Chydorus izvekovae Sinev & Novichkova & Chertoprud 2022, sp. nov.

Chydorus izvekovae sp. nov.

Figs. 1-6

Type locality. A small thermokarst lake near Yanskiy village, Olsky District, Magadan Area, Russia (N59°45.994' E149°29.622'), samples were collected on 10.07.2014 and 06.07.2015 by A.A. Novichkova.

Type material. Holotype. A parthenogenetic female from the type locality, deposited in the Zoological Museum of M. V. Lomonosov Moscow State University, Ml 251.

Paratypes. 30 parthenogenetic females from the type locality, deposited in the Zoological Museum of M. V. Lomonosov Moscow State University, Ml 252; 47 parthenogenetic females from lake near Yanskiy village, Olsky District, Magadan Area, Russia, N59°46.659' E149°29.4416'), 10.07.2014 and 06.07.2015, coll. A.A. Novichkova; 6 parthenogenetic females from small lake in vicinity of Klyopka village, Olsky District, Magadan Area, Russia (N59°47.9015' E151°24.7691'), 10.07.2015, coll. A.A. Novichkova.

Etymology: the species is named after our late teacher, colleague and friend, prominent Russian chironomidologist Evelina Ivanovna Izvekova (1939-2021).

Description. Parthenogenetic female. Body shape typical of the genus Chydorus, oval in juveniles (Fig. 1A) and rounded in adults (Figs. 1B, 2A–D) in lateral view, height/length ratio about 0.9, maximum height at midline. In frontal and posterior view (Figs. 2E–F), body subtriangular, with weakly convex dorsal surface and moderately developed egg locules, significantly narrowing in ventral half. Valves and head shield covered everywhere by numerous honeycomb-like meshes, not organized into ridges or collars (Figs. 2–4). Height of mesh walls low, about 10 µm in highest meshes. Surface of valves inside the meshes oblique or with irregular lines.

Valves. Anterior corner broadly rounded, with a submarginal flange at inner side. Ventral margin of valves (Fig. 1C) with about 50–55 setae; 10–12 anterior setae very thin, short, located on the inner side of valves close to margin, followed by about ten very short setae located maginally. Posterior group consists of over 30 long setae, armed bilaterally with long setules (Fig. 1D), located on inner side of valve; maximum length of setae at the middle of the group. Two-three posteriormoist denticles straight, thick, spine-like (Fig. 1F). Postero-ventral corner without denticles. Valves covered by honeycomb-like polygonal sculpture, over 350 cells per valve (Figs. 2D, 3B–D). Largest cells are located at egg locules, at the border with head shield, size of cell gradually decreases toward outer margins of valves.

Head with a short rostrum, protruding downward and posteriorly (Fig. 2C). Length of rostrum about 1.5 length of antennule. Ocellus 1.5 times smaller than eye. Head shield large, with maximum width at the middle of posterior portion; its posteriormost portion widely oval (Figs. 1F, 2A–B). Rostrum (Figs. 1G, 3F) wide triangular in frontal view, apex divided into two small asymmetric lobes. Head shield covered by a honeycomb-like polygonal sculpture, consisting of over 300 asymmetrically spaced, irregularly varying in size meshes (Fig. 4A). Largest meshes are located at posterior margin of head shield. Two major head pores located at the end of cuticular tubes of same height as height of surrounding mesh walls (Fig. 3E), PP = 1.2–1.5 IP. Lateral head pores minute, located asymmetrically, close to midline of head shield, a little closer to posterior major head pore.

Labrum with elongated narrow triangular labral keel (Figs. 1H–I) with narrow elongated apex. Height of keel about 2 widths. Anterior margin of labral keel convex, posterior margin concave, apex with rounded tip.

Postabdomen (Fig. 1J) short, rather narrow, weakly narrowing distally. Length about 3.5 heights. Ventral margin weakly concave. Basis of claws bordered from distal margin by clear incision. Distal margin convex, distal angle rounded. Dorsal margin straight in postanal portion and weakly concave in the anal one. Distal part of postabdomen about 1.5 times longer than the preanal portion; postanal portion longer than anal portion. Preanal angle well-expressed, triangular, prominent; postanal angle not defined. Preanal margin weakly concave. Dorsal margin with about 10 narrow sharp denticles and 2–3 broad groups of short setules in anal portion. Length of longest denticles 1.5–2 times greater than width of base of postabdominal claw. A row of 8–9 broad lateral groups of very short setules; in anal portion additional groups located above the main row. A single studied specimen had an abnormal armament of the postabdomen (Fig. 1L), with more numerous denticles and a single denticle placed laterally, in a row of lateral setules.

Postabdominal claw (Fig. 1K) slender, weakly curved, 1.5 times shorter than preanal portion of postabdomen, with distinctive pecten of setulae on dorsal margin. Basal portion of pecten consists of 7-10 short spinules, distal portion of about 30 longer setulae. Single long setula located ventrally near the end of the claw. Two basal spines; distal spine 0.2 length of the claw, proximal spine two times shorter. A pecten of spinules on dorsal side of claw and a longer spinule on its tip.

Antenna I (Fig. 1M) of moderate size; length about two widths. Antennular seta thin, slightly shorter than antennule, arising at half distance from the base. Nine terminal aesthetascs, two longest about 2/3 length of antennule.

Antenna II relatively short (Figs. 1N–O). Antennal formula: setae 0-0-3/0-1-3; spines 1-0-1/0-0-1. Branches relatively short; proximal segment of both branches 1.5 times longer and thicker than two others. Seta arising from middle segment of endopodite two times shorter than apical setae.Apical segments of both branches with two apical and a single subapical seta of similar size. All antennal spines very short.

Mandible morphology typical of the genus. Maxilla I with three short setae (Fig. 5A).

Limb I of moderate size (Figs. 5B–D). Epipodite oval, with a finger-like projection 1.5 times longer than exopodite itself. ODL with two setae, one of them very small. IDL with three setae and several clusters of setules. IDL setae 1–2 thin, about 1/3 and 1/2 length of longest ODL seta, respectively; seta 3 thick, strong, claw-like, little shorter than longest ODL seta, armed with about 25 hard setules in distal part. Base of IDL seta 3 two times wider than base of setae 1–2. A small sensillum located near bases of IDL setae 2–3. Endite 3 with four setae subequal in length, inner seta (1) slightly thinner than outer setae (a–c). Endite 2 with two long distally setulated setae (e–f); setae e longer than seta f; a shorter seta near their base (d) and an inner seta armed with spinules (2) on anterior face of limb. Endite 1 with three 2-segmented setae of similar size (g–i) setulated in distal part, a flat plumose seta (j) and an inner seta armed with spinules (3) on anterior face of limb. Six-seven rows of thin long setules on ventral face of limb. Two ejector hooks, first one slightly shorter than the other one.

Limb II subtriangular (Fig. 5E). Exopodite oval, with seta 1.5 times longer than exopodite. Eight scraping spines; spines 1–3 long, slightly decreasing in size basally, armed with fine spinules; spines 4–8 short, with length of setae 4–5 and 7–8 evenly decreasing basally, seta 6 much thinner and shorter than neighbours; seta 5 thicker than others, armed with thick spinules, seta 4 and 6–8 armed with spinules of moderate size. An elongated sensillum located between spines 3 and 4. Distal armature of gnathobase with four elements. Filter plate II with eight setae, the two posteriormost members much shorter than others, about 1/2 and 2/3 lengths of other setae, respectively.

Limb III (Figs. 5F–H). Epipodite oval, without projection. Exopodite subquadrangular, with three lateral (1–3) and four terminal (4–7) setae. Seta 4 being longest; setа 6 slightly shorter than seta 4; seta 7 about half length of seta 4; setae 1–3 and 5 subequal in length, about 1/3 length of seta 4. Setae 1–5 flattened, plumose; seta 6 slender, with row of very long, thick setules in basal portion and small spinules in distal portion; seta 7 slender, with short spinules in distal portion. Distal endite with 3 scraping setae (1–3) and two small sensillae located between their bases; setae 1–2 slender, of similar length, with small denticles in distal part; seta 3 small and thin, three times shorter than setae 1–2. Basal endite with six plumose setae (a–f) slightly increasing in size basally. Four pointed inner setae, seta 4–6 increasing in size basally, seta 7 as long as seta 5; an elongated sensillum near the base of seta 4. Distal armature of gnathobase with four elements: one very large sensillum with curved distal portion, strongly geniculated seta, short spine and sensillum of moderate size. Filter plate III with eight setae.

Limb IV (Fig. 5I–K). Pre-epipodite setulated; epipodite oval, with projection longer as epipodite itself. Exopodite rounded with seven setae; seta 1 being longest, length of setae decreases evenly from seta 1 to seta 4; seta 4 about 2/3 length of seta 1; seta 5 about 1/3 length of seta 1; seta 6 about 1/2 length of seta 1; seta 7 shorter than seta 5. Setae 1–5 flat, plumose; setae 6–7 setulated unilaterally in basal part, setulae of seta 7 shorter than setulae of seta 6. Inner portion of limb IV with four setae. Scraping seta (1) slender; three flaming-torch setae (2–4) with length of distal portion and thickness of setulae increasing from seta 2 to seta 4; seta 2 with numerous thin setules; setae 3–4 with about 10 thick setules each; small sensilla located near base of seta 3. Four inner setae (a–d) increasing in length basally. Gnathobase with one 2–segmented setae, a small hillock distally, and two small sensillae. Filter plate with six setae.

Limb V (Fig. 5L). Pre-epipodite setulated; epipodite oval, with projection two times longer than epipodite. Exopodite ovoid, with four plumose setae; setae 1–2 long, of similar length; length of setae 3–4 about 2/3 and 1/2 lengths of seta 2, respectively. Two small hillocks with thick setules located on basal side of exopodite near seta 4. Inner lobe long, narrow, with setulated inner margin. At inner face, two setae of similar length, in distal portion armed unilaterally with thick setules. Filter plate with four setae.

Ephippial female and male unknown.

Size. Length of ovigerous females 0.38–0.52 mm, height 0.32–0.43 mm; length of smallest juvenile female 0.29 mm, height 0.2 mm.

Differential diagnosis. Chydorus izvekovae sp. nov. has a honeycomb-like sculpture of the valves and head shield, typical of the faviformis -group of the genus. It clearly differs from C. bicornutus in the absence of lateral projections of valves and from C. bicollaris in uniform meshes of the valves, not forming lateral collars (see Frey 1982b). It clearly differs from all other species of the group (see Frey 1987) in: (1) narrow triangular labral keel with an elongated apex; in all other species labral keel is low oval with rounded or broadly triangular apex; (2) greater number of small-sized meshes on valve (over 350) and head shield (over 300), while in all other species number of meshes per valve and head shield not exceed 200, and size of meshes is much greater. C. izvekovae sp. nov. also differs from C. faviformis and C. obscurirostris in lower walls of meshes, in these two species height of the walls in dorsal part of valves is greater than eye diameter. C. izvekova e sp. nov. differs from C. angustirostris in bilobed tip of rostrum, from C. sinensis and C. opacus, in a longer and more narrow IDL seta 3, and from C. parvireticulatus, in shorter setules on IDL seta 3.

Distribution and ecology. To date the species is known only from the Olsky District, Magadan Area, Russia (Fig. 6), and the exact area of its distribution is unknown. But it is probably absent in Central Yakutia (Klimovski et al. 2015ab), North Yakutia (Novichkova et al. 2020; Chertoprud & Novichkova 2021), Khabarovsk Area (Garibian et al. 2019), Wrangel Island (Novichkova & Chertoprud 2015) and Kamchatka Peninsula (E.I. Bekker, personal comm.).

The type locality is a small thermocarst lake (area of approximately 75 m 2; depth more than 2 m). The lake has moss-covered shores with thickets of Phragmites sp. and Menyanthes sp. The bottom sediments near the shore are represented by boggy detritus. The temperature of water at the time of sampling was 18.8°C, pH 6.3; and total mineralization 9 ppm. The species was quite abundant in the samples, comprising up to 8% of the total abundance of microcrustaceans. Other cladoceran species were present in the samples (Alona quadrangularis, Alona guttata, Biapertura affinis, Chydorus cf. sphaericus, Eurycercus pompholygodes, Graptoleberis testudinaria, Macrothrix sp., Ophryoxus sp., Sida ortiva), as well as two cyclopoid species of the genus Acanthocyclops.

Specimens were found also in a two more lakes. First, located just near the type locality, has approximate area of 1200 m 2, depth more than 5 m, and moss-covered shores with moss- Carex hummoсks. The bottom sediments near the shore was represented by detritus. The temperature of water at the time of sampling was 18.9°C, pH 4.5, and total mineralization 20 ppm. The species made up to 18% of the total abundance of microcrustaceans. Moreover, cladoceran species Camptocercus streletskayae, Chydorus sp., Eurycercus macracanthus, Ophryoxus kolymensis, Scapholeberis microcephala, Sida crystallina and S. ortiva were present in the samples, as well as five more species of Cyclopoid copepods.

The second water body, a small lake (thermokarst pond) located more than 100 km to the East, contained C. izvekovae. It was only about 10 m 2 area with the depth of 4– 5 m. The pond has moss-covered shores and detritus bottom sediment as well. The temperature of water at the time of sampling was 25.8°C, pH 6.4, and total mineralization 12 ppm. Together with C. izvekovae, were Acroperus harpae, Alonella excisa, C. cf. sphaericus, E. macracanthus, Pleuroxus yakutensis, Polyphemus pediculus, S. microcephala, Acanthodiaptomus pacificus, Bryocamptus arcticus and six more Cyclopoida species.