Lathonura bekkerae, Dadykin & Pereboev, 2025

Lathonura bekkerae sp. nov.

Figs 7 View Figure 7 , 8 View Figure 8 , 9 View Figure 9 , 10 View Figure 10 , Suppl. materials 1, 5: figs S 1 G – J, S 5

References.

Du (1973): 59, fig. 45 A, B; Smirnov (1992): 109–111, fig. 469–474 ( rectirostris ); Dadykin et al. (2025): fig. S 2 ( cf. rectirostris ).

Type locality.

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

Type material.

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 GoogleMaps . Paratypes. Russia • 11 parthenogenetic ♀; access number MGU MI 286; as for holotype • 19 parthenogenetic females; as for holotype; access number IEE AAK 2025–007.

Other material examined.

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 GoogleMaps • 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 GoogleMaps • 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 GoogleMaps • 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 GoogleMaps • 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 GoogleMaps • 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 GoogleMaps • 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 GoogleMaps • 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 GoogleMaps • 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 GoogleMaps ; • 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 GoogleMaps • 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. GoogleMaps • 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 GoogleMaps .

Diagnosis.

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.

Description.

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 7 A, B View Figure 7 , 8 A View Figure 8 ). Body with a low dorsal keel originating posteriorly to headshield margin (Fig. 7 B View Figure 7 ). Ventral valve armature typical for the genus (Fig. 7 A, H View Figure 7 ). 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 7 I, J View Figure 7 , 8 J View Figure 8 , Suppl. material 1: fig. S 1 G – J). Carapace light brown, almost smooth, lacking reticulation (Fig. 8 A View Figure 8 ).

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

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

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 7 G, H View Figure 7 , 8 K View Figure 8 ). 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 7 G View Figure 7 , 8 L View Figure 8 ). Postabdominal seta typical for the genus (Fig. 7 A View Figure 7 ).

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. 7 A, E, F, K, L View Figure 7 ). 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. 7 K View Figure 7 ). 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. 7 K, L View Figure 7 ). One or two shorter scale-like setae located at the posterior face of the antenna I close to its distal end (Fig. 7 L View Figure 7 , black arrow; Fig. 8 D, E View Figure 8 , white arrows). Nine aesthetascs typical for the genus (Figs 7 K View Figure 7 , 8 E View Figure 8 ). A distal crown of spinulae as typical for the genus (Figs 7 K, L View Figure 7 , 8 D, E View Figure 8 ). The whole antenna surface is covered by transverse rows of one to six flattened wide triangular spinulae (Figs 7 K, L View Figure 7 , 8 D, E View Figure 8 ).

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. 7 M – O View Figure 7 ). Armature of antenna II typical for the genus (Figs 7 M – O View Figure 7 , 8 F, G View Figure 8 ).

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

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

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

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. 9 A, B View Figure 9 ). Ephippium ornamented by folds forming obscure irregular polygonal sculpture (Fig. 9 A – C View Figure 9 ); 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. 9 D View Figure 9 ). 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. 10 A – C View Figure 10 ). Posteroventral valve margin armed by groups of five to eight thin denticles alternating with solitary larger and thicker denticles (Fig. 10 C, D View Figure 10 ); the dorsalmost portion of the posteroventral margin lacking well-defined groups of denticles (Fig. 10 C View Figure 10 ). 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. 10 E View Figure 10 ). 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. 10 E View Figure 10 ).

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. 10 F, H View Figure 10 ); 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. 10 F, H View Figure 10 ); 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. 10 F, H View Figure 10 ). One additional scale-like seta at the posterior side of the antenna I, closely to its tip (Fig. 10 G, H View Figure 10 , 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. 10 A View Figure 10 ). 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. 10 I View Figure 10 ).

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. 8 J – L View Figure 8 , see Suppl. material 5: fig. S 5 A – 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. 8 K View Figure 8 ). Subdistal lobe bearing five transverse rows of short closely spaced spinulae; the SDL surface lacking long setulae (Fig. 8 L View Figure 8 ). Additional seta (X) located at anterior side of IDL ( SX / S 1 = 0.8). Additional anterior seta (Y) located at outer side of endite 3 (SY / S 4 = 0.4).

Thoracic limbs II – V typical for the genus.

Differential diagnosis.

The observed differences between Lathonura rectirostris s. l. and L. bekkerae sp. nov. are summarized in Table 2 View 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. 8 C View Figure 8 ), 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. S 1 A – 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.

Etymology.

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).

Distribution and ecology.

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).