Alona sibirica Sinev & Karabanov & Kotov 2020, sp. nov.

Alona sibirica sp. nov.

(Figs. 1–5, 6 A–F)

Lilljeborg, 1900: 459, Pl. 17: Fig. 12 (affinis partum); Werestschagin, 1911a: 555 (affinis var. dentata); Werestschagin, 1912b: 131 –133, Fig. 54, 55 (affinis var. dentata); Behning, 1941: 315 (affinis var. dentata); Smirnov, 1971: 472–474, Fig. 590 (affinis dentata); Sinev, 1997: 53, Fig. 4C (affinis var. dentata); Garibian et al., 2019: 49–52, Fig. 8 (cf. affinis).

Etymology. The species is named after Siberia, the region of its primary distribution.

Type locality. Lake near road between Amga river and Churapcha, Sakha (Yakutia) Republic, Russia, N 61.80453°, E 133.2409°, coll. on 26.08.2010 by A. A. Kotov (number of original sample is AAK 2011-034).

Type material. Holotype. Parthenogenetic female from type locality, MGU Ml186.

Allotype. Adult male from type locality, MGU Ml187.

Paratypes. 1 parthenogenetic female, 3 ephippial females, 1 adult male, 1 juvenile male, 2 juvenile females & adult male from type locality, MGU Ml188.

Other material examined. Samples, where Alona sibirica sp. nov. coexists with A. affinis are marked by aestherics. Russia: Moscow Area: 1 parthenogenetic female, 4 ephippial females, 6 adult males from Glubokoe Lake, Ruza district, N 55.75361°, E 36.50417°, 26.09.2019 and 06.10.2019, coll. A.Y. Sinev, N.M. Korovchinsky. Khanty-Mansi Autonomous Area: 11 parthenogenetic females from a lake in Posiolok Divniy, town of Nizhnevartovsk, N 60.92362°, E 76.49347°, 23.07.2005, coll. A. A. Kotov, AAK 2005-321; 1 parthenogenetic female from an oxbow lake near Pavlovskie Dachi, Stariy Vartovsk, N 60.85948°, E76.66573°, 23.7.2005, coll. A. A. Kotov, AAK 2005-326. Tomsk Area: 2 parthenogenetic females from an oxbow water body of River Chulym N 57.8082°, E 84.2458°, 13.07.2005, coll. A. A. Kotov, AAK 2005-280; 2 parthenogenetic females from an oxbow lake of River Gordiushka, N 57.81653°, E 84.27721°, 13.7.2005, coll. A. A. Kotov, AAK 2005-281; 2 parthenogenetic females from an oxbow lake of River Gordiushka, 57.81653° 84.27721°, 13.07.2005, coll. A. A. Kotov, AAK 2005-282; 6 parthenogenetic females from Lake Svetloe near River Chulym, N 57.81782°, E 84.18414°, 14.07.2005, coll. A. A. Kotov, AAK 2005-285; 6 parthenogenetic females from a puddle with Geum, N 56.5881°, E 84.8564°, 16.07.2005, coll. A. A. Kotov, AAK 2005-288; 38 parthenogenetic females, 2 ephippial females, 8 adult males, 1 juvenile male II from a boggy depression, N 56.586°, E 84.79308°, 16.07.2005, coll. A. A. Kotov, AAK 2005-289; * 3 parthenogenetic females from a mine-lake near road Tomsk-Mel’nikovo, N 56.53575°, E 84.13392°, 20.07.2005, coll. A. A. Kotov, AAK 2005-298; 5 parthenogenetic females from an oxbow lake of River Chaia, road Molchanovo-Parabel’, Kolpashevo District, N 58.08153°, E 82.80453°, 21.07.2005, coll. A. A. Kotov, AAK 2005-305; 41 parthenogenetic females from a roadside ditch near bridge through River Istok, the road Kolpashevo-Parabel’, N 58.41123°, E 82.14425°, 21.07.2005, coll. A. A. Kotov, AAK 2005-307; 2 parthenogenetic females from Lake Diadi Stepino near village of Tibinak, N 59.14938°, E 80.97303°, 22.07.2005, coll. A. A. Kotov, AAK 2005-320; Sakha (Yakutia) Republic: 11 parthenogenetic females from an un-named lake on Melnikova island, River Lena, N 63.85302°, E 127.4713°, 22.08.2010, coll. A. A. Kotov, AAK 2011-020; 8 parthenogenetic females, 2 males from a lake 2 on Melnikova island, Lena River, N 63.8512°, E 127.4738°, 22.08.2010, coll. A. A. Kotov, AAK 2011-021; 2 parthenogenetic females from Atlassovka lake near Yakutsk, N 61.96702°, E 129.6204°, 24.08.2010, coll. A. A. Kotov, AAK 2011-024; 3 parthenogenetic females, 1 male from a forest lake near River Aldan, N 63.11416°, E 134.0172°, 26.08.2010, coll. A. A. Kotov, AAK 2011-038; 2 parthenogenetic females from oxbow lake near River Khandyga, N 63.112°, E 134.0446°, 26.08.2010, coll. A. A. Kotov, AAK 2011-039; Kamchatka Area: * 4 parthenogenetic females from a small pool in Sphagnum bog near Bolshaya River, N 52.76419°, E 156.2536°, 24.08.2009, coll. A. A. Kotov, AAK 2009-098. Mongolia: * 1 parthenogenetic female from Uhegiin Gol (river) near Tes town, Uvs Aimag, N, 50.49443°, E 93.60029°, 30.08.2006, coll. C. Jersabek, AAK 2008-082.

Material studied earlier. See Sinev (1997) for the list of material from Eurasia (A. affinis var. dentata) and Garibian et al. (2109) for list of material from Bolon’ lake (Khabarovsk Area, Asian Russia).

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Description. Parthenogenetic female. In lateral view, body oval (Fig. 1 A–B, 2A–B), of moderate height, moderately compressed laterally, maximum height at middle. In adults height/length ratio about 0.6. Dorsal margin of valves convex; posterodorsal and posteroventral angles broadly rounded. Posterior margin convex. Anteroventral angle widely rounded. Ventral margin (Fig. 1C) weakly convex to straight, with 70–85 setae. About 25 anterior setae very long, next 10 seta short, posterior 40 setae of moderate length, decreasing in length posteriorly. Postero-dorsal angle (Fig. 1 D–G, 3A) with 1–4 closely spaced denticles of variable shape, at least one with broad base, and onetwo rows of short setulae of similar length and thickness, with 3–6 setulae in each. A row of about 100 setulae of variable length along posterior margin on inner side of carapace, these setulae not organized into groups. Carapace ornamentation as weakly developed longitudinal lines and dense longitudinal striae between them.

Head relatively small, low triangular-rounded in lateral view. In lateral view rostrum relatively narrow, protruding downwards. Ocellus of moderate size, eye larger than ocellus. Distance from tip of rostrum to ocellus 1.5–2 times greater than that between ocellus and eye.

Head shield (Fig. 1 H–I) with maximum width behind mandibular articulation, covered by fine striae as on valves. Rostrum short and rounded. Posterior margin as prominent, acute distal angle. Two major head pores (Fig. 1J, 2C, H) of same size, with a narrow connection between them. PP slightly less than 2 IP in adults. Lateral head pores located in small depressions more than 1 IP distance from midline, at level before anterior major head pore. Striae absent around head pores. A small pore-like thickening behind the posterior angle of head shield. Labrum (Fig. 1 K–L) of moderate size. Labral keel moderately wide, with a blunt apex. Anterior margin of keel in studied specimens polygonal, posterior margin weakly convex to straight, with two clusters of short setulae.

Thorax two times longer than abdomen. Dorsal surface of abdominal segments not saddle-shaped. Abdominal joint not developed.

Postabdomen (Fig. 4 A–B) large, subrectangular, moderately high, with parallel margins, weakly narrowing at the end. Length about 2.5 height. Ventral margin straight. Basis of claws bordered from distal margin by clear incision. Distal margin almost straight; distal angle rounded, sometimes weakly protruding. Dorsal margin weakly convex to straight in postanal portion and weakly concave in anal one, with distal part about 2.5 times longer than preanal one, with postanal portion 2.5–2.8 times longer than anal one. Preanal angle well expressed, postanal angle not defined. Preanal margin (Fig. 3B, 4C) with 11–13 well-developed, sharp composite denticles, each with 2–6 spinules along anterior margin; size of denticles increasing distally. Length of longest denticles about 1.5 the width of base of postabdominal claw, more than 3 times exceeding the width of the denticle base. Postanal portion with 10–12 broad lateral fascicles, posteriormost setula of each fascicle longest, thicker than others, 1.5 times shorter than neighboring marginal denticles. Anal portion with several smaller fascicles, spaced irregularly.

Postabdominal claw (Fig. 3C, 4D) of moderate length, slightly longer than preanal portion of postabdomen. Basal spine long and slender, about 0.35 length of the claw. A row of 7–10 long setulae on basal spine and the base of postabdomen.

Antenna I (Fig. 1M) of moderate size, length about 3 widths, with a cluster of 2–4 long setulae about one third the length of antennule at anterior face. Antennular seta thin, more than 1/2 length of antenna I, arising subterminally. Nine terminal aestetascs, two of them long and thick, longer than antenna I itself, all other much shorter, about 1/2–1/3 the length of antenna I.

Antenna II relatively short (Fig. 1N, 2G). Antennal formula, setae 0-0-3/1-1-3, spines 1-0-1/0-0-1. Basal segment robust, with very short seta between branches; branches relatively short; all segments cylindrical, slender. In each branches basal segment being longest, middle segment slightly shorter than apical segment. Seta arising from basal segment of endopodite thin, reaching above the end of endopodite. Seta arising from middle segment of endopodite, of similar size to apical setae. Two shortest apical setae on exopodite and two on endopodite with a very long spinule at the point of articulation. Spine on basal segment of exopodite slightly longer than middle segment. Spines on apical segments much shorter than apical segments.

Trunk limb I of moderate size (Fig. 5 A–D), with ovoid epipodite bearimng a short process. A long accessory seta only slightly shorter than ODL seta, with long setulae in distal part. ODL with one seta. IDL with three setae and several clusters of hard setulae. IDL seta 1 large and broad, claw-like, strongly curved, slightly longer than seta 2; IDL setae 2 and 3 with thin setulae in distal part, seta 3 slightly shorter than ODL seta, seta 2 about 2/3 the length of seta 3. Endite 3 with four setae subequal in length. Endite 2 (Fig. 3C) with two long distally setulated setae (e–f), a short- er seta near their base (d) and a naked inner seta (2) and small sensillum on anterior face of limb; seta (e) long, two times longer than seta (f), almost as long as limb body. Endite 1 with two 2-segmented setae (g–h), both setulated in distal part; a flat plumose seta pointed the limb base (i), and a naked inner seta (3). Inner seta 3 1.5 times longer than inner seta 2. Seven-eight rows of thin long setulae on ventral face of limb. Two ejector hooks, one slightly shorter than other. Maxillar process elongated, with a short setulated seta in distal part (not represented in Fig. 5A).

Trunk limb II subtriangular (Fig. 5E). Exopodite elongated, with a slender seta as long as exopodite body itself, and a cluster of very long setulae distally. Eight scraping spines, increasing in length distally; in some populations scraper 4 being significantly shorter than scrapers 3 and 5. Size of denticles on basal part of spines decreasing from basal to distal spines. Distal armature of gnathobase with four elements. Filter plate II with seven setae, the posteriormost member shorter than others.

Trunk limb III (Fig. 5 F–G) with ovoid epipodite lacking a projection. Exopodite of irregular shape, with seven setae. Seta 3 being longest, seta 6 about 1/3 length of seta 3, seta 7 1.5 times shorter than seta 6, other setae very short. Setae 1–5 plumose, seta 6 with three rows of thick setulae in distal part, seta 7 with thin setulae on its distal part. Distal endite with 3 setae, two distalmost members scraping, slender, sharp, with denticles in distal part; two short bottle-shaped sensillum located between their bases; basalmost seta flattened, as long as scraping seta 2, bilaterally armed with long setulae. Basal endite with 4 plumose setae increasing in size basally. Four pointed soft setae increasing in size basally; a small bottle-shaped sensillum near the distalmost seta. Distal armature of gnathobase with four elements, the first one elongated, narrowing distally sensillum; second strongly geniculated seta, third and fourth spines. Filter plate III with seven setae.

Trunk limb IV (Fig. 5 H–J) with setulated pre-epipodite; epipodite oval, with a finger-like projection shorter than exopodite itself. Exopodite rounded, with six setae. Seta 1 and 2 being longest, equal in length; seta 3 slightly shorter; seta 4 about 2/3 length of seta 1, bending toward seta 3; setae 5–6 subequal in length, slightly shorter than seta 4. Setae 1–4 plumose, setae 5–6 with short setulae. Inner portion of limb IV with four setae and narrowing distally sensillum. Scraping seta (1) slender, three flaming-torch seta (2–4) decreasing in size basally, with 7–9 long setulae each. Small sensillum between the base of middle and basal flaming-torch setae. Three soft setae (a–c) increasing in size basally. Gnathobase with one long 2-segmented setae, a small hillock distally and a sensillum. Filter plate IV with five setae.

Trunk limb V (Fig. 5 J–K) with setulated preepipodite; epipodite oval, with a finger-like projection as long as epipodite itself. Exopodite divided into two lobes, with four plumose setae: setae 1–3 long, subequal in length; seta 4 three times shorter. Inner lobe broad, rounded with setulated inner margin. At inner face, two setae densely setulated in distal part, one of them very long, slightly longer than exopodite setae; the other two times shorter. Filter plate V with three setae, broad-based hillock and a sensilla-like structure between inner face setae and filter plate.

Trunk limb VI (Fig. 5L) as elongated oval lobe with setulated margin.

Ephippial female (Fig. 1O, 2 D–E) with body slightly higher than parthenogenetic female; dorsal margin without depression between valves and head shield. Ephippium (Fig. 2F) yellow-brown in preserved specimens, with weakly developed egg locules, covered by weak polygonal sculpture and longitudinal striae.

Male. Body in juvenile male of instar II (Fig. 1R) is similar to that of female. In adult, body (Fig 1P, 3 D–E, 6A) low oval, with maximum height at the middle or in the third quarter of the body, height/length ratio about 0.54.

Postabdomen in juvenile male of instar II (Fig. 4G) similar to that of female; gonopore located at ventral margin, at 2/3 distance from the base. Armament of postabdomen same as in female. In adult (Fig. 3F, 4 E–F, 6C–D) of moderate size, with maximum height at postanal angle, evenly narrowing in postanal portion. Length about 2.5 height. Ventral margin almost straight. Postabdominal claw situated on small protrusion in ventral portion of distal margin. The sperm duct opens above the protrusion; posteroventral and posterodorsal angles rounded. Dorsal margin weakly convex in postanal portion and weakly concave in anal one. Preanal angle weakly defined; postanal angle not defined. Preanal margin almost straight. Distal portion of postabdomen 2–2.5 times longer than preanal one, postanal portion 2.2–2.4 times longer than anal one. Clusters of thin setulae in place of female marginal denticles on postanal margin. Lateral fascicles of setulae as in female. Postabdominal claw weakly curved, shorter than in female, significantly shorter than preanal margin. Basal spine about 0.25–0.3 length of claw.

Antenna I in juvenile male of instar II (Fig. 1S) slightly broader than in female, with 9 terminal aesthetascs, with anlage of male seta about 1/4 length of antenna I. In adult (Fig. 1Q) it is broader than in female, with 12 terminal aesthetascs, two longest almost as long as antenna I itself, others about 2/3 length of antenna I. Male seta subterminal, about 1/3 length of antenna I.

Trunk limb I in juvenile male of instar II (Fig. 5O) with curved copulatory hook, about 2/3 length of limb itself, and anlage of copulatory brush seta. IDL with anlage of male seta, IDL setae 1–3 similar to these of female. Ventral face of the limb under the copulatory brush seta with row of about 5 stiff setulae of moderate length. Inner seta (1) of endite 3 same as in female. In adult (Fig. 5 M–N, 6E–F) with U-shaped copulatory hook, about 2/3 length of the limb itself. The IDL has four setae: IDL seta 1 not hook-like, but still well-developed, about 2/3 length of IDL seta 2; IDL setae 2 and 3 much thinner than in female; the male seta curved, about 2/3 length of IDL seta 2. Copulatory brush seta about half length of IDL seta 2. Ventral face of the limb under the copulatory brush with row about 5 stiff setules of moderate length, outer face of endite 3 with row of about 15 shorter setulae, there is a narrow gap between these two rows. Inner seta (1) of endite 3 thinner than in female, with long setulae distally.

Size. In females of first juvenile instar, length 0.47–0.52 mm, height 0.26–0.29 mm; in females of second juvenile instar, length 0.58–0.65 mm, height 0.30–0.35 mm. In adult females length 0.68–0.94 mm, height 0.40–0.52 mm. Length of adult male 0.60–0.67 mm, height 0.32–0.38 mm.

Differential diagnosis. A. sibirica sp. nov. is a typical member of the affinis -group of Alona s. lat. and shares all distinctive characters of the group: (1) large size, (2) two major head pores, (3) angular posterior portion of the head shield, (4) massive claw-like IDL seta 1, (5) exopodite III with seven setae, (6) presence of limb VI (Sinev 2013). It clearly differs from all other species of the group (see Sinev 1997, 1998, 2009, 2013) in presence of denticles on the posteroventral angle of valves. Note that populations of the affinis -group with denticles on postero-ventral corner of valves were never reported outside the temperate regions of Eurasia (Smirnov 1971; Sinev 1997, 1998, 2009, 2013), so A. sibirica sp. nov. is presumably the only species of the group with denticles on the posteroventral corner of the valves. The latter also differs from A. ossiani in male postabdomen evenly narrowing in postanal portion (males are unknown for other species of the group).

Alona sibirica sp. nov. clearly differs from the other common Palaearctic taxon, A. affinis, in the adult male morphology. The latter has (1) a completely different postabdomen shape, with a right ventrodistal angle and parallel dorsal and ventral margins in basal part of postabdomen, (2) limb I with a thin, short IDL seta 1 located laterally, (3) smaller number of setulae on outer face of endite 3, and (4) greater size (length of adult male of A. affinis is 0.63–0.73 mm). Females of A. sibirica sp. nov. significantly differ from those of A. affinis only in morphology of the posteroventral angle of valves, and have somewhat longer seta (f) on the endite 2 of limb I; therefore the differences in female morphology between species of the affinis -group are not numerous (Sinev 1997, 1998, 2009, 2013).

Distribution. A. sibirica sp. nov. is common in Asian Russia; it is also recorded in Mongolia and Kyrgyzstan. In Europe the species is rare, it was recorded in a few water bodies of Sweden, Finland, Ukraine, and European Russia.