Rheoalona mekongensis, Sinev & Tiang-Nga & Sanoamuang, 2017

Sinev, Artem Y., Tiang-Nga, Supatra & Sanoamuang, La-Orsri, 2017, New genus of Cladocera of subfamily Aloninae (Anomopoda: Chydoridae) from the Mekong River, Zootaxa 4276 (3), pp. 416-426 : 418-423

Rheoalona mekongensis sp. nov.

( Figs 1–3 )

Sinev & Kotov, 2012: 23–26, Figs. 12–13 ( Coronatella sp.); Kotov, Van Damme, Bekker, Siboualipha, Silva-Briano, Ortiz, de la Rosa & Sanoamuang, 2013: 90, Figs. 7A–B ( Coronatella sp.); Sinev, 2016: 467, Figs. 9M–O ( Coronatella sp.)

Etymology. The species is named after its type locality, the Mekong river.

Type locality. South coast of Mekong river at Bueng Kan Province, Thailand, N 18 02.676', E 103 04.999', 19.01.2016, coll. A.Y. Sinev & S. Tiang-nga.

Type material. Holotype. Parthenogenetic female from the type locality, deposited at Zoological Museum of M.V. Lomonosov Moscow State University, Ml-156.

Paratypes. Over 40 parthenogentic females from type locality deposited at Zoological Museum of M.V. Lomonosov Moscow State University, Ml-157. Twelve parthenogenetic females from type locality were dissected for analysis of appendages or used for SEM, and not deposited afterwards.

Material studied earlier and excluded from type series. See Sinev & Kotov (2012) for the list of material from Thailand.

Diagnosis. As for genus.

Description. Parthenogenetic female. Body compressed laterally, low oval ( Figs. 1 A–C, 2A–B); in juvenile females lower than in adults, maximum height at the middle of body; in adults height/length ratio about 0.58. Dorsal margin curved, postero-dorsal and postero-ventral angles broadly rounded. Posterior margin convex. Ventral margin ( Fig. 1 D) almost straight, with about 40 setae; first 10–12 setae long; next 7–10 setae very short; setae in posterior portion of margin of moderate length. Valves with weakly developed linear sculpture in ventral part, without striae. Postero-ventral corner of valves with about 15 small setulae. A row of about 100 setulae, alternating very long and short setulae along posterior margin at some distance from one on inner side of carapace ( Fig. 1 E).

Head triangle-round in lateral view. Eye and ocellus large, of similar size. Rostrum elongated, with rounded tip. Two major head pores with broad connection between them ( Fig. 1 F, 2C); PP about 0.7–0.8 IP. Lateral pores minute, without cosmaria, located at 1.5 IP distance from midline, at level of anterior major pore.

Labrum ( Fig. 1 G–I) relatively small, with oval keel with broadly rounded apex, keel height is less than 1.5 keel widths. Posterior margin of keel without clusters of setulae, no lateral indentations found.

Postabdomen ( Fig. 1 J–K, 2D) elongated, with parallel margins, length about 2.3–2.4 height. Ventral margin weakly convex. Distal margin convex, distal angle rounded. Dorsal margin straight in postanal part and convex in anal part; distal part about 2.3 times longer than preanal one; postanal portion two times longer than anal one.

Preanal and postanal angles well-defined. Preanal margin weakly concave. Postabdomen provided with 6–8 clusters of 1–3 very small elementary marginal denticles on postanal margin, and with 4 groups of marginal spinules on anal margin. Ten-twelve lateral fascicles of setulae; posteriormost setae of postanal fascicles very thick and long; distal fascicles about 3 width of claw base; all other setulae in fascicles much thinner and shorter. Postabdominal claw weakly curved, thin, longer than preanal margin of postabdomen. Basal spine very short, less than 0.1 length of claw itself.

Antennule ( Fig. 3 A) relatively small, not reaching the tip of rostrum by far. Antennular seta thin, of about half length of antennule, arising at 2/3 distance from the base; a cluster of long setulae located near its base. Nine terminal aestetascs of similar size, about half length of antennule. All aesthetascs not reaching the tip of rostrum.

Antenna relatively short and massive ( Fig. 2 E, 3B); antennal formula setae 0-0-3/1-1-3; spines 1-0-1/0-0-1. Basipodite robust, with cluster of long setulae. Branches relatively short, with basal segments 1.5 times longer and more massive than middle and apical segments. Setae arising from basal segment of endopodite short, not reaching the end of endopodite. Setae arising from middle segment of endopodite of same size than apical setae. Spine on basal segment of exopodite 1.3–1.4 times longer than middle segment. Spines on apical segments 1.5 times longer than apical segments. Basal and middle segments of exopodite with clusters of long hard setulae in distal portion.

Maxilla ( Fig. 3 C) with two curved setae, setulated in distal part.

Five pairs of thoracic limbs.

Limb I ( Fig. 3 D–F). Epipodite small, rounded. Accessory seta about 1/3 length of ODL seta. IDL with two setae (2 and 3), both of them long, slender, armed with long setulae distally; seta 3 about 3/4 length of ODL seta; seta 2 about 2/3 length of ODL seta. Endite 3 with four setae; seta c significantly longer and thicker than others. Endite 2 with three outer setae of different length, middle of them (e) as long as ODL seta, and rudimentary inner seta (2). Endite 1 with two 2-segmented setae (g–h), a long flat seta (i) shifted to the limb base, and rudimentary inner seta (3). Five-six rows of long setulae on ventral face of limb. Two ejector hooks, one of them 1.5 times longer than the other.

Limb II ( Fig. 3 D–H) Exopodite elongated, without seta. Inner portion of limb with eight scraping spines; spines 1–5 long, decreasing in size basally, armed with thin setulae; scrapers 6–8 short, of similar size, armed with elongated denticles. Inner lobe of gnathobase inflated. Distal armature of gnathobase of only 3 elements. Filter plate II with seven setae of similar length.

Limb III ( Fig. 3 I–K). Exopodite small, rounded, with six setae. Seta 3 longest; seta 4 and 5 about 1/3 and 1/2 length of seta 3, respectively; other setae short. Setae 1–4 plumose; setae 5 with short thick setulae in distal part; seta 6 naked. Distal endite with 3 setae, two distalmost setae (1–2) scraping, slender, sharp, with spinules in distal part; seta 1 1.3–1.5 times longer than seta 2; basalmost seta (3) short, flattened. Basal endite with four stiff setae (4–7), increasing in size toward the base; a small sensillum near the base of distalmost seta. Four soft setae (a–d) increasing in size basally, setulated in distal part. Gnathobase not clearly separated from basal endite. Distal armature of gnathobase with three elements: an elongated, cylindrical sensillum; a long thin, bent seta; and sharp spines. Filter plate III with seven setae.

Limb IV ( Fig. 3 L–M). Exopodite rounded, with six setae; seta 3 being longest; setae 1–5 plumose; seta 6 naked. Inner portion of limb IV (Fig. 12I) with only three setae, a scraper (1) and two flaming torch setae (2–3), and a sensillum. Both flaming-torch setae broad, with short distal part; seta 2 of peculiar morphology, with setulae located on both sides of distal part; seta 2 of typical shape for subfamily. Three soft setae (a–c) (Fig.12J), increasing in size basally; seta c very large. Gnathobase with a long 2-segmented seta and a large projection distally. Filter plate with 5 setae.

Limb V ( Fig. 3 N). Pre-epipodite setulated. Epipodite oval, without finger-like projection. Exopodite oval, not subdivided into two lobes, with four plumose setae. Setae 1–3 long, evenly decreasing in size basally; seta 4 short, 4 times shorter than seta 1. Inner lobe broad oval, with setulated inner margin. At inner face, two setae, one equal in length to seta 2 of exopodite, other 2.5 times shorter. No filter plate.

Ephippial female and male. Not known.

Size. In juvenile females of instar I, length 0.24–0.27 mm, height 0.13–0.14 mm; in juvenile females of instar II, length 0.29–0.3 mm, height 0.16–0.18 mm; in adult females, length 0.34–0.4 mm, height 0.2–0.24 mm.

Distribution. So far known from North-West and North-East Tahiland and Vientiane province of Laos, but can be a common species in the rivers of South-East Asia, we expect it to be present within whole Mekong basin.

Ecology. The species was found at the coastal zone of Mekong river, at depth of 10–30 cm, on hard clay bottom covered by a layer of fine clay sediments, with some rocks covered by clusters of filamentous algae; the water at the time of sampling was extremely turbid. The studied sample contained only one other cladoceran species, a single female of the rheophylous Aloninae species Nicsmirnovius eximius (Kiser, 1948) .

Figures

FIGURE 1. Rheoalona mekongensis sp. nov. from the Mekong River (type locality). A – B, juvenile females of instars I – II. C, adult parthenogenetic female. C, lateral view. D, ventral margin of valves. E, posterior margin of labrum. F, head pores. G – I, labrum. J – K, postabdomen.

FIGURE 2. Rheoalona mekongensis sp. nov. from the Mekong River (type locality), adult parthenogenetic female. A, lateral view. B, dorsal view. C, head pores (lateral head pores and posterior margin of head shield indicated by arrows). D, postabdomen. E, antenna.

FIGURE 3. Rheoalona mekongensis sp. nov., appendages of adult parthenogenetic female. A, antennule and outline of rostrum. B, antenna. C, maxillae. D – N, thoracic limbs. D, limb I. E – F, ODL and IDL of limb I. G – H, inner and outer parts of limb II. I, exopodite of limb III. J – K, inner portion of limb III. L, exopodite of limb IV. M, inner portion of limb IV (gnathobase filter plate not shown). N, limb V. (A – B, D – E, I, N—from Sinev & Kotov 2012; C, F, G – H, J – L—orig.)