Rhynchotalona Norman, 1903

Sinev, Artem Y. & Kotov, Alexey A., 2014, Revision of the Holarctic genus Rhynchotalona Norman, 1903 (Anomopoda: Chydoridae), Zootaxa 3841 (2), pp. 188-210 : 190-191

Genus Rhynchotalona Norman, 1903

Sars, 1861: 139 ( Harporhynchus ); 1862b: 289 ( Alona, Harporhynchu s); Herrick, 1884: 114 (section Leptorhynchus ); Norman, 1903: 367 ( Rhynchotalona instead of Harporhynchus , Leptorhynchus – nomina preoccupata); Smirnov, 1971: 489; Flössner, 1972: 321–322; Negrea, 1983: 322; Alonso, 1996: 373.

Type species: Harporhynchus falcatus Sars, 1862 = Rhynchotalona falcata ( Sars, 1862) now. Typified by monotypy ( Norman, 1903).

Emended diagnosis. Female. General. Small-sized Aloninae , body low, oval, moderately compressed laterally, with maximum height in at the middle of the body (see Fig. 1 A–D, 5A–C). Height/length ratio about 0.55. Dorsal and posterior margins of valves convex, postrodorsal and postero-ventral angles broadly rounded. Moulting may be incomplete with valves from previous instars retained. Postero-ventral angle with a single denticle or without it (see Fig. 1 F). Antero-ventral corner of valves an elongated protrusion with rounded tip (see Fig. 1 E). Ventral margin of valves with about 40–45 densely spaced setae, about 7 anterior setae very long, next 15–10 setae short, posteriormost 20–25 setae of moderate length or very long. Sculpture of valves as longitudinal lines.

Head relatively large. Ocellus and eye large, of similar size. Rostrum long, about 1.5–3 length of antennule, strongly curved downward and backward (see Fig. 1 G–I, 6G–F). Head shield short and broad, rostrum triangular, with bifurcated tip, posterior portion of head shield as broad elongated protrusion with rounded tip (see Fig. 5 F). Major head pore as a rimmed field, compressed laterally (see Fig. 2 F, 5G–H). Lateral pores minute, located close to main head pore. Labral keel moderately broad, with rounded tip.

Thorax 2–3 times longer than abdomen.

Abdomen. No abdominal joint, middle segment of abdomen not saddle-shaped.

Postabdomen short and narrow, with almost parallel margins (see Fig. 1 J–L, 2H–D, 3A, 6H–J, 7B–C). Distal angle obtuse, rounded. Dorsal margin with anal and postanal portions of similar length. Preanal and postanal angles weakly defined. Preanal margin with shallow, numerous notches. Postanal margin with 2–3 large, sharp, slender, single marginal denticles, followed by several groups of 1–3 much smaller denticles. Postanal part with about 5–6 well-developed lateral fascicles of 3–5 thick setules, as long as distal marginal denticles; in anal portion fascicles consist of much shorter and more numerous setules. Postabdominal claw weakly curved, shorter than preanal portion of postabdomen. Basal spine short, straight, about 0.15 length of claw.

Antennule long and narrow (see Fig. 1 N, 6J, 7D). Antennular sensory seta slender, about half length of antennule, arising at 1/3–1/2 distance from the base. Nine terminal aesthetascs, two longest, their length about halflength of antennule.

Antenna of moderate length (see Fig. 1 M, 2J–K, 6K, 7E), antennal formula: setae 0-0-3/0-1-3, spines 1-0-1/0- 0-1. Basipodite without a cluster of very long setules. Proximal segments of both branches short and broad, two times shorter than other segments. Spine on proximal segment of exopodite less than 1/2 length of middle segment. Spines from apical segments almost as long as these segments. All antennal setae of similar thickness and length.

Thoracic limbs: six pairs.

Limb I with moderately long accessory seta (see Fig. 4 B–C, 8A–B). ODL with a single seta. IDL with three setae, seta 1 very short, setae 2 and 3 moderately thick, armed distally with thin setules, seta 3 without annulus. Soft setae are present on each of limb endites, soft setae of endite 1 long, of endite 2 short. Setae of endite 2 short, of similar length, only slightly longer than setae of endite 3. Endite 1 with two two-segmented setae and a long, flat seta pointed to the limb base. Ventral face of limb with 5–6 clusters of setae. Maxillar process short and broad, bearing a single seta longer than process itself.

Limb II with narrow, elongated exopodite and relatively long seta (see Fig. 4 D, 8C). Eight scraping spines decreasing in length proximally, scraper 3 broad and armed with blunt denticles. Distal armature of gnathobase with four elongated elements. Filter plate with seven setae, two posteriormost setae remarkably shorter than others.

Limb III with quadrangular exopodite provided with seven setae (see Fig. 4 E–G, 8D). Setae 1–5 flattened, plumose, setae 6–7 slender, thin, without long setules. Distal endite with three setae, two small sensillae located between their bases. Distal two setae long and thin, adapted for scraping, proximalmost seta flattened, with long setules unilaterally. Basal endite with 4 stiff, setulated distally setae of similar size. Four soft setae increasing in size proximally, small sensillum near the base of distalmost seta. Distal armature of gnathobase with 4 elements: an elongated, cylindrical sensillum, large geniculated seta, and two spines with fused bases. Filter plate of seven setae.

Limb IV with subrectangular exopodite bearing six setae (see Fig. 4 H, 8E). Setae 1–4 flattened, plumose, setae 5–6 slender, thin, with short setules, seta 3 longest, all other setae more than two times shorter than seta 3. Inner portion of limb IV with four setae and small elongated sensillum (see Fig. 4 I, 8F). Scraping seta (1) moderately long, three flaming-torch setae elongated, first of them (2) much thicker than two others (3–4), armed with longer, thicker setules. Small setae located between bases of two proximalmost flaming-torch setae. Soft setae of similar size, shorter than setae of filter plate. Filter plate with five setae.

Limb V with oval exopodite of moderate size (see Fig. 4 J, 8G), with four plumose setae, seta 4 about 2/5 length of seta 1. Inner limb portion as elongated oval lobe with setulated inner margin. At inner face, two setae, distalmost 1.5 times longer than other. Filter plate with three small setae.

Limb VI an oval setulated lobe (see Fig. 4 K, 8H).

Male. General. Body low oval, lower than in female (see Fig. 3 G–H, 5K–J).

Head. Ocellus larger than eye. Rostrum long, strongly bent in proximal third, straight afterward, in ventral view broad, asymmetric, widening distally, with broadly rounded tip (see Fig. 3 I).

Postabdomen short, narrowing distally (see Fig. 3 J–K, 7F). Gonopore at the end of postabdomen, above the base of claws. No distal angle, dorsal margin directly at base of claws. Dorsal margin with anal and postanal portions of similar length. Preanal and postanal angles weakly defined. Postanal margin without any denticles or setules. Lateral fascicles of setules as in female. Postabdominal claw very short, 1.5 times shorter than preanal portion of postabdomen. Basal spine very short.

Antennule slightly broader than that of female (see Fig. 4 L, 7G), with twelve terminal aesthetascs, three of them much broader than others, longest aesthetascs about half-length of antennules. Male seta short.

Limb I proximal portion as in female (see Fig. 4 M–N, 8I –K). Copulatory hook U-shaped, 2 times shorter than limb itself. IDL seta 1 absent, setae 2 and 3 short and thin, much smaller than in female, male seta thin, 1.5 times shorter than IDL seta 3.

Figures

FIGURE 1. Rhynchotalona falcata from Glubokoe Lake, Ruza District, Moscow Area, Russia, parthenogentic female. A – D, lateral, ventral, dorsal and anterior view of an adult female. E – F, antero-ventral and postero-ventral corners of valve. G – H, head in lateral view. I, head in ventral view. J – L, postabdomen. M, antennule. N, antenna. Scale bars: 0.1 mm for A – D, G – M; 0.05 mm for E – F, N.

FIGURE 2. Rhynchotalona falcata from Glubokoe Lake, Ruza District, Moscow Area, Russia, parthenogentic female. A – С, lateral, latero-ventral and anterior view. D, postero-ventral angle of valves and distal portion of postabdomen. E, head. F, head pores. G – H, postabdomen. I, postabdominal claw. J – K, antenna. Scale bar 0.1 mm for A – C; 0.01 mm for D – E, G – K; 0.001 mm for F.

FIGURE 3. Rhynchotalona falcata from Glubokoe Lake, Ruza District, Moscow Area, Russia. A, postabdomen of parthenogenetic female. B, ephippial female. C – F, juvenile male of instar I. C, lateral view. D, rostrum. E, postabdomen. F, antennule. G – L, adult male. G – H, same specimen in lateral view and ventral view. I, rostrum. J – K, postabdomen. L, distal part of antennule. Scale bars: 0.1 mm for B – C, G – H; 0.05 mm for A, D – F, I – K.

FIGURE 4. Rhynchotalona falcata from Glubokoe Lake, Ruza District, Moscow Area, Russia. A – H, parthenogenetic female, labrum and thoracic limbs. A, labrum. B, limb I. C, ODL and IDL of limb I. D, limb II. E, exopodite of limb III. F – G, inner portion of limb III. H, exopodite of limb IV. I, inner portion of limb IV. J, limb V. K, limb VI. L, juvenile male of instar II, ODL, copulatory hook and IDL of limb I. M – N, adult male, thoracic limb I. M, general view. N, ODL, copulatory hook, and IDL. Scale bar 0.05 mm.

FIGURE 5. Rhynchotalona weiri sp. nov. from Cranberry lake, Adirondack mountains, New York State, USA. A, juvenile female of instar II. B, adult parthenogenetic female. B – C, lateral view. D, posterior part of ventral margin of valve. E, posteroventral angle of valves. F, head shield. G – H, head pores. I, ephippial female. J – L, adult male. J – K, lateral view. L, ventral view. Scale bars: 0.1 mm for A – D, F, I – L; 0.05 mm for E, G – H.