Chydorus breviceps ( Stingelin, 1905 )

Sinev, Artem Y., 2014, A new and a rare species of Chydorus Leach, 1816 (Branchiopoda: Cladocera: Anomopoda) from Cat Tien National Park, Vietnam, Zootaxa 3861 (2), pp. 127-144 : 136-140

Chydorus breviceps ( Stingelin, 1905)

Stingelin, 1905: 334–335, Taf. 13, fig. 29, 29a ( Alonella ); Smirnov, 1971: 318, fig. 335; Smirnov, 1996: 97 ( Alonella ) Sinev & Korovchinsky, 2013, p. 134, fig. 3D–I ( Chydorus sp.)

Material studied. 17 parthenogenetic females from Cat Tien National Park, small forest pond near Bau Sau lake, N 11° 27.178’, E 107°20.328, 0 5.2009, coll. A.Y. Sinev; 8 parthenogenetic females from the same location, 10.2010.

Description. Parthenogenetic female: General. Body of preserved specimens weakly yellowish to colorless, relatively opaque. In lateral view body egg-shaped ( Fig. 5 E–F, 6A), elongated posteriorly, height/length ratio about 0.75, maximum height at the second quarter of the body. Body only weakly compressed laterally ( Fig. 6 B–D), posterior portion of valves flattened.

Valves. Anterior corner broadly rounded with a submarginal flange at inner side. Ventral margin ( Fig. 7 A) with about 15 long setae in anterior portion followed by about 12 short setules, and 40–45 setules in anterior half on inner side. In anterior group, setae thin, increasing in length posteriorly, without any setules. In middle group setae increasing in length posteriorly, without any setules. In posterior group, setae ( Fig. 7 B) thick, armed with long setules; length of setae decreasing posteriorly. Postero-ventral corner ( Fig. 6 F, 7C–E) with one-two denticles with narrow bases. Valves with a polygonal sculpture in antero-ventral portion only ( Fig. 6 E), covered by small circular depressions in other regions.

Head with a very short rostrum, shorter than antennule. Ocellus 2–3 times smaller than eye. Rostrum slightly protruding, with rounded tip without spinules ( Fig. 6 I–J). Head shield ( Fig. 6 H–G) large, with maximum width at the middle of posterior portion, its posteriormost portion wide oval. Head shield mostly oblique, along posterior margin with small depressions, same as on valves. Only anterior major head pore present ( Fig. 6 K). Lateral head pores minute, normally located asymmetrically, close to midline of head shield.

Labrum ( Fig. 7 H–J) with fleshy main body, small distal labral plate and a large medial labral keel. Labral keel of moderate width, with rounded or blunt apex. Anterior margin of keel convex in most specimens, with distinctive depression near the tip; posterior margin concave.

Thorax three times longer than abdomen.

Postabdomen ( Fig. 6 L, 7F–G) long and narrow with parallel margins at anal region, narrowing distally in postanal portion; maximum height at postanal angle. Length about 3.5–3.8 heights. Ventral margin weakly convex. Basis of claws bordered from distal margin by clear incision. Distal margin very short; distal angle broadly rounded. Dorsal margin weakly concave in postanal portion and concave in the anal one, with distal part 3.1–3.2 times longer than the preanal portion. Postanal portion 1.5 times longer than anal portion. Preanal angle well expressed, prominent; postanal angle well-defined. Preanal margin concave. About 10–13 narrow sharp denticles in dorsal margin and two-three groups of short setules at postanal angle. Length of longest denticles little more than width of base of postabdominal claw. About 8 broad lateral fascicles of very short setules; in anal portion additional fasciles located above the main row. Postabdominal claw small and slender, weakly curved, 1.5 times shorter than preanal portion of postabdomen; two basal spines, distal spine 0.25 length of the claw, proximal spine 3 times shorter. A pecten of short spinules on dorsal side of claw.

Antennule ( Fig. 8 A) elongated, length abouth 3 widths. Antennular seta thin, about half length of antennule, arising at 1/3 antennule length from the base. All aestetascs terminal, longest of them of less than 1/2 length of antennule.

Antenna relatively short ( Fig. 8 B). Antennal formula: setae 0-0-3/1-1-3; spines 1-0-1/0-0-1. Branches relatively short, in both branches basal segment 1.5–2 times longer and thicker than two others. Basal segment of endopodite without seta. Seta arising from middle segment of endopodite of similar size of apical setae. Apical segments of both branches with two apical and one subapical setae; all setae on exopodite of similar size; on endopodite, one of apical setae much thinner and shorter than others. All antennal spines very small.

Limb I of moderate size ( Fig. 8 C–D). Epipodite oval, without projection. ODL with just one long seta. IDL with three setae and several clusters of hard setules. IDL seta 1 thin, about 2/3 length of ODL seta, with setules in distal part; setae 2 and 3 of similar shape, long and slender, armed with short setules distally, seta 2 slightly shorter than ODL seta, seta 3 longer than ODL seta. Endite 3 with four setae subequal in length; inner seta (1) much thinner than outer setae (a–c). Endite 2 armed with pecten of spinules on anterior face, with two long distally setulated setae (e–f), a shorter seta near their base (d) and an inner seta (2); seta (e) almost 2 times longer than seta (f), slightly shorter than limb itself. Endite 1 with three 2-segmented setae of similar size (g–i) setulated in distal part, and an inner seta (3) on anterior face of limb,without setules (j). Five-six rows of long setules on ventral face of limb, one setule in each group two times thicker and much longer than others. Two ejector hooks, one slightly shorter than other.

Limb II ( Fig. 8 E) similar to that of the previous species, but scraping spines longer and more slender, with very fine setules. Sensilla located between spines 3 and 4 much longer than in previous species. Distal armature of gnathobase with only three elements, outer spine absent.

Limb III ( Fig. 8 F). Epipodite oval, without finger-like projection. Exopodite narrow, rectangular, with three lateral (1–3) and four terminal (4–7) setae. Seta 4 being longest; setae 6 and 7 of same length, about 3/4 length of seta 4; setae 5 and 1 about 3/5 and 1/2 length of seta 4; respectively, setae 2 and 3 short. Setae 1–5 plumose; setae 6–7 naked. Inner portion of limb III as in the previous species.

Limb IV ( Fig. 8 G–I). Pre-epipodite setulated; epipodite oval, without projection. Exopodite of irregular shape with protrusion on inner side, 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 and 6 about 1/2 length of seta 1; seta 7 about 1/3 length of seta 1. Setae 1–5 flat, plumose; seta 6 setulated unilaterally in basal part; seta 7 naked. Inner portion of limb IV with four setae: scraping seta (1) slender; three flaming-torch setae (2–4) long and slender, increasing in size basally; seta 4 almost 2 times shorter than seta 4, armed with 5–7 thick setules each. Small sensillum located near bases of setae 2–4. Four inner setae (a–d) slightly increasing in size basally. Gnathobase with one 2-segmented setae, a small hillock distally and a sensilla. Filter plate IV with six setae.

Limb V ( Fig. 8 J) large, much bigger than in the previous species. Pre-epipodite setulated; epipodite oval, with very short projection. Exopodite very large, oval, with four plumose setae, their length evenly decreasing basally. Two small hillocks with long 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: outer seta densely setulated in distal part; inner one with long thin setules unilaterally in basal part and thicker and longer setules in distal part. A small sensillum located between inner seta and filter plate. Filter plate V with four setae.

Ephippial female and male unknown.

Size: length of adult female 0.39–0.44 mm, height 0.29–0.33 mm.

Differential diagnosis. C. breviceps differs from most species of the genus by elongated body, presence of denticle on posteroventral corner of valves, and by the absence of posterior major head pore. It differs from C. tilhoi , C. nitidulus and C. dentifer (species with elongated body and denticle on posteroventral corner on valves) by a very short rostrum, which is shorter than the antennule, from C. dentifer by sharp denticles of the valves and by the absence of posterior major head pore, from C. tilhoi by the labrum without denticle and by the shape of postabdomen, and from C. nitidulus by the globular body and presence of anterior major head pore ( C. nitidulus lack both major head pores).

Distribution and ecology. Presently known from Java, Singapore ( Stingelin, 1905) and Vietnam, but probably distributed over all Oriental region. In Cat Tien National Park, C. breviceps was found in a single locality, a small forest pond near Bau Sau lake, deep within protected area (locality 2 in Sinev & Korovchinsky, 2013). The pond was relatively shallow, bordered with swamp, with humic water, bottom covered by dead leaves, and surface partially covered with Salvinia plants. C. breviceps was present in the pond during both spring and autumn. Gamogenetic stages were not found in both seasons. For the list of accompanying species see Sinev & Korovchinsky (2013).

Figures

FIGURE 6. Chydorus breviceps (Stingelin, 1905) from small forest pond near Bau Sau lake, Cat Tien National Park, Dong Nai province, Vietnam, parthenogentic female. A – D, specimens in lateral, ventro-lateral, antero-lateral and dorso-lateral views. E, anteroventral part of valves. F, postroventral andgle pof valves. G – H, head shields, major head pore indicated by arrow. I – J, rostrum. K, head pores, lateral head pores indicated by arrows. L, distal part of postabdomen. Scale bars— 0.1 mm for A – D, H, K, 0.03 mm for E, G, I, L, 0.01 mm for F, J – K.

FIGURE 7. Chydorus breviceps (Stingelin, 1905) from small forest pond near Bau Sau lake, Cat Tien National Park, Dong Nai province, Vietnam, parthenogentic female. A, ventral margin of valve. B, setae of distal group of ventral margin of valves. C – E, postero-ventral angle of valve. F – G, postabdomen. H – J, labrum.

FIGURE 8. Chydorus breviceps (Stingelin, 1905) from small forest pond near Bau Sau lake, Cat Tien National Park, Dong Nai province, Vietnam, appendages of parthenogentic female. A, antennule. B, antenna. C – D, limb I and its IDL and ODL. E, limb II. F, exopodite of limb III. G, exopodite of limb IV. H – I, inner portion of limb IV. J, limb V.

FIGURE 5. A – D. Chydorus cf. eurynotus Sars, 1901, parthenogenetic female from Bau Sau lake, Cat Tien National Park, Dong Nai province, Vietnam A, lateral view. B, rostrum. C, postabdomen. D, IDL of limb I. E – H, Chydorus breviceps (Stingelin, 1905), parthenogenetic female. E, specimen from drawing of Stingelin (1905), no scale bar. F – H, from small forest pond near Bau Sau lake, Cat Tien National Park, Dong Nai province, Vietnam, lateral, dorsal and frontal view.