Camptocercus fennicus Stenroos 1898

Camptocercus fennicus Stenroos, 1898

(Figs. 9–11)

Stenroos, 1898: 198–200, 202, Table 3, Fig. 19–24; Krasnodebsky, 1937: 426–430, Fig. 1; Smirnov, 1971: 436, Fig. 529, 530. Smirnov, 1998: 66–70, Fig. 22–27.

Material examined. 17 parthenogenetic females from Russia, Moscow Area, Ruza district, lake Glubokoe, 27.06.2014; coll. P.A. Garibyan. Parthenogenetic female, from Russia, Yakutia Autonomous Republic, a swamp at right side of the "Kolyma" Federal Road near crossing with the Suola River, 62,09933º N, 130,1467º E; 25.08.2010, coll. A. A. Kotov; AAK 2011-059. Adult male from Russia, Yakutia Autonomous Republic, swamp lake near lake Beloe, 63,40659º N 128,90326º E; 5.09.2012; coll. E.I.Bekker & A.I. Klimovskij; AAK-M-2601.

Description. Parthenogenetic female. Body (Fig.9 A,B, J) with well-expressed dorsal keel, moderately high, fusiform in lateral view (height/length ratio about 0.65 in adult); maximum height in the second quarter of body; strongly compressed laterally. Dorsal margin highly arched, without depression between valves and head shield. Posterodorsal angle broadly rounded; posterior margin convex. Posteroventral angle broadly rounded. Ventral margin (Fig. 9 C,D) unevenly convex in anterior half, almost straight in posterior half, with about 50–60 short ventral setae. Row of ventral setae ends at about 2/3 length of ventral margin, followed by numerous very short setules, subdivided into unclear groups. Posteroventral angle (Fig. 9 E,F) with 1–3 small denticles; distance between denticles 2–3 widths of denticle base. Anteroventral angle rounded. Valves with prominent sculpture in shape of diagonal, almost never anostomosing lines; in anteroventral portion of valves, curved upward. Keel oblique on both valves and shield.

Head (Fig. 10 A,B,H) with acute, triangular rostrum, protruding downward. Head keel wide, distance from eye to keel margin about 2–3 diameters of eye. Ocellus smaller than eye. Distance from tip of rostrum to ocellus 3 times greater than distance between ocellus and eye. Head pores as in C. uncinatus.

Labrum (Fig. 10 D,E, I) of moderate size. Labral keel moderately wide (height about 1.7 widths), with a blunt or rounded apex. Anterior margin of keel irregularly convex; posterior margin weakly convex, with 2–4 clusters of short setules.

Thorax and abdomen (Fig. 10 C) of similar length. Dorsal surface of middle abdominal segment saddle-shaped. Abdominal joint well-developed.

Postabdomen (Fig. 9 G, K) very long and narrow, evenly narrowing distally, without defined distal margin. Length about 5 heights. Ventral margin straight. Basis of claws bordered from dorsal margin by clear incision. Dorsal margin straight in postanal portion and weakly concave in anal one, with distal part about 4 times longer than preanal one, and postanal portion 3 times longer than anal one. Preanal angle well expressed, prominent, postanal angle not defined. Preanal margin convex. Postanal margin (Fig. 9 H, L) with 18 single, subrectangular denticles with serrated distal margin; distal denticle very long, with length about 1.5 widths of postabdominal claw base and two times longer than neighbors. Postanal portion with 14 lateral fascicles of 10–15 short setules; distalmost setule in each fascicle being longest. Postabdominal claw (Fig. 9 I, M) weakly curved, scythe-like; very long, about 1.5 lengths of preanal portion of postabdomen, with distinctive pecten of setules on dorsal margin. Only distalmost setule in basal pecten spine-like. Basal spine of unusual shape, broad, lancet -like, with maximum width at 2/3 of length, about 1/4 length of the claw, with short spinules on dorsal margin.

Antennule (Fig. 10 F, J) very long, curved at the base; length about 5–6 widths. Antennular seta short, about 1/5 length of antennule itself, arising at the middle of antennule from well-defined tubercule. Nine terminal aesthetascs. Length of two longest aesthetascs varies: in Glubokoe lake specimens (Fig. 10 F), longest one 1.3 times longer than antennule, second one as long as antennule; in Yakutia specimens (Fig. 10 J), length was about 0.8–0.9 length of antennule; all other much shorter and thinner, less than 1/2 length of antennule.

Antenna relatively short (Fig. 10 G). Antennal formula: setae 0-0-3/0-1-3; spines 1-0-1/0-0-1. Basal segment robust, with very short seta between branches. Branches long and slender; in both branches basal segment 1.5 times longer and thicker than two others. Seta arising from middle segment of endopodite of similar size of apical setae. All apical setae of similar thickness. All antennal spines very short.

Limb I (Fig. 11 A, C). Similar to that of C. uncinatus, but ODL seta armed with longer setules; IDL setae 2 and 3 with distal portion shorter and armed with smaller denticles than in C. uncinatus.

Limb II (Fig. 11 D). Similar to that of C. uncinatus, but scraper 3 shorter, about 2/3 lengths of neighbors; scrapers 1–2 and 3–4 somewhat shorter than in C. uncinatus.

Limb III (Fig. 11 E,F). As in C. uncinatus, but scraping spines of distal endite somewhat more slender.

Limb IV (Fig. 11 G, H). Similar to that of C. uncinatus, but exopodite seta 4 of normal morphology, not reduced to stub. Flaming-torch setae more robust, with shortened distal part.

Limb V (Fig. 11 I, J). Similar to that of C. uncinatus, but exopodite setae 1–3 of almost same length; seta 4 three times shorter. Sensillum of gnathobase elongated. No filter plate found.

Ephippial female unknown.

Male. Body lower than in female, fusiform (Fig. 9 N), with well-expressed dorsal keel on head and valves; height/length ratio 0.6; maximum height at 1/3 of the length. Body strongly compressed laterally.

Head (Fig. 10 K) with shortened, blunt rostrum of characteristic shape, similar to these of females of C. lilljeborgi (see below). Head keel less high than in female; distance from eye to the keel margin about 1.5 diameters of eye. Ocellus and eye of same size as in female.

Postabdomen (Fig. 9 O) long and narrow, narrowing distally in anal portion and with parallel margins in postanal portion, without defined distal margin. Length about 5 heights. Sperm ducts opening located close to the end of postabdomen on ventral margin. Basis of claws bordered from dorsal margin by clear incision. Dorsal margin straight in postanal portion and weakly concave in anal one, with postanal portion 2.5 times longer than anal one. Preanal angle well expressed, prominent, postanal angle not defined. Preanal margin convex. Distalmost seven marginal denticles replaced with groups of 7–10 thick spinules on small hillocks; spinules in distalmost group longer and thicker than others; in basal half of preanal margin, marginal denticles similar to those of female, but smaller (Fig. 9 P). Lateral fascicles of setules as in female. Postabdominal claw similar to that of female (Fig. 9 Q).

Antennule (Fig. 10 L) shorter than in female, curved in lateral view and longer than rostrum; length about 5 widths. Male seta short, arising at 4/5 distance from the base and reaching below the end of antennule. Twelve terminal aesthetascs of varying length, the longest ones about 2/3 length of antennule; the shortest about 1/4 length of antennule.

Limb I (Fig. 11 K, L) with U-shaped copulatory hook, 2 times shorter than limb itself. IDL setae 1–3 strongly reduced in size in comparison with those of female; setae 2 and 3 not hook-like, but armed with spinules; male seta straight, of moderate thickness, as long as IDL seta 2. Ventral face of limb below copulatory hook with a row of about 25 setules of moderate length and thickness.

Size. In females: length 0.66–0.91 mm; height 0.40–0.59 mm (in smallest ovigerous female length 0.78 mm, height 0.5 mm). In a single studied adult male: length 0.58 mm; height 0.34 mm.

Notes. Morphology of head and thoracic appendages of C. fennicus was studied for the first time, male of the species is reported for the first time. The species is distributed in Germany, Poland, Scandinavia, North European Russia (the southernmost location here is Moscow Area), and Central Yakutia. Species is rare, never encountered in large numbers. In lake Glubokoe, which was sampled extensively from 1980s by N.N. Smirnov, N.M. Korovchinsky, A. A. Kotov and the author, it was found only few times; studied sample was the only finding of C. fennicus here in last 20 years.