointed out that the specimens of this taxon were remarkably distinguished from Cytherissa lacustris ( Sars, 1863) by the carapace morphology and defined it as a separate species— Cytherissa dubitabilis Bronstein, 1947 . In the work of Meisch et al. (2024), we find the species name: Cytherissa dubitabilis ( Bronstein, 1947) Martens & Savatenalinton 2011 .

Z.S Bronstein (1930; 1947) did not separate type series of Baikal ostracods. G.F. Mazepova (1990) indicated syntypes of C. dubitabilis species ( Mazepova 2001: pp. 541). The sample these “ syntypes ” were taken from, was collected only in 1971, i.e. in 24 years after the publication of the original description (Z.S. Bronstein collected the samples for the description from about 1927–1928). Thus, the specimens from the collection cannot be syntypes according to ICZN (1999). Therefore, we isolate a neotype based on G.F. Mazepova’s specimens.

Geographic distribution. Endemic to Lake Baikal, found in all basins and in the Maloye More Strait at depths from 2 to 95 m (for details see Mazepova 1990: p. 309). Inhabits sands, rarely algae thickets and stones.

Figures

FIGURE 1. Cytherissa glomerata Mazepova, 1990, schematic drawings of the valves of females (A–D) and males (E–H), outer (A, B, E, F) and inner (C, D, G, H) sides, laterally. Arrows point towards the anterior. A–D—paralectotype No. 17; E–H—paralectotype No. 19. Scale 500 µm.

FIGURE 2. Cytherissa glomerata Mazepova, 1990, SEM photographs of carapaces (A–D) and valves (E, F) of females: A, B—laterally, outer side; C—ventrally; D—dorsally; E—LV laterally, inner side; F—RV laterally, inner side. Arrows point towards the anterior.A—paralectotype No. 9; B—paralectotype No. 10; C—paralectotype No. 12; D—paralectotype No. 11; E, F—paralectotype No. 17. Scale 500 µm.

FIGURE 3. Cytherissa glomerata Mazepova, 1990, SEM photographs of carapaces (A–D) and valves (E, F) of males: A, B—laterally, outer side; C—ventrally; D—dorsally; E—LV laterally, inner side; F—RV laterally, inner side. Arrows point towards the anterior. A—paralectotype No. 13; B—paralectotype No. 14; C—paralectotype No. 16; D—paralectotype No. 15; E, F—paralectotype No. 19. Scale 500 µm.

FIGURE 4. Cytherissa dubitabilis (Bronstein, 1947) Mazepova 1990, schematic drawings of the valves of females (A–D) and males (E–H), outer (A, B, E, F) and inner (C, D, G, H) sides, laterally.A–D—specimen No. 20; E–H—specimen No. 21.Arrows point towards the anterior. Scale 500 µm.

FIGURE 5. Cytherissa dubitabilis (Bronstein, 1947) Mazepova 1990, SEM photographs of carapaces (A–D) and valves (E, F) of females: A, B—laterally, outer side; C—ventrally; D—dorsally; E—LV laterally, inner side; F—RV laterally, inner side. Arrows point towards the anterior. A—specimen No. 13; B—specimen No. 12; C—specimen No. 15; D—specimen No. 14; E, F—specimen No. 20. Scale 500 µm.

FIGURE 6. Cytherissa dubitabilis (Bronstein, 1947) Mazepova 1990, SEM photographs of carapaces (A–D) and valves (E, F) of males: A, B—laterally, outer side; C—ventrally; D—dorsally; E—LV laterally, inner side; F—RV laterally, inner side. Arrows point towards the anterior. A—specimen No. 17; B—specimen No. 16; C—specimen No. 19; D—specimen No. 18; E, F—specimen No. 21. Scale 500 µm.

FIGURE 7. Cytherissa glomerata Mazepova, 1990 (A–C) and Cytherissa dubitabilis (Bronstein, 1947) Mazepova 1990 (D–F), SEM photographs of some elements of carapace structure of females (B, C, F) and males (A, D, E): A—central part of RV, laterally; B—central part of LV, laterally; C—microrelief, pore canals and broken sensilla; D—central part of carapace, ventrally; E—microrelief, pore canals and sensilla; F—posterior part of RV, laterally.Arrows point towards the anterior.A—paralectotype No. 13; B, C—paralectotype No. 10; D—specimen No. 19; E—specimen No. 17; F—specimen No. 13. Scale, µm: A, B, D, F—100; C, E—20.

FIGURE 38. Microphotographs of male hemipenes of the studied species: A—Cytherissa glomerata Mazepova, 1990, paralectotype No. 19; B—Cytherissa dubitabilis (Bronstein, 1947) Mazepova 1990, specimen No. 11; C—Cytherissa derupta Mazepova, 1984, paralectotype No. 11; D—Cytherissa plena Mazepova, 1985, paralectotype No. 29; E—Cytherissa cytheriformis Bronstein, 1947, specimen No. 20; F—Cytherissa golyschkinae Mazepova, 1990, paralectotype No. 26; G— Cytherissa obrutshevi Mazepova, 1990, paralectotype No. 16; H—Cytherissa calva Mazepova, 1990, paralectotype No. 20; I—Cytherissa bisetosa Mazepova, 1984, paralectotype No. 17; J—Cytherissa sinistra Mazepova, 1984, paralectotype No. 26. Scale 100 µm.

FIGURE 39. Microphotographs of valves of the studied species: A—Cytherissa glomerata Mazepova, 1990; B—Cytherissa dubitabilis (Bronstein, 1947) Mazepova, 1990; C—Cytherissa derupta Mazepova, 1984; D—Cytherissa cytheriformis Bronstein, 1947; E—Cytherissa golyschkinae Mazepova, 1990; F—Cytherissa obrutshevi Mazepova, 1990; G—Cytherissa calva Mazepova, 1990; H—Cytherissa plena Mazepova, 1985; I—Cytherissa bisetosa Mazepova, 1984; J—Cytherissa sinistra Mazepova, 1984. Sex designations for A are as for B–J. Arrows point towards the anterior. Specimens figured are the same as those used to prepare the schematic darwings of the valves. Scale 500 µm.

FIGURE 40. Drawings of non-geniculate L5 (A), L5 endopods (B–F), geniculate L6 (G), endopods of geniculate L6 (H–P) and brush organs(Q–S) ofmales:A,G,Q—Cytherissaglomerata Mazepova,1990,paralectotype No.19;B,H—Cytherissaobrutshevi Mazepova, 1990, paralectotype No. 16; C, K—Cytherissa dubitabilis (Bronstein, 1947) Mazepova 1990, specimen No. 21; D, O—Cytherissa sinistra Mazepova, 1984, paralectotype No. 27; E, N—Cytherissa plena Mazepova, 1985, paralectotype No. 29; F, J—Cytherissa derupta Mazepova, 1984, paralectotypes Nos. 11 (F) and 22 (J); I, S—Cytherissa cytheriformis Bronstein, 1947, specimen No. 20; L—Cytherissa calva Mazepova, 1990, paralectotype No. 20; M, R—Cytherissa bisetosa Mazepova, 1984, paralectotype No. 17; P—Cytherissa golyschkinae Mazepova, 1990, paralectotype No. 26. On A–F, the arrows show clear separation (D, F) and partial fusion (A–C, E) of distal segments of non-geniculate L5. The arrangement of pseudochaetae on A is characteristic of B–F but is not drawn. Scale 100 µm.