Eurycercus chernovi, Bekker, Eugeniya I. & Kotov, Alexey A., 2016

Bekker, Eugeniya I. & Kotov, Alexey A., 2016, A revision of the subgenus Eurycercus (Teretifrons) Frey, 1975 (Crustacea: Cladocera) in the Holarctic with description of a new species from Russian Arctic, Zootaxa 4147 (4), pp. 351-376 : 364-371

Eurycercus chernovi sp. nov.

( Figs 9–12 )

Etymology. This species is named after a renown Russian zoologist Yu.I. Chernov (1934–2012), because the taxon was first detected in the samples collected by him in 1969–1971.

Type locality. A lake about 150 m in diameter, north of settlement of Ust' Tareya , Taimyr Autonomous Area, Krasnoyarsk Territory, Russia. Coordinates: N73.25792º ; E 90.59356º.

Type material. Holotype. A parthenogenetic female in 96% alcohol, MGU ML149.

Paratypes. 16 parthenogenetic ♀♀ in 96% alcohol, MGU ML150. 23 parthenogenetic ♀♀ in 96% alcohol, AAK M-2764. All paratypes are from the type locality. The type series was collected in 0 8.08.2010 by O.L. Makarova.

Other material excluded from type series: all from Taimyr Autonomous Area , Krasnoyarsk Territory, Russia: 2 parthenogenetic ♀♀ from a small foodplain lake near River Tareya, a tributary of River Pyasina, coll. in 0 9.08.1969 by Yu.I. Chernov, AAK 1999-047; 3 parthenogenetic ♀♀ from River Tareya, a tributary of River Pyasina, coll. in 0 9.08.1969 by Yu.I. Chernov, AAK 1999-051; 2 parthenogenetic ♀♀ from unknown water body near Ust' Tareya, coll. in 12.08.1971 by Yu.I. Chernov, AAK 2013-050; 50 parthenogenetic ♀♀ from a small lake (N73.26169°, E90,57555°) in valley inside hummocky tundra, coll. in 0 8.08.2010 by O.L. Makarova, AAK M- 2765; 30 parthenogenetic ♀♀ from springs (N73.2553°, E90.58605°) in gullies within hummocky tundra, coll. in 0 8.08.2010 by O.L. Makarova, AAK M-2767; 8 parthenogenetic ♀♀ and 2 ♂♂ from a small lake 1 (N73.25873°, E90,58963°) at the margin of a polygonal bog, near outlier (depth about 2–2.5 m), coll. in 0 7.08.2010 by O.L. Makarova, AAK M-2768 ; 15 exuvia of parthenogenetic ♀♀ from Lake Taimyr (N74.66°, E100.9°), N portion of Taimyr Peninsula, coll. in 0 4.08.1999 by S.D. Pavlov, AAK 1999-092; 2 parthenogenetic ♀♀ from puddles 1–3 (N74.32°, E86.91°) near the creek, Arctic Coast of Taimyr Peninsula, coll. in 24.06.2007 by I.Yu. Popov, AAK M- 0570; 5 parthenogenetic ♀♀ from a swamp (N74.32°, E86.91°) at a creek, Arctic Coast of Taimyr Peninsula, coll. in 24.06.2007 by I.Yu. Popov, AAK M-0571.

Short diagnosis. Adult parthenogenetic female. In lateral view body sub-ovoid. In anterior view, body not compressed laterally, median dorsal keel absent. Dorsal head pores located on flat head shield, without any projection of the latter in this region, but a ring of head pores slightly projected above the level of head shield as a very low "pancake", and a surface of major head pore cuticle with a low projection in middle. Lateral head pores as longitudinal slits. Rostrum relatively short for subgenus. Ocellus very small. Labrum with a large median keel, with angled apex, projecting beyond distal end of antenna I. Postabdomen subovoid, with a deep anal embayment. Preanal teeth pointed, a small gap between basalmost teeth and bases of postabdominal setae. Teeth in distal part of postanal margin large, long, singular; spines at base of pre-claw portion predominantly single. Postabdominal claw relatively robust, with two basal spines. Antenna I elongated, with antennular sensory seta arising subdistally, at 1/ 4 antenna I length from end. Denticles in rows encircling antennular surface very small. On antenna II, spine situated on proximal segment of exopod shorter than second segment. Limb I IDL with a strong hook-like seta, the smallest IDL seta short. IDL with 11–15 distal spinulae, 10–16 proximal spinulae and 7–16 basal spinulae. Eight setae in filter plate II, 10 setae in filter plate III, 10 setae in filter plate IV, and 10 setae in filter plate V. Exopodite V with 7 setae. Intestine with a double loop, posterior intestinal caecum absent.

Male. Body more elongated as compared with female. Rostrum short, labral keel small, with a well-developed angular apex. On pre-claw portion of postabdomen there are postanal teeth, distalmost members particularly large, teeth at base of pre-claw portion short, predominantly single. Antenna I elongated, antennular sensory setae short, located subdistally at distance of about 1/4 antenna I length from its distal end; male setae short, located almost medially. Nine aesthetascs distally; additional aesthetascs on anterior and posterior surfaces on antenna I body. IDL of limb I shorter than that in female, with male seta about two times longer than shorthest IDL seta and with two groups few spinules. Limb I supplied by a strong clasping hook with two transverse ridges distally.

Description. Parthenogenetic female. In lateral view body sub-ovoid ( Fig. 9 A), maximum height of the body in its middle portion (BH/BL= 0.63–0.74); dorsal margin in general convex; postero-dorsal angle well expressed; posterior margin slightly convex; postero-ventral angle broadly rounded. In larger adults ventral margin with a slight prominence immediately anterior to the margin middle. In anterior view, body somewhat compressed laterally, maximum width of body at level of mandibular articulation; median dorsal keel absent ( Fig. 9 B). Intestine has a double loop, posterior intestinal caecum absent ( Fig. 9 A).

Head large, with dorsal margin regularly arched from rostrum to region of dorsal head pores. Border line between head shield and valves obscure in preserved animals observed in lateral view. Rostrum short ( Fig. 9 C). Compound eye rather large, located near dorsal margin of head markedly closer to rostrum extremity than to head pores. Ocellus very small, located at antenna I base, closer to eye than to tip of rostrum. A single major “head pore” as a ringed, sub-oval field of special cuticle without any projection of head shield in this region, but a ring of head pores slightly projected above the level of head shield as a very low "pancake", and a surface of major head pore cuticle with a low projection in middle ( Fig. 9 D–G). Lateral pore minute, slit-shaped and located at either side of major pore, closer to border of the head pore ( Fig. 9 H).

Labrum fleshy body, with a large median keel ( Fig. 9 C), terminating in a well-developed apex, which seems to be angular; anterior margin convex, without a setulation; posterior margin almost straight; distal labral plate with a rich setulation ( Fig. 9 C). Paired lateral projections on labrum well-developed, horn-like, with apexes directed anteriorly ( Fig. 9 I).

Valves generally subovoid (VL/BL= 0.76–0.80), with a very obscure reticulation. Antero-ventral portion of valves slightly prominent anterior, with a special narrow flap ( Fig. 9 J). Continuous row of setae along ventral rim of valves; setae in anterior portion short ( Fig. 9 J, M), then sharply increasing in size posterior to the prominence on ventral margin ( Fig. 9 M, L), and finally gradually decreasing in size to postero-ventral valve portion. Posteroventral angle with a row of spinulae, with minute setules between them. This row continues on ventral portion of posterior margin ( Fig. 9 M, N).

Thorax without external traces of segmentation, with six pairs of limbs. Abdomen thick, no abdominal projections on dorsal part of all segments.

Postabdomen as a large (PL/BL= 0.40–0.44), relatively broad ( PH /PL= 0.44–0.51), flattened plate ( Fig. 10 A); ovoid, with dorsal and ventral margins coming together distally. Dorso-distal (preanal) angle well expressed, distal anal embayment relatively deep, dorsal portion of distal (=anal) margin slightly concave; postanal angle obtuse, rounded ( Fig. 10 B). Pre-claw portion of postabdomen as a conical prominence. Ventral margin of postabdomen slightly convex. Armature of the preanal margin as a series of preanal teeth, slightly and fluently increasing in size in distal direction; a gap lacking any teeth at base of postabdominal setae; teeth in middle of preanal margin with sharp tips (NT= 88); distal most tooth somewhat larger than the others, located just on dorso-distal angle of postabdomen. On pre-claw portion of postabdomen there are clusters of postanal teeth, distalmost members (located on postanal portion of postabdomen) particularly large, predominantly clustered, teeth at base of pre-claw portion short, predominantly single ( Fig. 10 B).

Postabdominal setae short (with length less than third of preanal margin length), bisegmented, distal segment slightly shorter than basal one and bilaterally setulated. Setae located on a low, nut-like base.

Postabdominal claw relatively robust (CL/PL= 0.23–0.26); with massive base; weakly and evenly tapered in distal direction, and slightly curved. Two basal spines, first (distal) long (DS/CL= 0.30–0.35), second (basal) short (BS/CL= 0.1–0.17; BS/DS= 0.30–0.50), located dorsally immediately at base of claw.

Antenna I relatively long (AL/BL = 0.11–0.18; AL/DA= 3.6–4.4); maximal width in basal half, with its distal 2/3 portion evenly tapering distally ( Fig. 10 C); protruding greatly beyond tip of rostrum ( Fig. 9 C). Slender antennular sensory setae relatively short (about third of antenna I length), arising subdistally, at 1/3 antenna I length from end ( Fig. 10 C). Nine bisegmented esthetascs, pointed teeth around them. Numerous short rows of minute denticles encircling antennular surface.

Antenna II relatively short ( Fig. 10 D). In coxal region, a projection with two bisegmented setae, unequal in length, next projection with semi-circular row of setules and a distalmost projection with numerous strong spinulae. Massive basal segment with a relatively long seta distally on anterior surface, and rows of short setules. Both branches with elongated segments, being basalmost members particularly elongated; all segments with rows of short setules. Setae 0–0–3/1–1–3; both apical and lateral setae long, clearly bisegmented, with long hairs on both basal and distal segments. Spines 1–0–1/0–0–1. No additional spines on distal parts of any segments of either branch. Length of apical spines and of segments from which they arise sub-equal; spine situated on proximal segment of exopod shorter than second segment.

Mandibles relatively elongated, articulated with integument between head shield and valves.

Maxilla I with three densely setulated setae and a fourth short seta-like structure.

Maxilla II absent in adults and in juveniles ( Fryer 1963).

Limb I large. Epipodite (not represented in Fig. 10 E) without a finger-like projection. Two accessory setae, unequal in size and setulated in distal parts, are the distalmost structures of the distal portion of limb I. ODL distally with two setae of very unequal size. IDL with three bisegmented setae, one of them a relatively strong hook-like seta (but greatly less strong as in E. macracanthus ); the smallest IDL seta short. IDL supplied with long distal spinulae (about 11–15 in largest adults), long proximal spinulae (about 10–16 in largest adults), and short basal spinules (about 7–16 in largest adults), marginal spinules absent; a field of minute denticles on IDL basally ( Fig. 10 F, G). Endite 3 with three setulated, bisegmented posterior setae of similar size (a–c), and a setulated, stiff anterior seta 1. Endite 2 with three posterior setae (d–f) and a stiff, setulated anterior seta 2, a very small sensillum near its base. Endite 1 with 3 posterior setae (g–i), and a stiff anterior seta 3. Two ejector hooks anteriorly on outer portion of limb corm. Well-developed maxillar process bearing three slender, fully and densely setulated setae on inner side of limb base.

Limb II with ovoid epipodite lacking a finger-like projection; exopodite as a small lobe ( Fig. 10 H). At inner side of limb, a row of eight stiff scrapers ( Fig. 10 H, 1–8); setae 1–2 with more delicate feathering, setae 3–8 with relatively robust denticle. Posteriorly on limb corm eight soft setae (a–h): distalmost one (a) short; next two ones (b–c) longest; basalmost ones (d–h) similarly feathered by long hairs. Distal armature of gnathobase with four setae, one of them a relatively large sensillum ( Fig. 10 I, 1), located far from the others (2–4); a row of denticles (about 10 in largest adults) crossing near it. Filter plate with eight long, densely setulated setae; distalmost seta of filter plate clearly smaller than the others, second – third ones slightly shorter than others.

Limb III with relatively large epipodite bearing a finger-like projection (not represented in Fig. 11 A). Exopodite flat; distally, five setae of unequal size ( Fig. 11 A, 1–5); lateral group consists of three setae (6–8) increasing in size basally. Distal endite with three bisegmented anterior setae (1–3); basal endite with 4 stiff setae (4–7). Seven long soft setae of subequal size (a–h) on limb corm posteriorly. Gnathobase weakly demarcated from basal endite; distal armature with four members as in previous species ( Fig. 11 B, 1–4). Ten setae in filter plate.

Limb IV with epipodite large, ovoid, bearing a finger-like projection ( Fig. 11 C). Exopodite oval, with two distal, relatively long, bisegmented setae of unequal size, armed by short setules (1–2); other six-seven setae feathered bilaterally by very long setules (3–9). Marginally on inner limb face, a row of four stiff anterior setae (1– 4). Seta 1 long, setae 2–4 approximately of equal size, feathered by long setules in distal part. Posteriorly, five soft setae with subequal length. Distal armature of gnathobase with four members ( Fig. 11 D, 1–4). One of them a long, bisegmented seta, densely feathered in distal part (2); two others small (3–4); a large, bottle-shaped sensillum (1) is a fourth member of gnathobasic armature (similar to limb III). Filter plate IV with 10 setae, middle ones longer than marginal ones.

Limb V with pre-epipodite as a setulated hillock; epipodite with a finger-like projection ( Fig. 11 E). Exopodite very large, with four short distal (1–4) and three large lateral (5–7) setae. Inner portion of limb with a protruding flap-like distal projection, fringed by long setules. Three marginal setae on inner face of limb; distal member slightly protruding behind distal endopodite projection; a sensillum near basalmost seta. Distal armature of gnathobase with three members ( Fig. 11 F). Filter plate with 10 setae.

Limb VI triangular-shaped, with epipodite bearing a long finger-like projection and a row of setules somewhat distal to it; its inner margin setulated ( Fig. 11 G).

Ephippial female. Unknown (absent in our samples).

Male. In lateral view body almost subovoid, but more elongated as compared with female (BH/BL=0.57; VL/ BL=0.78) ( Fig. 12 A).

Head small, rostrum short ( Fig. 12 B). Labral keel small (somewhat shorter than antenna I), with a welldeveloped angular apex ( Fig. 12 B).

Postabdomen large (PL/BL=0.38; PH /PL=0.49) with convex ventral margin ( Fig. 12 C). Postabdominal claw relatively robust (CL/PL=0.25; DS/CL=0.31; BS/CL=0.1; BS/DS=0.35). On pre-claw portion of postabdomen there are postanal teeth (NT=83); distalmost members particularly large, teeth at base of pre-claw portion short, predominantly single ( Fig. 12 D).

Antenna I elongated (AL/BL=0.14; AL/DA=3.7); anterior border in basal half convex ( Fig. 12 E). Slender antennular sensory setae relatively short, about 1/5–1/4 of antenna I length, located subdistally at distance of about 1/4 antenna I length from its distal end. Small male setae ( Fig. 12 E, aml) located almost medially, its length also about 1/4 of antenna I length. Nine bisegmented aesthetascs located in antenna I distal part; additional aesthetascs on anterior and posterior surfaces on the body of antenna I. Numerous short rows of minute denticles encircling antennular surface.

Limb I, IDL shorter than that in female ( Fig. 12 F–G), with three setae on female type and a stiff strong male seta ( Fig. 12 G–G, mls) which is about two times longer than shorthest seta; IDL supplied with two groups of few spinulae, one of them located closer to basal part of IDL, the other in middle part of IDL ( Fig. 12 G); their homology unclear. Limb I supplied by a strong clasping hook located between IDL and ODL. Clasping hook relatively strong, with two transverse ridges at distal part.

Size. Juvenile females 2.5–3.2 mm; adult parthenogenetic females 3.3–4.5 mm; adult male 2.2 mm.

Differential diagnosis. Among the species of Eurycercus (Teretifrons) , E. chernovi has a unique morphology of the head pores: a ring of major head pore projected above the level of head shield, and a low projection in the middle of the surface of major head pore cuticle. This taxon is also unique for the subgenus having only 9 setae in exopodite IV and 7 setae in exopodite V.

Distribution. E. chernovi has been found to date only in the region of Taimyr Peninsula in Asian Russia (Siberia). Real range of its distribution is unknown.

Comments. At least some populations from Arctic Russia previously identified as E. glacialis ( Smirnov 1971) in reality belong to E. chernovi sp. nov. Also some populations from Taimyr previously identified as E. lamellatus ( Smirnov 1978) belong to the latter.

Figures

FIGURE 9. Eurycercus chernovi sp. nov., parthenogenetic female from a lake about 150 m in diameter, north of settlement of Ust' Tareya, Taimyr Autonomous Area, Krasnoyarsk Territory, Russia (type locality). A, lateral view. B, anterior view. C, head, lateral view. D, head shield. E – H, head pores. I, labrum, ventral view. J, valve margin. K, armature of its antero-ventral portion. L, armature of its ventral portion. M, armature of its postero-ventral portion. N, armature of its posterior portion. Scale bar denotes 1 mm for A – C, E, J; 0.1 mm for D, F – I, K – N.

FIGURE 10. Eurycercus chernovi sp. nov., parthenogenetic female from a lake about 150 m in diameter, north of settlement of Ust' Tareya, Taimyr Autonomous Area, Krasnoyarsk Territory, Russia (type locality). A, postabdomen. B, its distal portion. C, antenna I. D, antenna II. E, limb I. F – G, IDL of limb I. H, limb II. I, distal armature of its gnathobase. Scale bar denotes 1 mm for A – C, F – G, I; 0.1 mm for D – E, H.

FIGURE 11. Eurycercus chernovi sp. nov., limbs of parthenogenetic female from a lake about 150 m in diameter, north of settlement of Ust' Tareya, Taimyr Autonomous Area, Krasnoyarsk Territory, Russia (type locality). A, limb III. B, distal armature of its gnathobase. C, limb IV. D, distal armature of its gnathobase. E, limb V, F, its gnathobase. G, limb VI. Scale bar denotes 0.1 mm.

FIGURE 12. Eurycercus chernovi, adult male from a small lake at the margin of a polygonal bog, Taimyr Autonomous Area, Krasnoyarsk Territory, Russia. A, lateral view. B, head. C, postabdomen. D, its distal portion. E, antenna I. F, limb I. G, its distal portion. Scale bar denotes 1 mm for A – B; 0.1 mm for C – G.