Pleuroxus annandalei ( Daday, 1908 )

Kotov, Alexey A. & Sheveleva, Natalia G., 2008, Separation of Pleuroxus pamirensis (Werestschagin, 1923) from P. annandalei (Daday, 1908) (Cladocera: Chydoridae), Zootaxa 1775, pp. 25-38 : 26-32

Pleuroxus annandalei ( Daday, 1908)

Figs 1–4

Euryalona annandalei Daday 1908: 328 –329, fig. 3a–c.

Sinopleuroxus annandalei (Daday) in Chiang Sieh-chih 1963: 61–62 (in Chinese), 69 (in English), figs 41–46; Shen Chia-jui & Sung Ta-hsiang 1964: 64 (in Chinese), 68 (English).

Cornuella annandalei (Daday) in Chiang Sieh-chin & Du Nan-shan 1979: 263–264, fig. 187.

Pleuroxus pamirensis (Werestschagin) in Smirnov 1996: 67 –68 (only Mongolia), figs 252–253.

Pleuroxus sp. in Pomazkova & Sheveleva 2006: 186, fig. 12.1.

Type series. Type locality. "Rham-Tso, altitudine 4480.5 mtr., a superficie maris" ( Daday 1908), Chumbi Valley region, South Tibet. Type material. Apparently lost, absent in Daday's collection ( Forró & Frey 1982; rechecked by Dr L. Forró in 2007).

Material examined. Mongolia. Many parthenogenetic females from Lake Khövsgöl (=Khubsugul) (51º05' N, 100º29' E), Baikal Rift System, coll. in 25.07.1997 by N. G. Sheveleva & Dulma A., AAK 2007- 322; a juvenile female from Achit Nuur (nuur = lake)(49º29' N, 90º30' E), unknown collector, AAK 2007-323 (from Smirnov's slide 3608).

Diagnosis. Parthenogenetic female with body relatively transparent, of moderate height, postero-ventral angle without denticles. No medial keel on dorsum, valves with paired lateral projections of size greatly differing among populations. Rostrum long, protruding ventrally to apex of labral keel, with a pair of tubercles on tip. Ocellus and compound eye of subequal size. Labral keel relatively small, with rounded apex. Head shield posteriorly widely rounded, postpore distance = about 1.5–1.8 interpore distance. Valve with reticulation in its ventral portion, all setae of valve ventral margin exactly marginal, setules on posterior margin of valve exactly marginal. Postabdomen wide, narrowing distally, its preanal margin approximately as long as anal margin, preanal and postanal angle well-defined, postanal margin with series of 2–3 relatively small postanal teeth. Anntenna I relatively slender, not reaching tip of rostrum, with a strong basal peg. On antenna II, all apical "swimming" setae subequal in size, lateral seta on basal segment of endopod shorter than other setae. Spine on proximal segment of exopod significantly shorter then half of next segment length. On limb I accessory seta present, smallest ODL seta well-defined, distally with minute setules, two largest IDL setae subequal in size, on endite 1 seta i short. On inner-distal portion of limb II size of scrapers regularly decreasing basally, filter plate II with eight setae. On exopodite III setae 1 shortest among distal setae, filter plate III with eight setae. On inner portion of limb IV setae 2-4 with thick setules, filter plate IV with six setae. Exopodite V without special projections distally to distal seta 1. Juvenile female with solitary postanal teeth. Ephippial female and male unknown. Size up to 1 mm.

Redescription. Parthenogenetic female. Body relatively transparent, in lateral view oval, of moderate height for the genus (body height/ body length = 0.70–0.75 in large adults), maximum height in middle ( Fig. 1 A). Dorsal margin evenly arched from tip of rostrum to postero-dorsal angle, which is well-defined, posterior margin almost straight, postero-ventral angle broadly rounded, without any denticles, ventral margin with a slight projection somewhat anterior to its middle. Body moderately compressed laterally, without a dorsal medial keel, with lateral projections of size greatly differing among populations from small hillocks ( Fig. 1 B– C) to distinct horns. Chinese authors (Chiang Sieh-chin & Du Nan-shan 1979) illustrated another, smaller, pair of projections in posterior half of valves, which were absent in our specimens. Parthenogenetic female carries one-two eggs.

Head with a long rostrum, protruding downward ( Fig. 1 D), its tip with a pair of tubercles ( Fig. 1 E–F). Ocellus and compound eye of subequal size, the former lies near base of antenna I, closer to eye that no tip of rostrum. Labrum with fleshy main body supplied with lateral outgrowths, a relatively large distal labral plate and a relatively small (for the genus) medial labral keel, having rounded apex ( Fig. 1 G–I) not reaching tip of rostrum. Head shield elongated, with maximum width anterior to level of mandibular articulation, its posteriormost portion rounded ( Fig. 1 J). Two major head pores, postpore distance = about 1.5–1.8 interpore distance ( Fig. 1 K). Lateral head pores minute, normally located asymmetrically to midline. Valve with a reticulation expressed in its ventral half ( Fig. 1 M); valve portion contacting head shield with a special flap on its inner side; in anterior portion of valve, setae located at a distance from valve margin ( Fig. 1 N–O); ventral margin armed with numerous setae of different size in different regions, all plumose and located exactly marginally ( Fig. 1 P–S); posterior valve margin with additionally chitinised rim and a row of fine setules.

Abdomen short, its first (basalmost) segment with convex dorsal margin. Postabdomen wide, its ventral margin almost straight ( Fig. 2 A–B). Preanal margin slightly concave, approximately as long as anal margin, preanal and postanal angle well-defined, postanal margin equal or slightly longer than anal margin, dorso-distal angle widely rounded and slightly prominent distally, inflated basis of claws bordered from postanal margin by a distinct depression. Postanal margin provided with series of 2–3 relatively small postanal teeth. Laterally on postabdomen fascicles of short, fine setules. Postabdominal seta as long as preanal plus anal margin or shorter, with distal segment setulated. Postabdominal claw shorter anal margin, massive, evenly curved, with setules along ventral margin, and two basal spines, proximal one about half size of distal one ( Fig. 2 C– D).

Antenna I small, its length more than half labral keel length, slightly narrowing distally, with a welldefined basal peg ( Fig. 2 E–F). Antennular sensory seta slender, longer than half the antenna I, arising at middle to one third of antennular length from distal end directly from antenna I body, on a small prominence. Nine aestetascs of differing size. Antenna II ( Fig. 2 G) relatively short, coxal part with two sensory setae, basal segment robust, with a rudimentary distal spine. Antennal branches relatively elongated, endopod longer than exopod, all segments cylindrical ( Fig. 2 H), antennal formula, setae 0-0-3/1-1-3, spines 1-0-1/0-0-1. All apical "swimming" setae of endopod subequal in size, with chitinous insertions within distal segments ( Fig. 2 I), lateral seta on basal segment of endopod shorter than other setae. Spine on proximal segment of exopod significantly shorter then half of next segment length. Exopod and endopod apical spines subequal in size.

Limb I ( Fig. 3 A–B). Accessory seta present, ODL with a long seta armed with short setules distally, and a small seta with short, setulated distal segment. IDL of size similar with ODL, supplied with several series of setules; first IDL seta short, naked, second and third IDL setae subequal in size and similarly armed distally with fine setules. Endite 3 with three soft posterior setae (a–c) and stiff anterior seta 1 of similar length. Endite 2 with short posterior setae d and e, long seta f, and thin anterior seta 2 armed with minute setules distally. Endite 1 with long posterior setae g–h, a short seta i, a very short seta j, and anterior seta 3 similar to seta 2. Fascicles of thin setules on inner face of limb, plus bunches of larger setules at ventral margin of limb. Two slender ejector hooks of subequal size. A short seta, and a setulated hillocks, both remnants of gnathobase I, on limb base.

Trunk limb II. Exopodite subquadrangular, with a naked seta. Inner portion of limb with eight scrapers ( Fig. 3 C: 1–8), setae 1–2 specially long, with finer setules. A series of small projections posteriorly to distal setae, and a small sensillum near scraper 4. Distal armature of gnathobase with a bunch of setules and four setae ( Fig. 3 D: 1–4). Filter plate II with eight setae, two distal members shorter than the rest.

Trunk limb III. Exopodite sub-rectangular, with four distal setae ( Fig. 3 E: 1–4) and three lateral setae (5– 7). Distal endite with three anterior setae ( Fig. 3 F: 1–3), all with minute setules distally, of them two distal setae (1–2) long, basalmost seta (3) short. Small sensillae near bases of setae 2 and 3. Basal endite with four anterior setae (4–7), armed with fine setules distally, a sensillum with pointed tip near seta 4. On posterior surface of limb, six soft setae (a–f) bilaterally armed with sparse, fine setules. Gnathobase not clearly separated from basal endite. Distal armature of gnathobase a with large, bottle-shaped sensillum, three setae and a bunch of setules. Filter plate III with eight setae, distalmost seta shorter than the rest.

Trunk limb IV. Exopodite wide, subovoid, with seven setae of unequal size ( Fig. 3 G: 1–7). Inner-distal portion of limb IV with four marginal setae ( Fig. 3 H: 1–4). Distalmost seta (1) stout, with minute setules on distal segment, setae 2–4 with thick basal segments and slender, setulated distal segments, sensillae located near setae 2 and 3. On posterior surface, four soft setae (a–d), Gnathobase distal armature with four setae. Filter plate with six setae.

Trunk limb V. Exopodite large, subovoid, with a single distal seta ( Fig. 3 I: 1) and three lateral setae (2–4), no special projections distally to seta 1. Inner limb portion as elongated, flat lobe, with setulated inner margin, supplied with setulated setae 1 and 2. Distal armature of gnathobase as a single projection, filter plate V with four long setae.

Juvenile female. Body more elongated as compared with adult (body height/ body length = 0.64–0.68), with almost straight dorsal margin of carapace and less distinct postero-dorsal angle ( Fig. 4 A). Labral keel broad ( Fig. 4 B), relatively larger as compared with adult. Postabdomen with solitary postanal teeth ( Fig. 4 C, F), postabdominal claw ( Fig. 4 D, G) in general as in adult. Antenna I with sensory seta in distal half of its body and less distinct basal peg ( Fig. 4 E).

Ephippial female, male. Unknown.

Size. Parthenogenetic females in our material from Khövsgöl 0.46–0.85 mm, 0.9–1.0 mm according to Daday (1908).

Differential diagnosis. See differences from closest P. pamirensis in Table 1 .

Distribution. P. annandalei was also found by M. Alonso (pers. comm.) in a small unnamed lake (48º23'47'' N, 88º53'59'' E) near Dayan Nuur in Mongolian Altai. So, the species is distributed on the Tibetan Plateau (Tibet Autonomous Region and Qinghai Province of China), Baikal Rift System and Mongolian Altai.

Comments. Daday (1908) missed the presence of lateral projections on valves in his first description of P. annandalei . Earlier Kotov & Štifter (2005) found that Daday also missed the lateral horns in his description of " Ilyocryptus sordidus (Liévin, 1848) " from Paraguay, subsequently described as I. paranaensis Paggi, 1989 . Observations of a specimen in lateral position only, was characteristic of many earlier cladoceran investigators and led to mistakes.

Chiang Sieh-chih (1963) and then Chiang Sieh-chin & Du Nan-shan (1979) correctly found that their "horned" chydorids belong to Daday's taxon. But their illustrations are not fully adequate. For example, the posterior gut illustrated by them was too narrow. In reality, it is obvious from their figures that their female had an anal margin longer than the preanal margin, which is characteristic of P. annandalei .

Upon re-examination of material from Smirnov (1996), we found that a female from Achit Nuur with solitary postanal teeth on the postabdomen, which was identified by him as P. pamirensis , is actually a juvenile P. annandalei (length 0.60 mm).

Figures

FIGURE 1. Pleuroxus annandalei, parthenogenetic female from Lake Khövsgöl (A-B) Large adult, lateral and dorsal view. (C) Lateral projection of other individual. (D) Head, lateral view. (E-F) Tip of rostrum, anterior view. (G-H) Labrum, lateral view. (I) Head with labrum, ventral view. (J-K) Head shield and its posterior portion. (L) Dorsal head pores. (M) Valve. (N) Anterior portion of valve, external view. (O-P) Same, inner view. (Q) Medium portion of valve. (R-S) Postero-ventral portion of valve. Scale bars: 0.1 mm.

FIGURE 2. Pleuroxus annandalei, parthenogenetic female from Lake Khövsgöl (A-B) Postabdomen. (C-D) Postabdominal claw, external view. (E-F) Antenna I. (G-I) Antenna II, its endopod and apical swimming seta. Scale bars: 0.1 mm.

FIGURE 3. Pleuroxus annandalei, parthenogenetic female from Lake Khövsgöl (A-B) Limb I and its distal portion. (C- D) Limb II and distal armature of its gnathobase. (E-F) Limb III and its inner portion. (G-H) Limb IV and its inner portion. (I) Limb V. Scale bars: 0.1 mm.

FIGURE 4. Pleuroxus annandalei, juvenile female from Lake Khövsgöl (A-E) and Achit Nuur (F-G), both in Mongolia. (A) General view. (B) Labrum. (C-D) Postabdomen and postabdominal claw. (E) antenna I. (F-G) Postabdomen and postabdominal claw. Scale bars: 0.1 mm.

Tables

TABLE 1. Differences between adult parthenogenetic females of P. annandalei and P. pamirensis.

Character				P. annandalei	P. pamirensis
Preanal margin of postabdomen longer than anal margin				-	+
Postanal teeth predominantly solitary				-	+
Antenna I thick				-	+
Spine on basal exopod segment of antenna II longer than	half	of	second segment	-	+
Seta i on endite 3 of limb I long				-	+