Lathonura rectirostris (O.F. Müller, 1785).

Parthenogenetic female. Body length 0.5–1.3 mm. Body elongate, oval, slightly flattened laterally, valves with a low dorsal keel. Head lacking a rostrum, depression between head and valves weakly expressed or absent. Anterior headshield margin straight to slightly convex. A single dorsal head pore, resembling a large oval or rounded ‘fenestra’. Ventral valve margin weakly convex, bears lanceolate setae bilaterally armed by short spinulae. Gut with no loops. Abdomen lacking processes. Postabdomen small, bilobed in cross-section, with a large dorsal process bearing postabdominal setae; anal opening terminal. Preanal and anal portions are covered by transverse rows of spinulae. Postabdominal claws large, incurved, lacking basal spine; both outer and inner surfaces bear a longitudinal row of short spinulae. Antenna I long, cylindrical, ornamented by wide triangular scale-like spinulae; anterior surface bearing two scale-like serrate setae in its distal part; basal antennular seta located at a low prominence. Nine aesthetascs with acute, slightly incurved tips and subapical pores; one aesthetasc much longer than others. Antenna II long; basipodite with very short distal burrowing spine. Antenna II seta formula 0–1–1–3/1–1–3, spine formula 0–1–0–1/0–0–0. Five thoracic appendages. IDL of the limb I bearing four setae, each seta with unilateral armature of flattened setulae. Anterior setae of the limb I inner portion long, not forked. Ejector hooks small, one of them reduced to a tubercle. Limb II exopodite with one seta; inner portion with eight scraping setae in anterior row and three setae in posterior row. Limb III exopodite with four setae; distal endite with three long posterior setae and one long anterior seta. Limb IV exopodite with one seta; inner portion with three setae. Limb V with a total of four setae.

Ephippial female. Body length 1.1–1.2 mm. General body shape similar to that of parthenogenetic female, compressed laterally, dorsal keel low. Dorsal valve margin angulate in the anterior half, widely rounded posteriorly. Ephippium transparent to weakly melanized, ornamented by polygons or wrinkles. Two to eight eggs in the ephippium.

Male. Body length 0.5–0.8 mm. General body shape similar to that of young parthenogenetic female, body compressed laterally. Postabdomen shape similar to that of the female; gonopores slit-like, located subdistally on lateral surface of the postabdomen. Antenna I bearing an additional basal seta at the anteromedial side and six or seven transverse rows of more or less flattened setae along its anterior side. Antenna II basipodite with two additional distal setae at the outer side. Thoracic limb I with a prominent subdistal lobe and a hook at IDL base; anterior surface of the SDL bearing transverse rows of spinulae; posterior surface of the hook tip with three transverse cuticular ridges. IDL of the limb I and endite 3 both with one additional anterior seta.

Müller O.F. (1785): 90, pl. XII, figs 1–3 (Daphnia); Koch (1841): fig. 35.24 (Pasithea); Zaddach (1844): 23–24 (Daphnia brachyura); Liévin (1848): 42, pl. XI, figs 1–3 (Pasithea); Fischer (1848): 174–177, pl. VI (Daphnia mystacina); Lilljeborg (1853): 57–61, pl. IV, figs 8–11, pl. V, fig. 2; Schödler (1858): 27, pl. 1, fig. 10 (spinosa); Leydig (1860): 200–205, pl. VII, fig. 57 (Pasithea rectirostris, P. lacustris); Müller Р.Е. (1867): 139–140; Norman and Brady (1867): 14–16, pl. XXIII, figs 8–12; Lund (1870): 155, pl. IX, figs 1–4; Gruber and Weismann (1877): 103–113, pl. IV, figs 11, 12, 14, 15, 15a (Pasithea); Hellich (1877): 63; Birge (1879): 89–90, Herrick (1884): 71–72, pl. D; Matile (1890): 134; Herrick and Turner (1895): 216, pl. LVII; Lilljeborg (1900): 354–360, pl. LV, figs 15–18, pl. LVI, figs 1–14; Keilhack (1909): 64, fig. 147; Birge (1918): 716, fig. 1117; Berg (1929–1930): 65–66; Behning (1941): 207–209, fig. 90; Scourfield and Harding (1958): 29, figs 62, 63; Brooks (1959): 630, fig. 27.64; Herbst (1962): 72, fig. 45a–d; Sramek-Hušek et al. (1962): 292, fig. 106; Manuilova (1964): 201–202, fig. 93; Sergeev (1971): 1–7, figs 1–6; Flössner (1972): 242–245, figs 114, 115; Fryer (1972): 80–82, figs 1–3; Fryer (1974): 211–227, figs 91–109; Smirnov (1976): 158–162, figs 143–148 (rectirostris, lacustris); Negrea (1983): 196–198, fig. 78; Margaritora (1985): 183–185, fig. 75; Dodson and Frey (1991): fig. 20.50; Dumont and Silva-Briano (1998): figs 3, 5; Silva-Briano (1998): 156–168, figs 11–19, 68–102; Flössner (2000): 79–80; Kotov (2006): fig. 6H; Hudec (2010): 258–261, fig. 62; Kotov et al. (2010): 222, pl. 128, figs 7–8, pl. 130, figs 8–10; Kotov (2013): figs 27З, 65З, 81А–Б, 100В–Г, 151E, 163A; Błedzki and Rybak (2016): 248–249; Rogers et al. (2019): figs 16.2.23L–M, Korovchinsky et al. (2021): 235–237, pl. 68, figs 10–12.

Probably lost, as are all the materials of Otto Frederik Müller (Frey 1980).

Surroundings of Copenhagen, Denmark (Müller O.F. 1785).

Norway • 4 ephippial ♀, 3 ♂; Innlandet, Lake Randsfjorden; 60°47.70'N, 10°8.02'E; 17 Oct. 2015; A.Y. Sinev leg.; IEE AAK 2024–132 • 10 parthenogenetic ♀, 4 ephippial ♀, 4 ♂; same data as for preceding; 60°31.77'N, 10°17.67'E; 7 Oct. 2015; A.Y. Sinev leg.; IEE AAK M–3383. Russia • 2 parthenogenetic ♀; Murmansk Area, Teriberka; 69°11.80'N, 35°7.31'E; 4 Aug. 2020; small roadside lake with rocky sediment; P.G. Garibian leg.; IEE AAK M–5921 • 1 parthenogenetic ♀; Tver Area, Rivitsky village; 57°36.55'N, 35°54.84'E; 4 Sep. 2011; S.V. Pavlova leg.; IEE AAK M–2182 5921 • 5 parthenogenetic ♀; Moscow Area, Lake Glubokoe; 55°45.11'N, 36°30.65'E; 16 Aug. 2016; vegetated zone with water lily leaves; A.A. Kotov leg. IEE AAK M–3489 • 2 parthenogenetic ♀; Moscow Area, Lomonosov MSU biological station; 55°41.93'N, 36°43.83'E; 7 Oct. 2023; vegetated pond; E.K. Degtyareva leg.; IEE AAK M–8490 • 1 parthenogenetic ♀; Ryazan Area, Murmino; 54°34.95'N, 40°3.19'E; 15 July 2007; vegetated lake with water pineapple; A.A. Kotov leg.; IEE AAK M–0430 • 1 parthenogenetic ♀; Kostroma Area; 58°10.15'N, 44°30.57'E; 5 Sep. 2020; small vegetated pond in Unzha River valley; A.A. Neplyukhina leg.; IEE AAK M–6797 • 1 parthenogenetic ♀; Kostroma Area; 58°11.3'N, 44°33.24'E; 5 Sep. 2020; small vegetated pond in Unzha River valley; A.A. Neplyukhina leg.; IEE AAK M–6787 • 2 parthenogenetic ♀; Nizhny Novgorod Area; 56°12.69'N, 43°45.13'E; 19 Sep. 2018; Gnilichka River, vegetated shoreline with coontail; D.E. Gavrilko leg.; IEE AAK M–4471 • 5 parthenogenetic ♀; Nizhny Novgorod Area; 56°12.32'N, 43°45.41'E; 19 Sep. 2018; vegetated pond with water pineapple at Gnilichka River; D.E. Gavrilko leg.; IEE AAK M–4479 • 2 parthenogenetic ♀; Penza Area; 53°14.04'N, 45°5.28'E; 5 May 2010; vegetated oxbow lake; E.I. Bekker leg.; IEE AAK M–1808; • 2 parthenogenetic ♀; Penza Area; 53°10.36'N, 45°4.46'E; 5 May 2009; a small lake with water pineapple in the vicinity of Penza; E.I. Bekker leg.; IEE AAK M–0953 • 3 parthenogenetic ♀; Chelyabinsk Area; 55°0.68'N, 59°54.48'E; 8 Aug. 2006; a big creek on River Atlian; A.A. Kotov leg.; IEE AAK M–0325 • 2 parthenogenetic ♀; Chelyabinsk Area; 54°59.46'N, 59°50.38'E; 8 Aug. 2006; Lake Pestchanoe, Tobol River valley; A.A. Kotov leg.; IEE AAK M–0324 • 2 parthenogenetic ♀; Tomsk Area; 56°43.33'N, 84°41.52'E; 16 July 2005; a lake on right bank of Tom’ River; A.A. Kotov leg.; IEE AAK–0129 • 2 parthenogenetic ♀; Chelyabinsk Area; 56°32.63'N, 84°9.48'E; 20 July 2005; an oxbow lake near Melnikovo; A.A. Kotov leg.; IEE AAK M–0135. Mongolia • 1 parthenogenetic ♀; Uvs Aimag, Tes, Lake Uhegiin Gol; 50°29.67'N, 93°36.02'E; 30 Aug. 2006; C. Jersabek leg.; IEE AAK 2008–082. U.S.A. • 1 parthenogenetic ♀; Rhode Island, Bowdish Reservoir; 41°55.45'N, 72°13.04'W; 14 June 2004; D.J. Taylor, A.A. Kotov leg.; IEE AAK 2005–236 • 2 parthenogenetic ♀; New Hampshire, Lake Sunapee; 43°22.54'N, 73°55.80'W; 24 June 2004; W. Piel, A.A. Kotov leg.; IEE AAK 2006–165. See Suppl. material 6 for more information on localities.

Parthenogenetic female. Body length 0.5–1.3 mm. Body oval in lateral view (BL/BH = 1.5–1.6), slightly to moderately flattened laterally (BL/BW = 2.2–2.6) (Fig. 3A–C, Suppl. material 2: fig. S2A), with a well-expressed dorsal keel. Depression between head and valves is absent. Dorsal valve margin is evenly convex, maximum body height and width in the middle of the body (Fig. 3A, C, Suppl. material 2: fig. S2A). Posterodorsal valve angle weakly expressed. Posteroventral valve margin is widely rounded, bearing 90–135 uniform narrow denticles; the posteriormost 15–20 denticles might be somewhat larger than the others (Fig. 3D–G, Suppl. materials 1, 3: figs S1A–F, S3C–E). Ventral margin of intact specimen slightly convex in lateral view, slit between valves visible from ventral side expanded in its medial section (Fig. 3A, B). Ventral valve margin bears a row of 45–50 lanceolate marginal setae, slightly increasing in length posteriorly (Fig. 3D–G, Suppl. material 2: fig. S2D); the setae flattened in anteroposterior direction. Row of marginal setae reaching posterior quarter of the valve ventral margin (Fig. 3A, D–G); both inner and outer seta edges finely serrate (Fig. 3E, Suppl. material 2: fig. S2D). Anterior valve margin with a thickened cuticular flange (Fig. 3A, Suppl. material 2: fig. S2A). Carapace is almost smooth or finely reticulated (Suppl. material 2: fig. S2A), transparent to light brown, rarely dorsal body surface melanized.

Head large (HL/BL = 0.35–0.45), slightly narrowing anteriorly in dorsal view, lacking a cervical groove posteriorly. Headshield margins strongly extending laterally (HW/HL = 1.0–1.1), forming arch-shaped fornices; each fornix is concave in its medial portion in dorsal view, S-shaped in lateral view (Figs 3A, B, 4A). Dorsal head margin evenly convex in lateral view; anterior head margin concave in lateral view; anterior headshield margin straight to slightly convex in anterior view (Figs 3A, B, 4A, B). Rostrum reduced. Eye relatively small (ED/HL = 0.13–0.15), located close to dorsal head margin; ocellus small, shifted towards the anterior headshield margin (Fig. 4A, B). Ventral head margin posteriorly to rostrum evenly convex in dorsal view, roughly perpendicular to dorsal margin in lateral view (Figs 3A, 4A, Suppl. material 2: fig. S2A). Ventral head margin straight to slightly convex, bearing laterally a pair of conical prelabral outgrowths with rounded tips (Figs 3A, B, 4A, B).

Dorsal organ (= dorsal “head pore”) located at distance of 1–3× its diameter from the posterior headshield margin; approximately as large as the eye, almost rounded to oval and slightly widening posteriorly in dorsal view, smooth, sometimes weakly extending in lateral view (Figs 3A, C, 4A–C, Suppl. material 2: fig. S2B, C). Lateral head pores absent. A small horseshoe-shaped frontal pore located subterminally, close to anterior headshield margin (Suppl. material 2: fig. S2D).

Thorax long, bearing five pairs of appendages. A large, incurved midgut located in the dorsal head portion. Gut almost straight, without convolutions and caeca (Fig. 3A, C).

Abdomen strongly reduced, lacking processes, thoracic limb V closely spaced to postabdomen base (Fig. 3A).

Postabdomen small (PL/BL = 0.23–0.27), oval in lateral view, bilobed in cross-section (Fig. 3H), widening distally, with expanded postanal portion (Fig. 3I). Dorsal postabdomen margin forming a large conical process bearing postabdominal setae; apex of the process anteriorly with a semicircular row of five or six conical outgrowths surrounding postabdominal setal bases (Fig. 3H). Preanal margin 0.70–0.72× as long as the whole postabdomen, almost straight to slightly convex in lateral view, bearing groups of short spinulae. A shallow depression might present between preanal and anal margins. Anal opening terminal, shielded by expansion of postabdomen ventrally (Fig. 3I). Anal margin short, 0.25–0.26× as long as the whole postabdomen, slightly convex, bearing groups of setulae. Postabdominal claw long and thick (PCL/PL = 0.35–0.40), incurved, directed dorsally, lacking basal spine (Fig. 3H, K); base of a claw shielded by distalmost expanded postabdomen portion in lateral view (Fig. 3H). Bases of postabdominal claws closely spaced (Fig. 3I). A longitudinal row of five to nine short, triangular spinulae at outer and inner surfaces of the claw in its medial portion (Fig. 3H, I, K). Postabdominal seta 0.6–0.7× as long as the body, bi-segmented (Fig. 3A); basal segment lacking armature, distal segment with inner armature of circular cuticular thickenings, bearing four longitudinal rows of long, sparse, flattened setulae (setula length 40–60 µm, gap between the neighboring setulae in a row 25–30 µm) (Fig. 3J).

Antenna I attached closely to the anterior headshield margin, long and narrow (AL/ED = 2.9–3.0, AL/AW = 7.4–8.4), almost straight, cylindrical in cross-section, slightly narrowing distally (Fig. 4A, B, D, E, Suppl. material 2: fig. S2E, F). Basal seta of antenna I long (ASL/AL = 0.25–0.30), located at a low protuberance on the posterior side of appendage (Fig. 4D, E). Two scale-like setae located at anterior side of the antenna I in its distal half, approximately equidistant from the appendage base (Fig. 4D, E, Suppl. material 2: fig. S2F). Scale-like setae at ventral margin triangular, flattened, with serrated margins (Fig. 4H, Suppl. material 2: fig. S2G). Tip of antenna I armed by a semicircular row of spinulae similar in shape to the scale-like setae but much smaller; a row of five to eight thin spinulae located anteriorly at the tip of the antenna I (Fig. 4F, G, Suppl. material 2: fig. S2E, F). Nine terminal aesthetascs, posteriormost one 1.3–1.4× longer and thicker than the others (Fig. 4F, G); aesthetasc pores located subterminally, tip of each aesthetasc acute, hook-shaped (Suppl. material 2: fig. S2H). The whole antenna I surface is covered by transverse rows of 3–7 wide and short flattened triangular spinulae (Fig. 4D, E, Suppl. material 2: fig. S2E, F).

Antenna II large (ANL/BL = 0.40–0.50). Coxal portion folded, slightly flattened in anteroposterior direction, bearing two sensory setae subdistally. Basal sensory setae located at low conical outgrowth, one of the setae 0.65–0.85× as long as the other (Fig. 4I, Suppl. material 2: fig. S2J). Basipodite 0.3× as long as the antenna II, subconical; outer surface of basipodite covered by transverse rows of short flattened triangular spinulae (Fig. 4I, Suppl. material 2: fig. S2J). Outer side of basipodite bearing distally a very short distal burrowing spine, ~0.1× as long as basipodite itself (Fig. 4I, J, Suppl. material 2: fig. S2I). Inner side of basipodite bearing a distal sensory seta, 5× as long as the spine (Fig. 4J); distal portion of the seta with feather-like armature of fine setulae. Antenna II branches are covered by transverse rows of wide triangular flattened spinulae (Fig. 4I, Suppl. material 2: fig. S2K, M). Exopodite four-segmented, slightly longer than the endopodite (EXL/ENL = 1.1); basal exopodal segment shortened; the segments 2–4 subequal in length, each 0.3× as long as the exopodite. Endopodite three-segmented, segment 1 is the longest, 0.35–0.37× as long as the branch; segments 2 and 3 subequal in length, 0.31–0.33× as long as exopodite (Fig. 4I); the endopodal segment 3 apically with a small, rounded lobe, visible in outer view, and an anterior row of spinulae (Fig. 4K).

Antenna II seta formula 0–1–1–3/1–1–3 (Fig. 4I, K, L, Suppl. material 2: fig. S2K, M). All the swimming setae bi-segmented, with apical segment flattened dorsoventrally. Setae of basal exopodal and endopodal segments subequal in length, 0.9× as long as the exopodite; basal and distal setal portions with a bilateral armature of uniform, long, flattened setulae (20–30 µm length, gap between neighboring setulae ~6 µm, Suppl. material 2: fig. S2L). Apical setae are similar in both branches: the posteriormost seta is the shortest, 0.9× as long as the exopodite; seta structure similar to the setae described above (Fig. 4K, L, Suppl. material 2: fig. S2K, M). Two anterior setae of similar structure: basal portion lacking setae, bearing 2–5 spinulae along the posterior margin; distal portion setulated, similar to that of the remaining setae (Fig. 4K, L, Suppl. material 2: fig. S2K, M). The anteriormost apical seta is the longest, 1.2× as long as medial apical seta; the medial seta is as long as the posterior apical seta (Fig. 4I). Spine formula 0–1–0–1/0–0–0 (Fig. 4I, Suppl. material 2: fig. S2I). The spine of exopod segment 2 is relatively short, 0.25–0.27× as long as the corresponding segment; the apical exopodal spine is 0.45–0.50× as long as the corresponding segment (Fig. 4I, L).

Labrum relatively small (LL/ED = 1.5–1.6), lacking a labral keel; anterior labral portion triangular, with rounded tip; posterior side of labrum tip l bearing few long setulae. Distal labral outgrowth rounded, finely setulated (Fig. 4M). A pair of small rounded paragnaths preceding mouth opening (Fig. 4M, O), bearing fine setulae.

Mandible small (ML/HL = 0.20–0.25). Two mandibles asymmetrical (Fig. 4N, Suppl. material 3: fig. S3A, B). Right mandibular molar plate oval (MPL/MPW = 2.2), slightly expanding posteriorly (Suppl. material 3: fig. S3A); dorsal margin of the right molar plate convex, bearing six or seven thin spines anteriorly and three robust teeth posteriorly; the teeth decrease in size posteriorly (Suppl. material 3: fig. S3A). Ventral margin slightly sinuous, bearing eight to ten transverse ribs anteriorly (Suppl. material 3: fig. S3A). Medial molar plate portion formed by 10–12 diagonal flattened outgrowths ornamented by tubercles, dense and prominent at anterior mandible edge and becoming lower and sparser towards posterior (Suppl. material 3: fig. S3A). Left mandibular molar plate subtriangular (MPL/MPW = 2.3), posterior end wider than the anterior one (Suppl. material 3: fig. S3B). Left molar plate dorsal margin convex, lacking outgrowths, ventral margin slightly concave, armed by 12–13 ribs. Medial molar plate portion armed by nine or ten diagonal rows of subrectangular outgrowths, the ventralmost outgrowths bearing digitate processes becoming longer towards posterior (Suppl. material 3: fig. S3B). The posterior edge is almost straight, armed by long digitate processes (Suppl. material 3: fig. S3B).

Maxilla I small, spatulate, incurved towards the mouth opening, bearing three bi-segmented setae densely covered by long setulae from base to tip (Fig. 4Q).

Maxilla II absent.

Five pairs of thoracic limbs, differing in size and structure (Fig. 5, see also Suppl. material 5: fig. S5).

Limb I is the largest, bent in half, consisting of outer distal lobe (ODL), inner distal lobe (IDL), three inner endites and inner lobe (IL) (Fig. 5A). Long thick accessory seta on outer side of limb, basally to the ODL; accessory seta densely covered by long setulae from base to tip (Fig. 5A). ODL short, cylindrical, bearing two setae (a, b), seta a very short, naked, located subapically; a short spine might present at base of the seta (Fig. 5C). Seta b located apically, thick, 2.6–2.7× as long as the accessory seta, naked; seta b bent medially, basal and distal portions of the seta almost straight (Fig. 5A, C). IDL short and wide, bearing four apical setae (1–3, 1’). Posterior setae (1–3) of similar structure, bi-segmented, with apical portion approximately as long as the basal one, the apical portion bearing a row of closely spaced long flattened spinulae with hook-like tips (Fig. 5A). Setae 1–3 slightly decreasing in length, seta 1 the longest (S1/Sb = 0.60–0.65). Anterior seta (1’) similar to that of posterior row but shorter (S1’/S1 = 0.70–0.75, S1’/S3 = 0.80–0.85), with apical segment exceeding the basal one (Fig. 5A). Endite 3 bearing three posterior setae (4–6) and one anterior seta (2’) (Fig. 5A). Setae 4 and 5 similar in length (S4/S1 = 0.85–0.90), seta 5 slightly shorter. All three setae bi-segmented; basal portion bilaterally armed by long setulae, apical portion bearing similar armature and additional row of 4–10 setulae at anterior surface (Fig. 5A). Anterior seta (2’) relatively long (S2’/S4 = 0.6), wide at base and narrowing distally, bilaterally armed by short setulae (Fig. 5A, D). Endite 2 bearing three posterior setae (7–9) and one anterior seta (3’). Setae 7–9 similar in size and structure (S7/S4 = 0.70–0.75), bi-segmented; basal portion densely covered by spinulae, apical portion similar to that of the setae 4–6 (Fig. 5A). Seta 3’ relatively long, wide at base and narrowing distally; basal portion bearing a row of 6–12 setulae of spines along the inner margin, apical portion bilaterally armed by long closely spaced setulae (Fig. 5A, E); a short spine located near the base of the seta laterally (Fig. 5E, F). Endite 1 bearing three bi-segmented posterior setae (10–12), a single unsegmented posterior seta (13) and a single anterior seta (4’) (Fig. 5A). Seta 10 slightly shorter than seta 11 (S10/S11 = 0.80–0.85), equal in length to this in setae 7–9; seta 10 basal portion covered by dense setulae, distal portion bearing bilateral armature of long setulae; the setulae of outer row are more closely spaced than that of inner row. Seta 11 with a basal portion bilaterally armed by long setulae; seta 11 distal portion similar to that of the seta 10. Seta 12 slightly shorter than the seta 10 (S12/S10 = 0.90–0.95), bi-segmented; seta 12 basal portion with bilateral armature of long sparse setulae, distal portion with two rows of closely spaced long setulae (Fig. 5A). Seta 13 is similar in length and structure to accessory seta (Fig. 5A). Anterior seta (4’) relatively short (S4’/S3’ = 0.85), bearing two rows of 8–15 short thick spinulae (Fig. 5G, H). Inner lobe is short, densely covered by long setulae. Two ejector hooks, first hook short, slightly incurved, armed by row of sparse setulae, second hook reduced to a tubercle (Fig. 5A, B). Anterior limb surface is covered by transverse rows of spinulae; base of endite 2 bearing a cluster of long setulae (Fig. 5A).

Limb II consists of a small rounded epipodite, cylindrical exopodite and wide inner portion (Fig. 5I). Exopodite bearing a single seta (a), long to relatively short (Sa/S1 = 0.45–0.80) wide at base and progressively narrowing distally, armed by two rows of long sparse setulae. Posterior side of exopodite armed by two groups of spinulae; exopodite apex bearing a transverse row of spinulae anteriorly, at base of the seta a. Inner portion bearing three posterior setae (1–3) and eight anterior scraping setae (1’–8’) (Fig. 5I, J). Scraper 1’ is the longest (S1’/S1 = 1.2), scraper 2’ is shorter than scrapers 1’ and 3’ (S2’/S1 = 0.80–0.85, S2’/S3’ = 0.9), scraper 4’ is the shortest (S4’/S1’ = 0.75–0.77, scrapers 5’–8’ progressively increase in length towards the gnathobase (S5’/S1’ = 0.8–0.82, S8’/S1’ = 0.93–0.95). Scrapers 1’–3’ armed by a row of thin spinulae; scrapers 4’ and 5’ with 15–20 thick spines; scrapers 5’–8’ with 25 or more relatively thin spines. Seta 1 located behind the scraper 1’ attachment, exceeds length of this scraper; seta 1 bi-segmented, naked, slightly incurved. Three lobes of different shape located behind bases of scrapers 1’–6’: outermost lobe long and narrow, expanding medially, with acute apex; medial lobe relatively short, oval; innermost lobe short and wide (Fig. 5J). Setae 2 and 3 shifted towards the gnathobase, short, bi-segmented; distal portion of each seta bearing dense short setulae (Fig. 5I, J). Gnathobase large, lobate, bearing four posterior elements (α–δ): a very short naked seta (α), a long bi-segmented seta (β), bent medially; the seta β basal portion inflated, distal portion thin, both bearing row of long setulae; hook-shaped short seta with inflated base (γ); very short naked seta (δ), slightly longer than seta α (Fig. 5I, K). Filtering comb consisting of seven or eight long bi-segmented setae, similar in length and structure, with distal portion armed by two rows of long closely spaced setulae (Fig. 5I). Outer gnathobase margin bearing a row of numerous short closely spaced setulae and 7–12 long setulae (Fig. 5I).

Limb III is large, bent in half, consisting of short oval epipodite, elongate exopodite, and wide inner portion (Fig. 5L–P). Exopodite flattened, widening distally, slightly incurved, bearing four unsegmented setae (a–d): setae a and b short (Sa/Sc = 0.47–0.50, Sb/Sc = 0.85–0.88), naked, wide at base, with narrow distal portion; setae c and d equal in length, progressively narrowing distally, bearing two rows of long sparse setulae from base to tip (Fig. 5M). Several transverse rows of long setulae at base of the exopodite (Fig. 5M). Inner portion distal endite bearing three posterior setae (1–3) and one anterior seta (1’). The setae 1–3 subequal in length, long (S1/EXL = 1.6–1.7), bi-segmented, with distal portion bearing a row of closely spaced flattened spinulae (Fig. 5L, N). Anterior seta (1’) roughly equal in length to the setae 1–3, unsegmented, slightly incurved, with basal portion bearing two rows of long sparse setulae; seta 1’ distal portion bearing a row of short closely spaced setulae (Fig. 5L, N). Inner endite bearing three unsegmented posterior setae (4–6) and two unsegmented anterior setae (2’ and 3’). Setae 4 and 5 similar in length and structure, relatively long (S1/S4 = 0.45–0.48), with distal portion bearing dense short setulae; the seta 6 short (S6/S4 = 0.40–0.45), bottle-shaped, with distal portion bearing short setulae (Fig. 5L, N). Setae 2’ and 3’ are similar in length and structure, short (S2’/S4 = 0.35–0.45), armed by dense setulae from base to tip (Fig. 5L, N). A short oval sensillum at base of the seta 6, almost equal in length to the seta (Fig. 5L, N). Gnathobase with three elements: a long seta bent medially (α) and two cylindrical sensillae with acute tips (β, γ) (Fig. 5L, O). Inner portion distal endite covered by transverse rows of long setulae; two clusters of long setulae located at inner portion closely to the distal sensillum; basal portion of the gnathobase with 12–20 long setulae armed by secondary processes (Fig. 5O, P).

Limb IV is small, consisting of a short setulated pre-epipodite, long epipodite, short cylindrical exopodite and wide inner portion (Fig. 5Q). Exopodite bearing a single unsegmented seta (a), armed by two rows of long sparse setulae (Fig. 5Q). Inner portion of distal endite with three unsegmented setae (1–3), progressively decreasing in size basally (S1/Sa = 0.8, S2/Sa = 0.7, S3/Sa = 0.5); basal portion of each seta bearing a row of setulae along the inner margin, ending with a short thick spine (Fig. 5R). Gnathobase with four elements (α–δ) (Fig. 5Q). Distalmost element (α) lobate, expanding distally, with distal margin armed by row of spinulae; a bunch of long setulae at anterior surface of lobe (Fig. 5R). Seta β long, bi-segmented, with basal portion inflated, bearing row of long setulae along the outer margin; distal portion with two rows of long sparse setulae (Fig. 5Q). Proximal elements (γ and δ) short, γ longer than δ (Fig. 5Q). Exopodite and inner portion with several transverse rows of setulae; cluster of setulae at base of gnathobase.

Limb V is as large as limb IV, consists of large epipodite and relatively small inner portion (Fig. 5S). Exopodite and endopodite merged, bearing two thick unsegmented setae (1 and 2) of similar structure; the seta 1 1.4× as long as the seta 2, both setae slightly incurved, with basal portion naked, distal portion bearing several closely spaced longitudinal rows of short spinulae along the posterior margin; (Fig. 5S). Gnathobase relatively small, rounded, bearing two setae (α and β), the seta α 3× longer than the seta β, both seta distal portions densely covered by short setulae, seta α basal portion bearing a row of long setulae along the outer margin (Fig. 5S). Gnathobase with two clusters of setulae basally (Fig. 5S).

Ephippial female. Body length 1.0–1.2 mm. General morphology is very similar to that of parthenogenetic females (Fig. 3L, M, Suppl. material 3: fig. S3F, G). Body high, moderately flattened laterally (BH/BL = 0.65–0.70, BW/BL = 0.30–0.35). Dorsal valve margin forming an angle in its anterior half, dorsal keel low (Fig. 3L, Suppl. material 3: fig. S3F). Ephippium thin, transparent to light brown, weakly melanized, containing two to eight eggs. The ephippium uniformly ornamented by well-defined irregular polygons (Fig. 3L, M, Suppl. material 3: fig. S3F–H). Ephippium surface inside the polygons is almost smooth, at most bearing shallow pits (Suppl. material 3: fig. S3H). Morphology of head and thoracic appendages similar to that of parthenogenetic females.

Male. Body length 0.5–0.8 mm. General body shape similar to that of parthenogenetic female, body compressed laterally (BH/BL = 0.63–0.67, BW/BL = 0.40–0.42) (Fig. 6A, B). Ventral and posteroventral valve armature similar to that of parthenogenetic female (Fig. 6C).

Head, thorax, and abdomen as in parthenogenetic females (Fig. 6A, B).

Postabdomen is weakly modified in comparison to that of parthenogenetic female (PL/BL = 0.22–0.25) (Fig. 6D, Suppl. material 4: fig. S4A). Postabdominal claw long and thick (PCL/PL = 0.45–0.48), incurved, directed dorsally (Fig. 6D). Gonopore opening subdistally at lateral surface of the postabdomen; gonopore small, slit-like (Fig. 6D, Suppl. material 4: fig. S4B).

Antenna I long and slender (AL/ED = 4.28–4.30, AL/AW = 0.12–0.15), cylindrical in cross-section (Fig. 6E, F, Suppl. material 4: fig. S4C, D). Basal seta of antenna I 0.18–0.20× as long as the antenna I, seta located posteriorly at low expansion (Fig. 6E, Suppl. material 4: fig. S4C). Additional male seta (mas) located at anterior side of appendage, male seta more distant from the antenna I base than basal antennular seta (Fig. 6E, F), 0.3–0.4× as long as basal seta. Anterior surface of antenna I bearing six additional clusters of modified setae, becoming wider and more flattened towards the antenna I apex (Fig. 6E, F, Suppl. material 4: fig. S4C–E, arrows): first group consists of six to eight closely spaced thin setae of similar structure, the next include two or three flattened setae forming a transverse row. A pair of scale-like setae (scs) in distal half of the antenna I, the setae similar in shape and position to that of parthenogenetic female (Fig. 6E, F, Suppl. material 4: fig. S4C). Nine terminal aesthetascs as in parthenogenetic females, the longest one 0.37–0.39× as long as the whole antenna, 1.2–1.5× as long as the other aesthetascs (Fig. 6E, F, Suppl. material 4: fig. S4C). Antenna I sculpture is similar to that of a parthenogenetic female (Fig. 6E–G, Suppl. material 4: fig. S4C–E).

Antenna II in general is similar to that of parthenogenetic female (ANL/BL = 0.50–0.55) (Fig. 6A). Two additional setae (mds) located at distal end of the basipodite, between exopodite and endopodite attachments (Fig. 6H, Suppl. material 4: fig. S4F, G); the anterior seta is 0.80–0.90× as long as the posterior one; both setae naked (Fig. 6H, Suppl. material 4: fig. S4F, G).

Mouth parts as in parthenogenetic females.

Thoracic limb I is highly modified, consisting of outer distal lobe (ODL), inner distal lobe (IDL), subdistal lobe (SDL), inner endites, and small inner lobe (IL) (Fig. 6I, J, Suppl. material 4: fig. S4H–M). Accessory seta and ODL morphology similar to that of parthenogenetic female. IDL bearing four apical setae (1–3, 1’) similar to that of parthenogenetic female (Suppl. material 4: fig. S4J, K) and additional seta (X) located anteriorly to the seta 1’. Seta X approximately as long as the seta 1’ (SX/S1’ = 0.90–0.95), slightly incurved in its distal portion, naked (Fig. 6I, Suppl. material 4: fig. S4J). Large melanized hook present at the IDL basal portion outer side (Fig. 6I, Suppl. material 4: fig. S4H, I); the hook moderately incurved, with slightly narrowed tip armed by three transverse cuticular ribs (Suppl. material 4: fig. S4H, M). SDL large, located at anterior face of the limb proximally to the IDL attachment; SDL approximately reaching distal hook portion, rounded, armed by 5–6 transverse rows of minute closely spaced spinulae (Fig. 6K, L, Suppl. material 4: fig. S4L, M); four groups of long setulae at outer surface of the SDL (Fig. 6K, L, Suppl. material 4: fig. S4I, L, arrows). General morphology of inner endites similar to that of parthenogenetic female (Fig. 6J). Endite 3 outer side bearing an additional seta (Y) directed towards the IDL (Fig. 6J); seta Y relatively short (SY/S4 = 0.60–0.65), with basal portion progressively narrowing distally and narrow distal portion, naked (Fig. 6J). Ejector hook morphology similar to that of parthenogenetic female. Anterior limb face with transverse rows of setulae; a group of long setulae located proximally to the SDL (Fig. 6J).

Limbs II–V similar to that of parthenogenetic females.

The known range of Lathonura rectirostris s. str. includes the whole territory of Europe except continental Portugal, Spain, and the Mediterranean islands, although it is found in Sicily (Błedzki and Rybak 2016), European Russia (Kotov et al. 2010; Korovchinsky et al. 2021), and West Siberia east to the Tomsk Area of Russia (i.e., predominantly east of the Ob’ River). The species is, most probably, rather common in North Eurasia (although rarely recorded in zooplankton monitoring studies due to ecological affinities), but the southern distribution borders of the species remain unclear. Lathonura rectirostris s. l. was also observed in North Canada and eastern part of the USA: Minnesota (Herrick 1884; Herrick and Turner 1895) and Wisconsin (Birge 1892, see also Smith 2001). Parthenogenetic females of at least some East Nearctic populations share diagnostic characters with L. rectirostris s. str. (Suppl. material 1: fig. S1F) but genetic data show they belong to a deeply divergent clade and might represent another species (Fig. 2A, B, clade A1).

Lathonura rectirostris s. str. prefers weakly acidic waters with pH 5.5–7.5 (Sergeev 1971; Fryer 1974; Błedzki and Rybak 2016). The species inhabits a variety of vegetated water bodies with sandy or stony sediments and organic debris (Błedzki and Rybak 2016). Lathonura rectirostris is able to swim but spends most of the time attached to the substrate by setae of limb I IDLs (Fryer 1974). For slow movement along the substrate, limbs I and III are used (Sergeev 1971; Fryer 1974) instead of pushing by means of the postabdomen or antenna II typical for most macrothricids (Fryer 1974). Lathonura rectirostris consumes a variety of epibiont algae scraped from the aquatic plant leaves or other substrates (Błedzki and Rybak 2016). The ephippial female attaches the ephippium to submerged plants with a sticky secretion (Fryer 1974).

Du (1973): 59, fig. 45A, B; Smirnov (1992): 109–111, fig. 469–474 (rectirostris); Dadykin et al. (2025): fig. S2 (cf. rectirostris).

Russia, Kamchatsky Krai, Parapolsky Dol, a small unnamed tundra lake (61°17.58'N, 164°42.30'E; Fig. 1, Suppl. material 6, loc. 31)

Holotype: Russia • 1 parthenogenetic ♀; Kamchatsky Krai, Parapolsky Dol; 61°17.58'N, 164°42.30'E; 5 Sep. 2017; small tundra lake; E.I. Bekker leg.; access number MGU MI 285; original number IEE AAK M–5887. Paratypes. Russia • 11 parthenogenetic ♀; access number MGU MI 286; as for holotype • 19 parthenogenetic females; as for holotype; access number IEE AAK 2025–007.

Russia • 2 parthenogenetic ♀; Krasnoyarsky Krai, Severtsov IEE biological station “Mirnoe”; 62°21.24'N, 89°2.49'E; 17 July 2011; oxbow lake; A.A. Kotov leg.; IEE AAK M–2146 • 1 parthenogenetic ♀; Irkutsk Area, Bazhei, Iret’ River; 52°58.28'N, 102°38.81'E; 18 Aug. 2019; A.A. Kotov, D.P. Karabanov leg.; IEE AAK M–5109 • 1 parthenogenetic ♀; Irkutsk Area, Angarsk, Bolshoi Kanal River; 52°29.06'N, 103°57.22'E; 18 Aug. 2019; A.A. Kotov, D.P. Karabanov leg.; IEE AAK M–5172 • 3 parthenogenetic ♀; Irkutsk Area; 52°1.64'N, 105°24.76'E; 16 Aug. 2023; small vegetated lake; I.A. Dadykin, A.A. Kotov leg.; IEE AAK M–8189 • 4 parthenogenetic ♀; Zabaikalsky Krai, Chasovinka, Shilka River; 53°29.20'N, 120°3.04'E; 24 Aug. 2018; D.P. Karabanov, A.A. Zharov, M.A. Gololobova, A.A. Kotov leg.; IEE AAK M–5000 • 10 parthenogenetic ♀, 1 ephippial female; Yakutia, Maragas; 62°12.28'N, 127°44.84'E; 19 June 2022; roadside puddle; L.V. Andreeva, P.G. Garibian leg.; IEE AAK M–7136 • 5 parthenogenetic ♀; Yakutia, Yakutsk; 62°1.11'N, 129°45.42'E; 5 Sep. 2011; lake; E.I Bekker, A.I. Klimovski leg.; IEE AAK M–2292 • 1 parthenogenetic ♀; Yakutia, Tuluna; 62°48.36'N, 131°4.50'E; 10 Sep 2011; E.I Bekker and A.I. Klimovski leg.; IEE AAK M–2315 • 1 parthenogenetic ♀; Magadan Area, Snezhny, Lake Dukga; 59°44.37'N, 150°51.36'E; 12 June 2016; N.G. Sheveleva leg.; IEE AAK M–6502; • 6 parthenogenetic ♀; Kamchatsky Krai, Parapolsky Dol; 61°22.62'N, 164°44.28'E; 5 Sep. 2017; small tundra lake; E.I. Bekker leg.; IEE AAK M–5890 • 2 parthenogenetic ♀; Kamchatsky Krai, Karaginsky Island; 58°52.58'N, 163°48.55'E; 6 Aug. 2022; vegetated ditch in peat lake; I.A. Dadykin leg.; IEE AAK M–7722. U.S.A. • 2 parthenogenetic ♀; Alaska, Seward Peninsula; 65°5.34'N, 165°4.63'W; 4 Aug 2011; tundra lake in Quatrine River basin, Seward Peninsula; A.A. Kotov and D.J. Taylor leg.; AAK M-6657.

Body length of parthenogenetic female 0.5–1.2 mm, dorsal keel well developed. Ventral valve margin with flattened setae typical for the genus. Posteroventral valve margin armed by groups of short thin denticles alternating with solitary larger denticles. The anterior headshield margin is slightly convex. Dorsal organ large, rounded, ornamented by a net-like sculpture. Antenna I distal end with one or two additional scale-like setae at the posterior side. Ephippial female with angulate dorsal margin and low dorsal keel. Ephippium ornamented by wrinkles, not forming regular polygonal sculpture. Subdistal lobe of male thoracic limb I lacking groups of spinulae at outer surface.

Parthenogenetic female. Body length 0.5–1.2 mm. Body oval in lateral view, slightly to moderately compressed laterally (BL/BW = 1.6–1.7, BL/BH = 0.35–0.50) (Figs 7A, B, 8A). Body with a low dorsal keel originating posteriorly to headshield margin (Fig. 7B). Ventral valve armature typical for the genus (Fig. 7A, H). Posteroventral valve margin bearing clusters of 5–12 short thin denticles alternating with solitary (very rarely double) longer and thicker denticles (10–15 denticles per valve); valve margin closely to posterodorsal angle usually lacking large denticles (Figs 7I, J, 8J, Suppl. material 1: fig. S1G–J). Carapace light brown, almost smooth, lacking reticulation (Fig. 8A).

Head typical for the genus (HL/BL = 0.30–0.35, HW/HL = 0.9–1.0) (Fig. 7D, E). Dorsal organ rounded, with net-like ornamentation formed by cells margins (Figs 7C, D, 8C). Frontal head pore horseshoe-shaped (Fig. 8B). Anterior headshield margin slightly convex (Fig. 7F). A pair of conical prelabral outgrowths at lateral head surface (Fig. 7E, F).

Thorax and abdomen typical for the genus. Gut straight, lacking convolutions (Fig. 7A).

Postabdomen typical for the genus (PL/BL = 0.18–0.22), with dorsal margin bearing transverse rows of five to seven spinulae; lateral postabdomen surface covered by groups of short thin spinulae (Figs 7G, H, 8K). Postabdominal claw long and thick (PCL/PL = 0.35–0.40, PCL/PCW = 4.7–4.8), slightly incurved, bearing a longitudinal row of six to ten short spinulae at both outer and inner margin (Figs 7G, 8L). Postabdominal seta typical for the genus (Fig. 7A).

Antenna I is long and narrow (AL/ED = 3.0–3.1, AL/AW = 6.9–7.0), cylindrical in cross-section, almost straight in anterior view (Fig. 7A, E, F, K, L). The basal seta of the antenna I located at low expansion, the seta 0.20–0.25× as long as the whole antenna I (Fig. 7K). Two scale-like setae located at distal half of the antenna I at its anterior face; the seta morphology is typical for the genus (Fig. 7K, L). One or two shorter scale-like setae located at the posterior face of the antenna I close to its distal end (Fig. 7L, black arrow; Fig. 8D, E, white arrows). Nine aesthetascs typical for the genus (Figs 7K, 8E). A distal crown of spinulae as typical for the genus (Figs 7K, L, 8D, E). The whole antenna surface is covered by transverse rows of one to six flattened wide triangular spinulae (Figs 7K, L, 8D, E).

Antenna II typical for the genus (ANL/BL = 0.55–0.65), exopodite equal in length to endopodite or slightly longer (EXL/ENL = 1.0–1.1) (Fig. 7M–O). Armature of antenna II typical for the genus (Figs 7M–O, 8F, G).

Labrum and paragnaths typical for the genus (Fig. 7E, F, P).

Mandibles and first maxillae typical for the genus (Fig. 8H, I).

Five pairs of thoracic appendages, differing in size and structure, typical for the genus (Suppl. material 5: fig. S5). Gnathobase of limb II with eight to ten long filtering setae and four posterior elements (Fig. 7Q).

Ephippial female. Body length 0.8–1.0 mm. Body largely similar to that of parthenogenetic female (BL/BW = 1.8, BL/BH = 2.6–2.7), dorsal keel low (Fig. 9A, B). Ephippium ornamented by folds forming obscure irregular polygonal sculpture (Fig. 9A–C); well-visible polygons present only in ventral and posterior edge of the ephippium; between the large folds, ephippium surface covered by smaller wrinkles and pits (Fig. 9D). Morphology of head, thorax, abdomen, and postabdomen typical for the genus.

Male. Body length 0.61 mm, body oval in lateral view, moderately compressed laterally (BL/BH = 1.6, BL/BW = 2.3). Ventral margin slightly convex, armed by flattened setulae as typical for the genus (Fig. 10A–C). Posteroventral valve margin armed by groups of five to eight thin denticles alternating with solitary larger and thicker denticles (Fig. 10C, D); the dorsalmost portion of the posteroventral margin lacking well-defined groups of denticles (Fig. 10C). Head, thorax and abdomen morphology typical for the genus. Postabdomen shape and armature similar to that of parthenogenetic female (PL/BL = 0.2), postabdominal claw large and thick, slightly incurved, directed dorsally (PCL/PL = 0.4, PCL/PCW = 4.5) (Fig. 10E). Postabdominal setal length and armature typical for the genus. Two gonopores; each gonopore slit-like, located subdistally at the lateral surface of the postabdomen (Fig. 10E).

Antenna I is relatively short, almost straight, cylindrical in cross-section (AL/ED = 3.3, AL/AW = 6.9). Basal antennular seta similar to that of parthenogenetic female. Anterior antenna I face with additional male seta (ms) located proximally to the basal seta (Fig. 10F, H); the male seta is relatively short, 0.6× as long as the basal seta. Three clusters of four to six long thin setae at the anterior face of the appendage basally (Fig. 10F, H); three transverse rows of one to three flattened setae of various length and width more distally; two scale-like setae similar in size and location to that of parthenogenetic female (Fig. 10F, H). One additional scale-like seta at the posterior side of the antenna I, closely to its tip (Fig. 10G, H, black arrows). Distal armature of antenna I as in parthenogenetic female; appendage surface covered by transverse rows of two to three short wide triangular flattened spinulae.

Antenna II typical for the genus (ANL/BL = 0.64) (Fig. 10A). Distal basipodite end bearing two additional setae (mds) at outer face; one of setae 0.9–1.0× as long as the other, both setae naked (Fig. 10I).

Mouth parts typical for the genus.

Five pairs of thoracic appendages, differing in size and structure, typical for the genus.

Thoracic limb I largely similar to that of female (Fig. 8J–L, see Suppl. material 5: fig. S5A–F for female limb I morphology) but modified as typical for L. rectirostris male. IDL hook with a rounded tip almost reaching subdistal lobe (SDL); the hook tip bearing three subdistal transverse rows of very thin closely spaced spinulae at the anterior side (Fig. 8K). Subdistal lobe bearing five transverse rows of short closely spaced spinulae; the SDL surface lacking long setulae (Fig. 8L). Additional seta (X) located at anterior side of IDL (SX/S1 = 0.8). Additional anterior seta (Y) located at outer side of endite 3 (SY/S4 = 0.4).

Thoracic limbs II–V typical for the genus.

The observed differences between Lathonura rectirostris s. l. and L. bekkerae sp. nov. are summarized in Table 2. Both parthenogenetic and gamogenetic stages of L. bekkerae clearly differ from those of L. rectirostris s. l., first of all, by the armature of the posteroventral valve margin. Also, all studied specimens of L. bekkerae possess one or two additional posterior scale-like setae on the antennae I. Moreover, the specimens of L. bekkerae studied by scanning microscopy have a sculptured dorsal organ in contrast to L. rectirostris s. l. (Fig. 8C), although this character cannot be revealed when using optical microscopy. The ephippial female of L. bekkerae differs from that of L. rectirostris s. str. in the external ornamentation of the ephippium (irregular wrinkles forming obscure polygons versus well-distinct polygons in L. rectirostris s. str.). Finally, the male of L. bekkerae has a rather different armature of the limb I subdistal lobe, which bears long spinulae at the outer side (absent in males of L. rectirostris s. str.). However, male morphology was studied in one individual only, thus the latter features need to be confirmed.

Lathonura bekkerae sp. nov. also differs from both species inquirendae of Lathonura, L. dorsispina and L. ovalis. Lathonura dorsispina, described from Romania, lacks figures in original description (Cosmovici 1901) and cannot be clearly attributed to Lathonura based on its diagnosis. The diagnostic features of L. dorsispina are: antenna II lacking transverse rows of denticles; numerous fine spinulae at “dorsal” (in fact probably referring to posterior) valve margin; small size (~0.4 mm, Cosmovici 1901). As noted above, valve morphology is unclear but, basing on description, seems to be more similar to L. rectirostris s. l. which has uniform closely spaced fine spinulae at posterior valve margin (Suppl. material 1: fig. S1A–F). Short spinulae present at antennae II of both L. rectirostris s. l. and L. bekkerae sp. nov. could easily have been missed by Cosmovici (1901). Finally, rather small individuals (~0.5 mm length) were also observed in both L. rectirostris s. l. and L. bekkerae sp. nov.

Lathonura ovalis, described from Pakistan (Mahoon and Sabjr 1985), can be attributed to the genus Lathonura basing on presence of flattened marginal setae at ventral valve margin and antenna II seta formula; however, most of characters included in the diagnosis of this taxon seem doubtful. The diagnostic features for this species are: incurved antenna I and straight postabdominal claw, seven “anal spines”, elliptical compound eye with only five ommatidia, six aesthetascs (Mahoon and Sabjr 1985: figs 43–47). None of these features can be referred to L. bekkerae and are observed in genus Lathonura in whole; it seems that Mahoon and Sabjr (1985) described a damaged or poorly fixed specimen. The individual depicted in Mahoon and Sabjr (1985: fig. 43) can be more likely attributed to Macrothrix based on body and antenna I shape and absence of flattened setae at ventral valve margin. Antenna II depicted in Mahoon and Sabjr (1985: fig. 45) is similar to that of Lathonura in seta formula, while some in Mahoon and Sabjr (1985: figs 44, 46, 47) cannot be referred to Lathonura with confidence.

The species is named after its collector, Evgenia I. Bekker, who strongly contributed to the studies of Eurasian cladocerans (Bekker et al. 2012, 2016, 2018).

Lathonura bekkerae sp. nov. is widely distributed in Northeast Asia, including Irkutsk Area, Zabaikalsky Krai, Yakutia, Magadan Area, and Kamchatka. Populations of Lathonura are relatively common in northeastern regions of Russia (Streletskaya 1975, 2010), and populations from southern Russian Far East (Kotov et al. 2011) and North China (Ji et al. 2015) likely belong to Lathonura bekkerae sp. nov. Note that Lathonura has not been observed in Korea (Jeong et al. 2014) and Japan (Uéno 1927), whereas records of “L. rectirostris” from South China apparently belong to Guernella raphaelis Richard, 1892 (Ji et al. 2015). Also, L. bekkerae occurs in Alaska, but its distribution in North America remains unclear. Ecological preferences of the species have not been studied but seem to be rather similar to that of L. rectirostris. Lathonura bekkerae sp. nov. inhabits mostly vegetated lakes, ponds, and oxbows, but can be also found in vegetated zones of rivers, ditches, and creeks (Suppl. material 6).