Alona werestschagini Sinev 1999

Alona werestschagini Sinev, 1999

(Figs. 1-4)

Smirnov, 1971: 380, Figs. 450-451 (guttata guttata partum); Chiang, 1963a: Plate VI: Fig. 39–40 (Alonella rostrata); Chiang & Du, 1979: 227, Fig. 155A–D (pulchella); Sinev, 1999: 23–30, Fig. 1–4; Sinev, 2002: 938, Fig. 1 л, 3и,с, 4и, 5д,л; Sarmaja- Karjonen & Sinev, 2008: 44–46, Fig. 2A–F; Kotov et al., 2010: 247, Fig. 141: 12–14; Korovchinsky et al., 2021b: 288–289, Fig. 85: 10–14.

Material studied here: Russia: 10 parthenogenetic females from Karelia Republic, Kem’ District, small lake in Letnyaya River floodplain, N 65.10323°, E 33,88282°, 16.08.2020, coll. P.G. Garibian, AAK M-5985; 12 parthenogenetic females, 5 males from Tyva Republic, 18.09.2020, groundwater-fed flow-through pond in Hjut River floodplain, N 52.653925°, E 94.547988°, coll. N. A. Kirova; 2 parthenogenetic females, 8 ephippial females, 5 males from Chukotka Autonomous Area, Providenskii District, small tundra lake near Kivak settlement, 7.09.2015, N 64.26075°, E 172.9531°, coll. E. Kuzmicheva, D. Vasyukov, AAK M-5844. Mongolia: 1 parthenogenetic female from Tuv Aimag, Ugii lake, N 47.75037°, E 102.7697°, 9.07.2005, coll. Ch. Jersabek, AAK 2008-005; 4 parthenogenetic females from a shallow pool in the basin of Khovsgol lake, N 51,28761°, E 100,8277°, 28.07.2005 coll. Ch. Jersabek, AAK 2008-014.

Material studied earlier: see Sinev (1999, 2002) for the list of material from, Kyrgyzstan, Tien Shan, and Russia (Altai Mountains, Komi Republic and Kola Peninsula).

Description. Parthenogenetic female. General. Body (Fig. 1A) oval in lateral view, strongly compressed laterally, of moderate height, maximum height at middle of the body; in adults height/length ratio 0.67–0.70. Dorsal margin uniformly curved; postero-dorsal and postero-ventral angles broadly rounded; posterior margin uniformly curved; ventral margin almost straight; antero-ventral angle rounded. Body moderately compressed laterally. Valves with weakly developed longitudinal lines. Ventral margin (Fig. 1B) with 40–50 setae; anterior setae short, not forming separate group from central setae (Fig. 1C); posterior 20 setae of moderate length, decreasing in length posteriorly (Fig. 1D). Postero-dorsal angle (Fig. 1E) bears about 70 short, thin, hair-like setules of similar length, not organized into groups. A row of about 80–100 thin setules along the posterior margin on inner side of valve.

Head of moderate size, triangular-rounded in lateral view, rostrum short, pointing downward. Eye and ocellus large, of similar size. Distance from tip of rostrum to ocellus in adults 1.5–2 times greater than that between ocellus and eye. Head shield (Fig. 2A) with maximum width behind mandibular articulation, without any sculpture; rostrum short, broadly rounded; posterior portion of head shield subtriangular with broadly rounded distal angle. Three connected major head pores of similar size, PP about 0.7–1.1 IP (Fig. 1F, 2B). Lateral head pores minute, located at about 0.8 IP distance from midline, at the level of middle major head pore. Labrum (Fig. 1G) of moderate size; labral keel narrow (height about 2 width), with rounded apex; anterior margin of keel convex, posterior margin with two clusters of short setules.

Thorax twice longer than abdomen, dorsal surface of abdominal segments not saddle-shaped.

Postabdomen (Fig. 1H–I, 4B) short, of moderate width, slightly narrowing distally, with a rounded, slightly prominent distal angle; length about 2.5 height. Ventral margin straight. Base of claws separated from distal margin by weak incision. Distal margin weakly convex to straight. Dorsal margin with distal part 1.6–1.8 times longer than preanal one, with postanal portion equal to or slightly shorter than anal. Postanal portion of dorsal margin straight, anal portion weakly concave. Preanal angle well-defined, protruding; postanal angle wealy defined.

Postanal and distal part of anal margin with 8–10 well-developed composite marginal denticles, decreasing in size basally, most bearing spinules on anterior margin; length of distal denticles equal to the width of the postabdominal claw base. Basal half of anal margin with 2–3 groups of very short setulae. Postanal margin with 9–12 lateral groups of setulae; six-seven distalmost groups wide, consisting of thin setulae, with longest setulae in the middle, slightly shorter than longest marginal denticles. Several additional groups above the main row in anal portion. Postabdominal claw curved, of moderate length, slightly longer than preanal portion of postabdomen. Basal spine short, almost straight, about 0.2 of claw length.

Antennule (Fig. 1J) of moderate size, length about 2.5 width, with four clusters of thin setules at anterior face; setae of two distalmost central clusters long. Antennular sensory seta slender, two times shorter than antennule, arising at 2/3 distance from the base. Nine terminal aesthetascs, length of two longest of them about half length of antennule.

Antenna (Fig. 1K) relatively short. Antennal formula, setae 0–0–3/1–1–3, spines 1–0–1/0–0–1. Basipodite robust, branches of moderate length and width, basal segments of both branches 1.5 times longer than others. Seta arising from basal segment of endopodite thin, not reaching the end of endopodite. Seta arising from middle segment of endopodite of similar size with shortest apical setae. Spine on basal segment of exopodite slightly shorter than middle segment. Apical spines slightly shorter or equal to apical segments.

Thoracic limbs: six pairs.

Limb I (Fig. 3A–C) of moderate size. Epipodite with long process 3–4 times longer than exopodite body. Accessory seta about 1/3 length of ODL seta. IDL with three setae and two-three clusters of small setules; setae 2 and 3 only slightly shorter than ODL seta, armed with thin setules in distal part; seta 1 very short, located laterally. Endite 3 with four setae, inner seta (1) shorter than setae a–c. Endite 2 with three setae (d–f), setae e–f of similar length, 1.5 times shorter than limb itself. Endite 1 with two two-segmented setae (g–h), both setulated in distal part, and very small, rudimentary triangular seta i. No inner seta on edites 1 and 2. Five-six rows of thin long setules on ventral face of limb. Two ejector hooks, one of them little larger than another one. Maxillar process elongated, with a short seta.

Limb II (Fig. 3D–E). Exopodite elongated, with setulated seta almost as long as exopodite body. Eight scraping setae (1–8), armed with small spinules increasing in length distally; scraper 3 thicker than neighbors and armed with thicker spinules. Distal armature of gnathobase with four elements. Filter plate with seven setae, the posteriormost seta three times shorter than others.

Limb III (Fig. 3F–G). Epipodite oval, without process; exopodite trapezium-shaped, with seven setae. Seta 3 being longest, setae 4 and 6 about 1/2 and 2/3 length of seta 3, setae 1 and 7 about 1/3 length of seta 3, setae 5 and 2 short. Setae 6–7 armed with short setules in distal portion, all other setae plumose. Distal endite with three setae (1–3), two distalmost members slender, sharp, with distal parts unilaterally armed with sharp denticles, seta 1 slightly longer than seta 2; basalmost seta (3) as long as seta 2, broad, bilaterally armed with setules. Basal endite with four setae (4–7), increasing in size towards the base; a small sensillum (s) near the base of distalmost seta. Four inner setae increasing in size basally (a–d). Gnathobase not clearly separated from basal endite. Distal armature of gnathobase with four elements: an elongated, cylindrical sensillum; thin, bent seta; two sharp spines. Filter plate III with seven setae.

Limb IV (Fig. 3H–I). Preepipodite setulated, epipodite with process two times longer than exopodite body. Exopodite subquadrangular, with six setae. Seta 3 longest, setae 1–2 about 4/5 lengths of seta 3, setae 4, 5 and 6 of 1/4, 1/3 and 1/3 length of seta 3, respectively. Setae 1–4 plumose, setae 5–6 with short setules in distal portion. Inner-distal portion of limb IV with four setae and sensillum of characteristic shape: seta 1 slender, sharp; first flaming-torch seta (2) broad, with about 10 thick setulae; two other thin, slender, with thin hair-like setulae. Small sensillum is located between the bases of setae 3 and 4. Three inner setae (a-c) of similar size. Gnathobase with a long 2-segmented seta, elongated sensillum and a small hillock distally. Filter plate with five setae.

Limb V (Fig. 3J). Preepipodite setulated; epipodite with process 1.5 times longer than exopodite body. Exopodite divided into two lobes, with four plumose setae. Setae 1–3 long,decreasing in size basally; seta 4 short, 2.5 times shorter than seta 1. Inner limb portion as a small oval lobe, with setulated inner margin. At inner face, two setae, one about 3/4 length of exopodite seta 1, and other 2 times shorter. Filter plate with three setae, large semicircular sensillum located near it.

Limb VI (Fig. 3K) as elongated oval lobe with setulated margin, as long as exopodite V; length about 2 widths.

Ephippial female (Fig. 2C, 4A) with body slightly higher than in parthenogenetic female, with defined posterodorsal angle and almost straight posterior margin of valves. Ephippium intensive yellow-brown in preserved specimens, with well-developed sculpture in shape of thick longitudinal, sometimes anastomosing lines, much thicker than those on the rest of valves.

Male. Body low oval to ovoid (Fig. 1L, 2D), with maximum height at the middle, height/length ratio about 0.6.

Postabdomen (Fig. 1M, 2G, 4D–E) elongated, strongly narrowing distally in anal portion; in postanal portion narrow, with almost parallel margins. Distal margin almost straight. Postanal margin of postabdomen evenly comes to the base of claws; dorso-distal and dorso-ventral angles obtuse. Length about three heights. Ventral margin weakly convex to straight. Sperm ducts open at the end of postabdomen above the base of claws. Dorsal margin weakly concave to straight in postanal portion and weakly convex in anal one. Preanal and postanal angles not defined. Preanal margin convex. Distal portion of postabdomen 2.5 times longer than preanal one, postanal portion 1.5 times longer than anal. Postanal margin with 7–8 clusters of strong spinules in distal portion. Distal half of postanal portion with clusters of thick, sharp spinules laterally; posterior half of postanal margin and anal margins with about 10 groups of lateral setules in a row, similar in morphology to those of female, but somewhat thicker. Postabdominal claw (Fig. 1G) short, curved, 1.5 times shorter than preanal margin of postabdomen. Basal spine absent.

Antennule (Fig. 2H) slightly broader than in female, with 12 long terminal aesthetascs; longest aestetascs about 2/3 length of antennule. Male seta located at 2/3 distance from the base, slightly shorter than half-length of antennule.

Thoracic limb I (Fig. 3L–M) with V-shaped copulatory hook, 1.5 times shorter than limb itself. IDL seta 1 absent, IDL setae 2 and 3 of similar size, much thinner and shorter than in female; male seta strongly curved, longer than setae 2–3. Copulatory brush seta slightly shorter than IDL setae 2–3. Ventral face of the limb under the copulatory brush with row of 20–30 long stiff setules; base of endite 3 with row of about 15 short setules. Inner seta (1) of endite 3 longer and thinner than in female, longer than seta c, unilaterally armed with long setules in distal portion.

Size. In adult females, length 0.42–0.54 mm, height from 0.29–0.36 mm. In adult male, length 0.32–0.38 mm, height 0.2–0.24 mm.

Distribution and ecology. Iceland: Svarbald Islands, north of Scandinavia. Russia: North Karelia, Kola Peninsula, Komi Republic, Wrangel island, Chukotka Autonomous Republic, north Kamchatka Peninsula, Bering island, Altai mountains, Sayan mountains. Kyrgyzstan: Tien Shan mountains. China: Tibet plateau. India: Hymalaya. Mountains of West Mongolia. Species is predominantly found in cold oligotrophic lakes, and in small floodplain water bodies.