Pleuroxus yakutensis, Garibian & Neretina & Klimovsky & Kotov, 2018

Garibian, Petr G., Neretina, Anna N., Klimovsky, Alexey I. & Kotov, Alexey A., 2018, A new case of West-East differentiation of the freshwater fauna in Northern Eurasia: the Pleuroxus trigonellus species group (Crustacea: Cladocera: Chydoridae), Zootaxa 4532 (4), pp. 451-482 : 463-475

publication ID	https://doi.org/ 10.11646/zootaxa.4532.4.1
publication LSID	lsid:zoobank.org:pub:F69AFA65-2777-499D-82D1-1EC7327A798B
DOI	https://doi.org/10.5281/zenodo.5950054
persistent identifier	https://treatment.plazi.org/id/D92B87B6-0B6E-FFF2-B2F7-F9D9FB804789
treatment provided by	Plazi
scientific name	Pleuroxus yakutensis
status	sp. nov.

Treatment

Pleuroxus yakutensis sp. nov.

( Figs. 8–20 View FIGURE 8 View FIGURE 9 View FIGURE 10 View FIGURE 11 View FIGURE 12 View FIGURE 13 View FIGURE 14 View FIGURE 15 View FIGURE 16 View FIGURE 17 View FIGURE 18 View FIGURE 19 View FIGURE 20 )

Pleuroxus trigonellus O.F. Müller in Du (1973) View in CoL : p. 78, fig. 84; Chiang & Du (1979): p. 247–248, fig. 172; Smirnov (1996): figs. 116–118.

Etymology. This new species is named after Sakha Autonomous Republic (Yakutia), the largest Autonomous Republic of Russian Federation, where it was discovered.

Type locality. A small dam lake near Lake Malaya Chabyda (N 61.92249°, E 129.4182°), Sakha Autonomous Republic (Yakutia), Russia. From this locality, 25 parthenogenetic females, 15 ephippial females, 15 males were collected in 0 2.09.2010 by A.A. Kotov and A.I. Klimovsky GoogleMaps .

Type material. Holotype: an adult male deposited at the collection of Zoological Museum of Moscow State University, MGU Ml 161. The label of holotype is: “ Pleuroxus yakutensis sp. nov., 1 ♂ from a small lake near Lake Malaya Chabyda , Yakutia, HOLOTYPE ” . Paratypes from a small dam lake near Lake Malaya Chabyda , preserved in 96% ethanol: 1 undissected parthenogenetic female ( MGU Ml 162: N 61.92249°, E 129.4182°, coll. 0 2.09.2010 by A.A. Kotov & A.I. Klimovsky); 5 undissected parthenogenetic females ( MGU Ml 163: N 61.92249°, E 129.4182°, coll. 0 2.09.2010 by A.A. Kotov & A.I. Klimovsky); 4 undissected parthenogenetic females ( AAK 2018-059 : N 61.92249°, E 129.4182°, coll. 0 2.09.2010 by A.A. Kotov & A.I. Klimovsky) GoogleMaps .

Other material studied: 2 parthenogenetic females from Lake Aly-Syrdah (N 63.9331°, E 127.4878°), Yakutia Autonomous Republic, Russia, coll. 21.08.2010 by A.A. Kotov, AAK 2011-016; 10 parthenogenetic females from Anga on Melnikov Island, Lena River (N 63.86428°, E 127.4509°), Yakutia Autonomous Republic, Russia, coll. 21.08.2010 by A.A. Kotov, AAK 2011-019; 2 parthenogenetic females from Lake 3 on Melnikov Island, Lena River (N 63.85395°, E 127.4678°), Yakutia Autonomous Republic, Russia, coll. 22.08.2010 by A.A. Kotov, AAK 2011-019; 2 parthenogenetic females from oxbow of Tuoluma River, Amga River basin (N 61.80082°, E 133.2925°), Yakutia Autonomous Republic, Russia, coll. 26.08.2010 by A.A. Kotov, AAK 2011-032; 1 parthenogenetic female from the lake in Usun Alass, the vicinities of Karbalah (N 62.15904°, E 134.2948°), Yakutia Autonomous Republic, Russia, coll. 28.08.2010 by A.A. Kotov, AAK 2011-046; 1 parthenogenetic female from Lake 1 near Nizhnekolymskoe village (N 68.5316°, E 160.9364°), Yakutia Autonomous Republic, Russia, coll. 0 8.08.2010 by A.A. Kotov, AAK M-1906; 1 parthenogenetic female from Lake 1 near Nizhnekolymskoe village (N 68.5316°, E 160.9364°), Yakutia Autonomous Republic, Russia, coll. 0 8.08.2010 by A.A. Kotov, AAK M-1907; 4 parthenogenetic females from Pond 1, the vicinities of Yakutsk (N 61.92249°, E 129.4182°), Yakutia Autonomous Republic, Russia, coll. 24.08.2010 by A.A. Kotov, AAK M-1935; 1 juvenile male from Yakutsk (N 62.03011°, E 129.7759°), Yakutia Autonomous Republic, Russia, coll. 0 3.09.2010 by A.A. Kotov, AAK M-1994; 5 adult males from the pool near Namsk-Yakutsk highway (N 63.02369°, E 129.5204°), Yakutia Autonomous Republic, Russia, coll. 0 6.09.2012 by E.I. Bekker & A.I. Klimovsky, AAK M-2587; 5 parthenogenetic females from the pool near Tube-Namtsy highway (N 63.19186°, E 129.4852°), Yakutia Autonomous Republic, Russia, coll. 0 6.09.2012 by E.I. Bekker & A.I. Klimovsky, AAK M-2589.

Diagnosis. Body transparent. Parthenogenetic and ephippial female with length up to 0.63 mm. Median dorsal keel absent. Sculpture of valves and head strongly developed, represented by hexagonal combs. Rostrum sharply pointed, directed downwards. Labral keel long (reaching tip of rostrum), subtriangular, its anterior margin slightly convex, distal angle acute. Posteroventral angle of valve with few denticles. Postabdomen moderately elongated, slightly tapered distally, postanal margin almost straight, with strong postanal teeth, on postabdominal claw proximal basal less about half size of distal one. Antenna I in female elongated, with a basal peg and seven terminal and two subterminal aesthetascs. Apical swimming setae of antenna II subequal in size. Filter plate of gnathobase II with eight setae; filter plate III with 8 setae; filter plate IV with 6 setae; filter plate V with 4 setae. Male with length up to 0.53 mm. Rostrum short. Postabdomen distal portion as a tube bearing sparsely located, thin setules, postanal teeth transformed into bunches of long, thin setules. Gonopore opens ventrally at level of postabdomen narrowing (base of the tube). Antenna I in male with a well-developed basal peg and nine aesthetascs almost subequal in size.

Description. Parthenogenetic female. General. Body transparent. In lateral view body ovoid, high (body height/ body length about 0.76 in adults and juveniles). Maximum height in middle of body ( Figs. 8 View FIGURE 8 A–C, 10A). In dorsal and anterior view body compressed laterally, a median dorsal keel absent ( Figs. 8D View FIGURE 8 , 10B View FIGURE 10 ). Dorsal margin evenly arched from tip of rostrum to posterodorsal angle, without depression between head and the rest of body. Posterior margin relatively straight, posteroventral angle almost straight, with teeth ( Figs. 8 View FIGURE 8 A–C, 10A, 11D-E), ventral margin ( Fig. 11F View FIGURE 11 ) with a slight prominence in thirst half of body behind head ( Figs. 8 View FIGURE 8 A–C). In most cases, sculpture of valves and head strongly developed, represented by hexagonal patterns ( Figs. 8 View FIGURE 8 A–C, 10A–H, 11A–C).

Head with a long rostrum, protruding downward and posteriorly ( Figs. 8A, E View FIGURE 8 , 11A, G View FIGURE 11 ). Eye only slightly larger than ocellus, distance from tip of rostrum to ocellus greater than that between ocellus and eye ( Figs. 8 View FIGURE 8 A–C, E). Head shield elongated, with maximum width immediately in middle ( Figs. 8D, G View FIGURE 8 , 10C View FIGURE 10 , 11A View FIGURE 11 ). In dorsal and anterodorsal views rostrum with acute apex ( Figs. 8 View FIGURE 8 E–G, 9D–E, G–H, 10A, G). Two major head pores ( Figs. 8 View FIGURE 8 G– H, 10C, 12A–B), PP = 1.6 IP. Lateral head pores minute, located at midline between major pores ( Figs. 8 View FIGURE 8 G–H).

Labrum large, with a fleshy main body, small distal labral plate setulated, labral keel short (not projected behind tip of rostrum), triangular, its anterior margin slightly convex, distal angle acute, with a drawn tip ( Figs. 8E View FIGURE 8 , 11G View FIGURE 11 ).

Valves large, ventral margin armed with numerous setae of different size in different regions, all plumose and located exactly marginally ( Figs. 8 View FIGURE 8 A–C, 9A–G, 11D–F). Anterior margin broadly rounded ( Figs. 8 View FIGURE 8 A–C, 9D, 12A). Ventral margin with setae covered by long prominent setules ( Figs. 9 View FIGURE 9 B–G, 11D–F). Posteroventral angle with several teeth ( Figs. 9 View FIGURE 9 B–G, 11D–E).

Postabdomen elongated, wide, its ventral margin almost straight ( Figs. 9 View FIGURE 9 H–L, 13A–B). Preanal margin slightly concave, and slightly longer than anal margin, preanal and postanal angle well-defined, postanal margin clearly longer than anal margin, dorsodistal angle almost straight ( Figs. 9 View FIGURE 9 K–L, 13B–D). Basis of claws inflated. Each side of postanal portion provided with a row of long and thickened postanal teeth, increasing in size distally ( Figs. 9 View FIGURE 9 K– L, 13A–D). Laterally to marginal denticles, a row of fascicles consisting of bunches of short, fine setules ( Figs. 9 View FIGURE 9 H–I, 13A–C). Postabdominal seta longer than postanal margin, with basal segment approximately as long as preanal margin and distal segment short than basal one, supplied with delicate setules (9H).

Postabdominal claw massive and long (subequal in length to anal and preanal margins), slightly curved ( Figs. 9 View FIGURE 9 K–L, 13A–C). Dorsal edge of claw armed with a bunch of fine setules decreasing in size distally. Two basal spines on each claw, distalmost basal spine long (two times longer than the base of claw), proximal basal spine relatively short, two times shorter than distal spine.

Antenna I not reaching tip of rostrum, cylindrical, with a well-defined basal peg ( Figs. 9 View FIGURE 9 M–N). Antennular sensory seta slender, longer than half of antenna I, arising at half of antennular body. Nine short aesthetascs unequal in length.

Antenna II relatively short, coxal part with two sensory setae, basal segment robust, with a small spine ( Figs. 8 View FIGURE 8 A–C, 9O, 10E, 11G). Antennal branches elongated, exopod and endopod brunches subequal in length, all segments cylindrical, covered by transverse rows of fine spinules. Antennal formula: setae 0-0-3/1-1-3, spines 1-0- 1/0-0-1. Proximal segment of exopod with a short spine. Exopod and endopod apical swimming setae and lateral endopod setae covered by fine long setules ( Fig. 9P View FIGURE 9 ). Distal segments of apical setae with chitin insertions. Apical spines on distal segments of exopod and endopod almost subequal in size ( Fig. 11G View FIGURE 11 ).

Maxilla I ( Fig. 14A View FIGURE 14 ) with three short setulated setae.

Maxilla II absent.

Limb I ( Figs. 13E View FIGURE 13 , 14 View FIGURE 14 B–C, 15A–C, G): accessory seta present, ODL relatively small, bears a long seta with naked distal segment and a short seta with short, setulated distal segment ( Figs. 14C View FIGURE 14 , 15G View FIGURE 15 ). IDL subequal in size to ODL, covered by transverse rows of setules. First IDL seta short, naked, second and third IDL setae not equal in size and similarly armed distally with short, fine setules ( Figs. 14C View FIGURE 14 , 15G View FIGURE 15 ). Endite 3 with three soft posterior setae (a–c) and stiff anterior seta 1 of similar length ( Figs. 14C View FIGURE 14 ). Endite 2 with short seta (anterior seta d), long setae (e and f), and delicate posterior seta 2 armed with minute setules. Endite 1 with long, slender posterior setae (g–i), a very short seta (j), and anterior seta 3 (it significantly shorter than seta 2). Fascicles of thin setules on inner face of limb, plus bunches of longer thicker setules at ventral margin of limb. Two slender ejector hooks of remarkably different size. A short seta, a remnant of the maxillar process, on the limb base.

Limb II ( Figs. 14D View FIGURE 14 , 15 View FIGURE 15 A–F) subrectangular. Exopodite subquadrangular, with a short seta. Inner portion of limb with eight setae: setae 1–3 especially long, setae 4–5 shorter, subequal in size, setae 6–8 short, also subequal in size. A series of small projections posteriorly to distal setae, and a small sensillum near scraper 4. Distal armature of gnathobase with a bunch of plumose setules, unique for the anomopods, and four setae. Filter plate with eight setae, two distalmost setae subequal in size and shorter than the rest; basal most seta of filter plate with inflated basal segment.

Limb III ( Figs. 14 View FIGURE 14 E–F, 15A) with ovoid elongated epipodite. Exopodite subrectangular, with four distal setae (1–4), and three lateral setae (5–7). Distal endite with three anterior setae ( Figs. 14 View FIGURE 14 E–F), 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), slightly increasing in size basally, not armed, small bottle-shaped sensillum near seta 4. On posterior surface, six soft setae (a–f) subequal in length, bilaterally armed with sparse, fine setules. Gnathobase not clearly separated from basal endite. Distal armature of gnathobase with large, bottleshaped sensillum, three setae, and a bunch of setules. Filter plate with eight setae.

Limb IV ( Figs. 14 View FIGURE 14 G–H, 15A) with ovoid elongated epipodite and rounded preepipodite. Exopodite wide, subovoid, with seven setae of unequal size. Innerdistal portion of limb IV with four marginal setae. Distalmost seta (1) slender and uncovered with minute setules on distal segment, setae 2–4 with thick basal segments and slender, setulated distal segments. On posterior surface, four soft setae (a–d), gnathobase well-separated, its distal armature with four setae. Filter plate with six setae.

Limb V ( Figs. 14I View FIGURE 14 , 15A View FIGURE 15 ) with ovoid elongated epipodite and rounded preepipodite. Exopodite large, subovoid, with a single distal seta 1 and three lateral setae (2–4), distally to seta 1 there are two projections bearing long setules. Inner limb portion as truncated, flat lobe, with setulated inner margin, supplied with setulated setae 1 and 2, the latter bears specially robust setules. Distal armature of gnathobase as a single projection. Filter plate with four long setae.

Ephippial female. Identical to parthenogenetic female ( Figs. 16 View FIGURE 16 A–D) in size and shape, but anterodorsal portion of valves modified into ephippium ( Figs. 16 View FIGURE 16 E–H). Ephippium not bordered from the rest of body, brownish, contains a single resting egg.

Adult male. General. Body transparent. In lateral view elongated (height/length ratio about 0.69 in adults) ( Fig. 17A View FIGURE 17 , 18A View FIGURE 18 ). Dorsal margin slightly convex, posterodorsal angle expressed, posteroventral angle broadly rounded. Ventral margin strongly convex.

Head relatively small, narrow ( Fig. 17A View FIGURE 17 , 18 View FIGURE 18 C–F). Rostrum pointed, relatively short, only slightly longer than antenna I ( Fig. 17A View FIGURE 17 , 18F View FIGURE 18 ). Compound eye slightly larger than ocellus. Distance from tip of rostrum to centre of ocellus somewhat larger than distance between centers of ocellus and eye.

Labrum with a short, triangular labral keel, similar in proportion to that in female ( Fig. 17A View FIGURE 17 ).

Armature of posteroventral angle of valve as in female ( Fig. 17 View FIGURE 17 B–D).

Postabdomen elongated, tapering distally ( Figs. 17E View FIGURE 17 , 18A View FIGURE 18 , 19 View FIGURE 19 A–E). Postabdomen length/height ratio about 2.8. Ventral margin broadly concave, somewhat undulated in postanal portion. Preanal and anal margins slightly concave. Postanal margin broadly convex. Postanal margin about 2 times longer than anal and preanal margins. Postanal and preanal angles smooth. Postanal margin armed by bunches of long setules of similar size through all the margin length, following by clusters of short setules on anal margin. Bunches of fine setules on lateral surfaces of postanal and anal margins. Distal portion of postabdomen as a narrow tube, thin sparsely located setules on it. Gonopores open ventrally on the level of postabdominal narrowing. Some short spinules near them ( Figs. 17E View FIGURE 17 , 19E View FIGURE 19 ).

Postabdominal claw almost straight, 2 times shorter than anal margin, with a thin basal spine subequal in length to claw diameter at base, and a tiny second basal spine ( Fig. 17E View FIGURE 17 , 19 View FIGURE 19 B–D).

Antenna I subconical, short (thicker than in female), with a well-developed basal peg, with nine terminal aesthetascs of subequal size ( Fig. 17F View FIGURE 17 ). Male seta long, arising at the middle of antenna I, slightly shorter than antennular sensory seta ( Figs. 17F View FIGURE 17 , 18 View FIGURE 18 B–F).

Antenna II similar to female ( Fig. 17G View FIGURE 17 , 18E View FIGURE 18 ).

Limb I with U-shaped copulatory hook ( Figs. 17H View FIGURE 17 , 18D View FIGURE 18 , 19F View FIGURE 19 ). Inner distal lobe with four setae (one of them especially long, three setae shorter). Male seta slender, slightly curved.

Juvenile male of instar II. General. Proportions of body in general similar with adult male ( Figs. 20 View FIGURE 20 A–G), but postabdomen more thicker, almost conical, without a distal narrowing ( Fig. 20F View FIGURE 20 ). Setae on inner distal lobe of limb I slightly different in length.

Size. Maximum length of adult parthenogenetic females up to 0.63 mm, maximum height 0.50 mm. Maximum length and height of ephippial females subequal to adult parthenogenetic females. Maximum length of adult males up to 0.53 mm, height 0.33 mm. Holotype 0.52 mm in length, 0.33 mm in height.

Variability. No significant variability between investigated individuals was found. Variability was observed in armature of posteroventral angle of valve and concerns both the number of denticles and their shape in females and males.

Differential diagnosis. P. yakutensis sp. nov. differs from its closes congener P. trigonellus in: (1) presence of basal peg in male antenna I; (2) distal narrowing of male postabdomen having sparsely located thin setules; (3) terminal aesthetascs on male antenna I of subequal size.

From sibling species, P. uncinatus ( Fig. 21 View FIGURE 21 A–G), both taxa differ based on shape of male postabdomen. In P. uncinatus the distal tube of the postabdomen is short; postabdomen gradually narrowing distally ( Fig. 21D View FIGURE 21 ). In the P. trigonellus species group the distal tube of the postabdomen is long.

Distribution. Based on original samples we found that this new species is widely distributed in different water bodies of Yakutia Autonomous Republic. Du (1973) and Chiang & Du (1979) clearly illustrated thin setules on the distal tube of the male postabdomen in “ P. trigonellus ” from North China. Probably, Pleuroxus yakutensis sp. nov. may be considered as a common species in water bodies of East part of Northern Eurasia, although these data are just only preliminary. P. cf. trigonellus is present in Japan ( Uéno 1937), but males are not described from this country. Yoon (2010) described " P. trigonellus " from South Korea, but in reality the female from his fig. 78 belongs to P. quasidenticulatus ( Smirnov, 1996).

Taxonomic comments. No trigonellus -like taxa were described from East Asia to date ( Smirnov 1971, 1996) except of P. csikii Daday, 1901 from "Sibiria: Abakansk" or "Abakanszk". This is most likely the Abakan River (or town of Abakan?) in the Yenisey basin, Khakassia Autonomous Republic of Russian Federation. Specimens in the slide of E. Daday (lectotype and paralectotypes selected by Forró & Frey (1982)) are in bad state and not helpful to clarify the taxon’s status. But according to a well-developed labral keel this taxon is closer to P. trigonellus s.str., moreover the specimen in Daday's figure 9 has disproportionally long distal postanal teeth (as compared with more proximal teeth), it could be even a separate taxon, or subject to Daday’s specific drawing styles.

Pleuroxus trigonellus brevicornis Brehm, 1933 View in CoL was described from Sumatra ( Brehm 1933) and then recorded from China (see in Ji et al. 2015). Most probably, it is not member of the trigonellus View in CoL -group due to it relatively small postanal teeth, moreover, a specimen from Brehm's (1933) fig. 36 has a really long posterior margin (its length is about half of body height), and it could be a member of Alonella, not Pleuroxus.