Chydorus idrisi, Sinev, Artem Y., 2014

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 : 129-136

Chydorus idrisi sp. nov.

( Figs 1–4 )

Idris & Fernando, 1981: 242–243, fig. 38–42 ( cf. pubescens ); Idris, 1983: 72–74, fig. 34 ( cf. pubescens ).

Etymology. Species name honors Dr. B. A. G. Idris, Malaysian cladocerologist, who produced the first description of this species.

Type locality: Vietnam, Dong Nai Province, Cat Tien National Park, small roadside forest pond, 11° 24.444’ N, 107° 24.748’ E, specimens collected during October 2010 by A. Y. Sinev.

Type material. Holotype: parthenogenetic female from type location, ZMOU Ml-127.

Allotype: male from type location, ZMOU Ml-128.

Paratypes: over 50 parthenogenetic and ephippial females from type location, ZMOU Ml-134. All other samples from Vietnam, Dong Nai Province, Cat Tien National Park, collected by A. Y. Sinev: 12 parthenogenetic females from littoral zone of eastern part of Bau Sau Lake N11° 27.55’, E 107°20.66’; 22 parthenogenetic females from littoral zone of Bau Chim lake, N 11° 28.829’, E 107° 22.608’; over 50 parthenogenetic and ephippial females, 2 adult males from small roadside forest pond, N 11° 26.755’, E 107° 23.180’; over 30 parthenogenetic females from roadside pond in a former sand quarry, N 11° 24.423’, E 107° 24.790'; over 50 parthenogenetic females, 3 ephippial females from small roadside pool in forest, N 11° 26.323’, E 107° 25.372’.

Description. Parthenogenetic female. General. Body of preserved specimens from weakly yellow-brownish to colorless, relatively opaque. In lateral view body regularly round in both juveniles ( Fig. 1 A–B) and adult ( Fig. 1 C, 2A–B); height/length ratio about 0.9, maximum height at midline. Body only weakly laterally compressed ( Fig. 1 D–E, 2C–D).

Valves. Anterior corner broadly rounded, with a submarginal flange at inner side. Ventral margin of valves ( Fig. 3 A) with about 15 long setae in anterior portion and with about 30 setules in posterior portion on inner side of valve; second quarter of ventral margin without setae. In anterior group, setae thin, increasing in length posteriorly without any setules. In posterior group, setules thick, armed with long setules ( Fig. 3 B), maximum length of setae at the middle of the group. Postero-ventral corner without denticles. Polygonal sculpture, margins of polygons wavy on most surface of valves ( Fig. 3 C) but straight along ventral margin.

Head with short rostrum, protruding downward and posteriorly. Length of rostrum about 1.5 length of antennule. Ocellus 2–3 times smaller than eye. Head shield large, with maximum width at the middle of posterior portion; its posteriormost portion wide oval ( Fig. 3 D). Rostrum ( Fig. 3 E–F) triangular in frontal view, apex with two small asymmetric spines at the end ( Fig. 2 E, 3G); under optic microscope tips of spines usually appear rounded. Head shield sculpture has polygons with wavy margins, similar to that of valves. Two major head pores ( Fig. 2 E, 3H), PP = 1.2 IP. Lateral head pores minute, normally located asymmetrically, close to midline of head shield, little closer to anterior major head pore.

Labrum ( Fig. 3 I–K) with fleshy main body, small distal labral plate and a rather small medial labral keel, projecting to the level of rostrum tip. Labral keel of moderate width, with rounded apex. Anterior margin of keel convex, in most specimens with distinctive depression near the tip; posterior margin almost convex.

Thorax three times longer than abdomen.

Postabdomen ( Fig. 2 G, 3L–M) short, rather narrow, with parallel margins at anal region, narrowing distally in postanal portion. Length about 3.5 heights. Ventral margin weakly concave. Basis of claws bordered from distal margin by clear incision. Distal margin very short. Distal angle broadly rounded. Dorsal margin straight in postanal portion and weakly concave in the anal one, with distal part about 2 times longer than the preanal portion. Postanal portion about 2 times shorter than anal portion. Preanal angle well expressed, prominent, postanal angle weakly defined. Preanal margin weakly concave. Dorsal margin ( Fig. 2 H) with about 10 narrow sharp denticles and two broad groups of short setules in anal portion. Length of longest denticles little more than width of base of postabdominal claw. About 10 broad lateral fascicles of very short setules; in anal portion fascicles bigger, horseshoe-shaped. Postabdominal claw ( Fig. 2 I) slender, curved, slightly shorter than preanal portion of postabdomen, with two basal spines: distal spine 0.2 length of the claw and proximal spine two times shorter. A pecten of spinules on dorsal side of claw and a longer spinule on its tip.

Antennule ( Fig. 4 A) of moderate size; length about 2 widths. Antennular seta thin, about 1/3 length of antennule, arising at 1/3 antennule length from the base. All aestetascs terminal, about 2/3 length of antennule.

Antenna relatively short ( Fig. 2 J, 3N). 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 with apical setae. Apical segments of both branches with two apical and one subapical setae of similar size. All antennal spines very short.

Limb I of moderate size ( Fig. 4 B–C). Epipodite oval, with projection 1.5 times longer than exopodite itself. ODL with two setae, one of them very small. IDL with three setae and several clusters of setules. IDL seta 1 moderately thick, about 1/2 length of longest ODL seta, with hard setules in distal part; seta 2 thin, about about 3/4 length of longest ODL seta; seta 3 thick, strong, claw-like, little shorter than longest ODL seta, with hard setules in distal part. Base of IDL seta 3 two times wider than base of seta 1. Endite 3 with four setae subequal in length, inner seta (1) much thinner than outer setae (a–c). Endite 2 with two long distally setulated setae (e–f) of similar length; a shorter seta near their base (d) and a naked inner seta (2) on anterior face of limb. Endite 1 with three 2- segmented setae of similar size (g–i) setulated in distal part, a flat plumose seta (j) and a naked inner seta (3) on anterior face of limb. Six-seven rows of thin long setules on ventral face of limb. Two ejector hooks, one slightly shorter than other.

Limb II subtriangular ( Fig. 4 D–E). Exopodite elongated of irregular shape, with seta 2 times longer than exopodite itself. Eight scraping spines with fine denticles; three basalmost spines (6–8) subequal in length, others increasing in length distally. A small sensillum located between spines 3 and 4. Distal armature of gnathobase with four elements. Filter plate II with eight setae, the posteriormost member only slightly shorter than others.

Limb III ( Fig. 4 F–H). Epipodite oval, without finger-like projection. Exopodite subrectangular, with three lateral (1–3) and four terminal (4–7) setae. Seta 4 being longest; setae 1, 6 and 7 subequal in length, about 3/4 length of seta 4; seta 2 about 2/3 length of seta 4; setae 3 and 5 of about 1/3 length of seta 4. Setae 1–5 plumose; seta 6 with row of very long, thick setules in basal part; seta 7 naked. Distal endite with 2 scraping setae (1–2), slender, sharp, with small denticles in distal part; small sensilla located near base of seta 2. Basal endite with 6 plumose setae (a–f) increasing in size basally. Four pointed inner setae increasing in size basally; a small bottleshaped sensillum near the distalmost seta. Distal armature of gnathobase with four elements: the first one large, elongated, narrowing distally sensillum; second strongly geniculated seta; third and fourth-spines with fused bases. Filter plate III with eight setae.

Limb IV ( Fig. 4 I–J). Pre-epipodite setulated; epipodite oval, with projection as long as epipodite itself. Exopodite rounded 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 about 1/3 length of seta 1; seta 6 about 1/2 length of seta 1; seta 7 little shorter than seta 6. Setae 1–5 flat, plumose; setae 6–7 setulated unilaterally in basal part, setules of seta 7 shorter than setules of seta 6. Inner portion of limb IV with four setae. Scraping seta (1) slender, three flaming-torch setae (2–4) long and slender of similar size; seta 2 with numerous thin setules; setae 3–4 with about 10 thick setules each; small sensillae located near bases of setae 3 and 4. Four inner setae slightly increasing in size basally. Gnathobase with one 2–segmented setae, a small hillock distally, and a sensillum. Filter plate IV with six setae.

Limb V ( Fig. 4 K). Pre-epipodite setulated; epipodite oval, with short projection. Exopodite small, rounded, 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 and thicker and longer setae in distal part. A small sensillum located between inner seta and filter plate. Filter plate V with four setae.

Ephippial female. In lateral view, similar to parthenogenetic female ( Fig. 1 F, 2K). Body more strongly compressed laterally ( Fig. 1 G–H, 2L), ephippum with weakly developed egg locules, yellow-brownish. Head shield ( Fig. 3 O) more narrow and more flattened than in pathenogenetic female.

Male: In lateral view body oval ( Fig. 1 I); height/length ratio about 0.8, maximum height before the middle of the body. Body moderately compressed laterally. Head with a short rostrum, protruding downward and posteriorly; length of rostrum exceed length of antennule. Ocellus 2–3 times smaller than eye. Rostrum ( Fig. 3 P–Q) triangular in frontal view, with broadly rounded apex with two small asymmetric spines at the end.

Postabdomen ( Fig. 3 R) short and narrow, with almost parallel margins in middle portion, narrowing distally in distal third, distal portion rounded, and concave at preanal angle. Length about 3.5 height. Sperm duct openings large, round, located above the bases of claws. Ventral margin almost straight. Dorsal margin evenly comes to the basis of claws, weakly concave in postanal portion and almost straight in anal one; distal part about 3–3.5 times longer than preanal one and postanal portion 2 times shorter than anal one. Postanal angle not defined. Preanal margin almost straight. Postanal margin with 1–3 thin spine-like marginal denticles; number of denticles on left and right sides of postabdomen can be different (in one specimen denticles were absent on one side); lateral fascicles setules same as in female. Postabdominal claw very short, 1.5 times shorter than preanal margin of postabdomen.

Antennule ( Fig. 4 L) larger than in female, with 6 terminal and 4 lateral aestetascs; longest aesthetascs almost as long as antennule. Male seta large, arising at the middle of antennule, almost as long as antennule.

Thoracic limb I ( Fig. 4 M) with U-shaped copulatory hook, about 1/3 length of limb itself. The IDL has four setae: seta 1 similar to that of female; setae 2 and 3 of similar morphology, slender, thin, armed with short thin setules in distal part, only slightly longer than seta 1; male seta curved, as long as seta 1. Copulatory brush seta large, as long as IDL seta 1.

Size. Minimum length of juvenile female was 0.24 mm and height 0.21 mm; in ovigerous females length 0.30–0.36 mm and height 0.28–0.34 mm. In adult male, length 0.28–0.30 mm and height 0.21–0.22 mm.

Differential diagnosis. Chydorus idrisi sp. nov. can be easily differentiated from the species of sphaericus - group of the genus by small labrum without elongated tip and from the species of faviformis -group and reticulatus - group by absence of honeycomb-like sculpture of carapace. It shares many diagnostic features with Oriental populations of C. eurynotus s. lato (for comparison see Fig. 5A–D ), but C. eurynotus s. lato lacks characteristic polygonal sculpture of carapace, lacks spinules at the end of the rostrum, has basal spines of postabdominal claw of equal length, and more massive and thick seta 3 of IDL (width of seta 3 base about 3 width of seta 1 base). South American populations of C. eurynotus s. str. also lack polygons on valves and spinules at the end of rostrum (see Paggi, 1972) and have different shape of male postabdomen (see Sars, 1901). C. idrisi sp. nov. clearly differs from the other species with rounded labral plate; from C. ventricosus in smaller size and in shape of valves; from C. parvus Daday, 1878 in the absence of cavities on the valves; from C. pubescens in lack of setules on the valves, presence of spinules on the end of rostrum, and in non-defined posterodorsal angle of valves; from C. brevilabris Frey, 1980 in round, not oval body, and in the shape of postabdomen.

Distribution and ecology. Presently known from Vietnam, Thailand and Malaysia, but probably is distributed over all Oriental region. In Cat Tien National Park C. idrisi sp. nov. was widely distributed, present in littoral zone of permanent lakes Bau Sau and Bau Chim (locations 1 and 3 in Sinev & Korovchinsky, 2013). It was abundant in autumn 2010 in several forest ponds without aquatic macrophytes (locations 6–8 and 10), and numerous ephippial females and males were found in locations 6–7 and 10. In contrast, its sibling-species, C. eurynotus s. lato, was found only in water bodies with abundant macrophytes, in Bau Sau lake, in both spring and autumn, and in spring in two village ponds (locations 4–5); its gamogenetic stages were never found. For the list of coexisting species see Sinev & Korovchinsky (2013).

Figures

FIGURE 1. Chydorus idrisi sp. nov. from type locality, forest pond in Cat Tien National Park, Dong Nai Province, Vietnam. A – B, outlines of juvenile females of instar I and II. C – E, parthenogenetic female in lateral, dorsal and frontal view. F – G, ephippial female in lateral and dorsal view. H, adult male.

FIGURE 2. Chydorus idrisi sp. nov. from type locality, forest pond in Cat Tien National Park, Dong Nai Province, Vietnam. A – J, parthenogenetic female. A – B, specimens in lateral view; C – D, specimens in anterior and posterior view. E, rostrum. F, head pores, anterior pore up, lateral pores indicated by arrows. G, postabdomen. H, marginal denticles of postabdomen. I, postabdominal claw. J, antenna. K – L, ephippial female in lateral and anterodorsal view.

FIGURE 3. Chydorus idrisi sp. nov. from type locality, forest pond in Cat Tien National Park, Dong Nai Province, Vietnam. A – N, parthenogentic female. A – B, ventral margin of valves and setae of its anterior portion. C, sculpture of central part of valve. D, undeformed head shield in dosal view. E – F, rostrum. G, tip of rostrum. H, head pores. I – K, labrum. L – M, postabdomen. N, antenna. O, head shield of ephippial female. P – R, adult male: P – Q, head shield an its rostrum. R, postabdomen.

FIGURE 4. Chydorus idrisi sp. nov. from type locality, forest pond in Cat Tien National Park, Dong Nai Province, Vietnam. A – K, parthenogenetic female. A, antennule. B – C, limb I and its IDL and ODL. D – E, limb II and its exopodite. F, exopodite of limb III. G – H, inner portion of limb III. I, exopodite of limb IV. J, inner portion of limb IV. K, limb V. L – M, adult male. L, antennule, M, limb I.

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.