Rhynchotalona falcata Sars 1862

Rhynchotalona falcata (Sars, 1862)

Sars, 1861 (1993): 139–141, Pl. 100: figs 1–3, Pl. 101: figs 1–7 (Harporhynchus falcatus, name in manuscript, published only in 1993); 1862a: 162 (Alona, Harporhynchus falcatus); Müller P.E., 1867: 183–184, Pl. 4: figs 13–14 (Alona falcata); Norman & Brady, 1867: 387, Pl. 18 (Lynceus falcatus); Hellich, 1877: 92, figs 52–53 (Alona falcata); Lilljeborg, 1901: 488–492, Pl. 69: figs 22–26, Pl 70: figs 1–5 (Leptorhynchus falcatus); Norman, 1903: 367; Herr, 1917: 112–113, fig. 36; Behning, 1941: 274–275, fig. 113; Šrámek-Hušek et al., 1962: 352–354, fig. 131; Manujlova, 1964: 236–237, fig. 121; Smirnov, 1971: 489–491, figs. 618–621; Flössner, 1972: 322–324, fig. 152; Negrea, 1983: 332–333, fig. 136; Margaritora, 1983: 158, fig. 104; Margaritora, 1985: 333–334, fig. 132; Alonso, 1996: 373–378, figs 168, 169; Flössner, 2000: 358–360, fig. 132; Kotov et al., 2010b: 255, fig. 145: 9–10.

Type locality. Lake Marisvand (Sars, 1862), the vicinity of Oslo, Norway.

Material studied. 5 females and 2 males from Dofinovsky Liman of Black Sea, near Odessa, Ukraine, slides NNS MGU 50–51, 540, 715, 977, 1926, 2022; 5 females from north portion of Lake Svyatoe, Island Bolshoy Solovetsky, Arkhangelsk Area, Russia, coll. 10.06.2012 by A. Makhrov & V. Artamonova, AAK M-2482; 3 females from Lakhta Bay, Chudskoe Lake, Pskov Area, Russia, coll. 18.08.2007 by A. A. Kotov & Y. R. Galimov, AAK M-0594; 2 females from Lake Shlino, Tver Area, Russia, coll. 14.08.2010 by T. P. Korobkova, AAK M- 1799; 10 females from Pukhlinskiy Stvor, Uglich Water Reservoir, Tver Area, Russia, coll. 0 6.06.2010 by A. Invanovsky, AAK M-1546; 20 females from Uglich water Reservoir, Yaroslavl Area, Russia, coll. 23.08.1962 by N. N. Smirnov, NNS 1999-005 & AAK 1999-042; numerous parthenogenetic females, several males and ephippial females from Lake Glubokoe, Ruza district, Moscow area, Russia, coll. 08-09.1999, AAK 1999-088 - 089; 5 females from Lake Turgoyak near Island Svyatoy Very, Chelyabinsk Area, Russia, coll. 2011 by T. S. Dorofeeva, AAK M-2169; 6 parthenogenetic females from Usu lake, Yakutia Autonomous Republic, Russia, coll. 0 4.08.2010 by A. A. Kotov, AAK 2011-003; 3 parthenogenetic females from lake Bayan Nuur near town Zuungovi, Uvs Aimag, Mongolia, coll. 10.09.2006 by Ch. Jersabek, AAK 2008-090; 2 females from Hoid Gol (backwater) near Zuungovi (town), Uvs Aimag, Mongolia, coll. 11.09.2006 by Ch. Jersabek, AAK 2008-088; 2 females from Khar Nuur (Lake), Khovd Aimag, coll. 0 8.2010 by D.P. Karabanov, AAK M-1730; 5 females from a bay with vegetation, Hurgan Nuur, Olgiy Aimag, coll. 10.08.2008 by D.P. Karabanov, AAK M-0783.

Redescription. Parthenogenetic female. General. Body shape (Fig. 1 A–D, 2A–C) and morphology of valves (Fig. 1 E–F, 2D) as for genus. Valves from previous molts retained rarely. Ventral margin of valves with posterior group of setae of moderate length.

Head as for genus. Rostrum long, more than two times longer than antennule, evenly curved (Fig. 1 G–H, 2E). Head shield as for genus. Major head pore as elongated, narrow rimmed field, slightly narrowing in the middle, length about 4 width (Fig. 2 F). Lateral head pores as for genus. Labrum as for genus, examination under high magnification reveals two lateral groups of thin setules in posterior half of the keel (Fig. 4 A).

Postabdomen (Fig. 1 J–L, 2G–I) clearly narrowing distally, length about 3 height. Postanal margin with 2–3 large, sharp, slender, single marginal denticles, followed by 3 groups of 2–3 much smaller thin denticles. Lateral fascicles of setules in postanal portion consisting of 4–6 long setules, distal setules in fascicles only little shorter than distal marginal denticle. Postabdominal claw as for genus.

Antennule (Fig. 1 N) as for genus, antennal seta arising at 1/2 distance from the base.

Antenna (Fig. 1 M, 2J–K) as for genus. Spine on proximal segment of exopodite about than 1/3 length of middle segment. Apical spines longer than apical segments.

Limb I (Fig. 4 B–C) as for genus, IDL setae 2–3 relatively short and robust.

Limb II (fig. 4d) as for genus, scraper 3 shorter and much thicker than scrapers 2 and 4.

Limb III (Fig. 4 E–G) as for genus, exopodite two times smaller than exopodite V, length of seta 3 about 2.5 heights of exopodite.

Limb IV (Fig. 4 H–I) as for genus, epipodite with long process, more than two times longer than epipodite itself, exopodite two times smaller than exopodite V, length of seta 3 about three heights of exopodite. Inner portion of limb with scraping seta (1) as long as largest flaming-torch seta (2).

Limb V (Fig. 4 J) as for genus.

Limb VI (Fig. 4 K) small, much smaller than exopodites III–IV, with short, thin setules.

Ephippial female. Body outline similar to parthenogenetic female. Ephippium yellow-brown, with a sculpture of moderately thick longitudinal lines.

Male. Both adult and juvenile male of instar II were studied.

General. Body shape low oval in both juvenile (Fig. 3 C) and adult males (Fig. 3 G–H), height-length ratio about 0.47–0.50. Males retaining valves from the previous molts were not observed, and not recorded in the literature.

Head. Rostrum of instar II juvenile male in lateral view shorter than in adult male, evenly curved in proximal portion and almost straight in distal portion, in ventral view (Fig. 3 D), similar to that of adult male but shorter and less wide. Ocellus as large as eye. In adult male, rostrum as for genus (Fig. 3 I), ocellus larger than eye.

Postabdomen of juvenile male of instar II (Fig. 3 E) similar to that of female, but have relatively shorter postanal portion and convex ventral margin. Marginal denticles and latral fascicles of setae same as in female. Postabdominal claw shorter than in female, basal spine shorter and more robust than in female. In adult male, postabdomen (Fig. 3 J–K)as for genus, evenly narrowing in postanal portion.

Antennule. In juvenile male of instar II, shape of antennule (Fig. 3 F) and aestetascs as in female, anlage of male seta located at 2/3 distance from the base. In adult male, antennule (Fig. 3 L) as for genus, male seta located close to the end of antennule.

Limb I. In instar II juvenile male limb I (Fig. 4 L) with short curved copulatory hook and anlage of copulatory brush seta. IDL with anlage of male seta, IDL seta 1 very small, setae 2 and 3 similar to these of female. Endites as in female. In adult male, limb I (Fig. 4 M–N) as for genus, ventral face of limb below copulatory brush with about 30 long, thin, densely spaced setules.

Size: length of adult parthenogenetic female 0.44–0.57 mm, height 0.31–0.36 mm, length of juvenile male of instar II – 0.34–0.35 mm, height 0.17–0.18 mm, length of adult male 0.37–0.38 mm, height 0.18–0.19 mm. According to the literature, maximum length of female 0.6 mm, of male - 0.4 mm (Alonso, 1996).

Distribution and ecology. Europe, Kazakhstan, Siberia, Mongolia. The species inhabits open littoral in oligo- and mesotrophhic lakes, and usually encountered on sandy bottom. According to Smirnov (1971), exopodites of limbs III–V in R. falcata lack mobility and do not perform pumping actions, thus the species require highly oxygenated waters.