Bythotrephes centralasiaticus, Korovchinsky, 2020

Korovchinsky, Nikolai M., 2020, Description of a new species in the genus Bythotrephes Leydig, 1860 (Crustacea Cladocera: Onychopoda), supplements to selected species, and concluding remarks on the genus, Zootaxa 4789 (2), pp. 441-465 : 443-449

publication ID	https://doi.org/ 10.11646/zootaxa.4789.2.5
publication LSID	lsid:zoobank.org:pub:7B9DD51D-1BD9-431B-A57E-901250DAB7F8
persistent identifier	https://treatment.plazi.org/id/03F20E34-FF88-FFD7-FF19-FD65FAA5FB49
treatment provided by	Plazi
scientific name	Bythotrephes centralasiaticus
status	sp. nov.

Treatment

Bythotrephes centralasiaticus sp.nov.

( Figs. 1–4 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 )

Bythotrephes sp.: Korovchinsky 2019, p. 346.

Etymology. The species is named for the region of its location (mountain lakes of Tyva Republic), which is situated in the geographical center of Asia.

Type locality: Lake Kadysh, Tyva Republic, Southern Siberia, Russia.

Type material. Holotype. A female with body length 2.30 mm from Lake Kadysh preserved in formalin and deposited in Zoological Museum of Moscow State University ( MGU ML 184 ).

Paratypes. All specimens, except holotype, enumerated below. Three of them from Lake Kadysh are deposited in Zoological Museum of Moscow State University ( MGU ML 185 ); others in author’s collection .

Material examined. Russia, Tyva (coll. E.I. Zuykova): 1) Lake Kadysh (52°35’54’’ N; 97°3’49’’ E), July 2008 GoogleMaps , 40 ad, 10 juv; 2) Lake Er-Kara-Hol (51°24’56’’ N; 89°29’13’’ E), July 7, 2004, 13 ad, 3 juv; 3) Lake Shuram- Hol (52°41’25’’ N; 96°47’0’’ E), July 19, 2004, 1juv.

Data on body and body parts measurements of specimens of some populations are presented in Table 1.

Description. Female. General body appearance and segmentation. Body elongated and divided into four parts: head, thorax, abdomen, and postabdomen with long caudal process ( Figs. 1A, 1M View FIGURE 1 ). Its longitudinal axis is conspicuously incurved when head is sometimes located at almost right angle to the thorax.Also highly movable abdomen can be either at straight line with the thorax or stays at different angles to it. Head large with rounded anterior part filled by the enormously developed compound eye and bearing small antennules ventrally. Posterior part of head bearing long swimming antennae and mouth parts consisting of mandibles, maxillules (mx I), and upper lip (labrum). Thorax with strongly developed muscular ventral side bearing four pairs of thoracic limbs of different size directed antero-ventrally. Dorsally, thorax bears sacklike carapace transformed into brood pouch sometimes reaching large size. Abdomen (metasome) is elongated, cylindrical, inconspicuously three-segmented (see Korovchinsky 2015) and flexible, connected with small postabdomen, bearing ventrally a pair of straight claws and posteriorly very long straight caudal process with a pair of similar claws proximally. General body length of females (without caudal process) may reach 2.30 mm (in the examined specimens it ranges from 1.70 to 2.30 mm) while the length of caudal process may surpass the body length in 1.5–2.5 times (on average, in about two times).

Head. Comparatively very large ( Fig. 1A View FIGURE 1 ) and subdivided into two parts: rounded anterior part mostly filled by large compound eye, and posterior part bearing dorsally a large saddle-shaped neckorgan, swimming antennae and mouth parts. Large pigment spot occupies about one-third or, at most, half of the eye’s volume. Ocellus (naupliar eye) is absent.

Antennules. Small and situated on the ventral side of the anterior head part beneath the eye. They are bulbous ( Fig. 1B View FIGURE 1 ) and sit on the joined basis slightly split anteriorly. Terminally they bear five regular aesthetascs in two groups, with three and two in each one, and one shorter and thinner aesthetasc-like structure, situated in the group with two regular aesthetascs, and having slightly widened apical end (“accessory simple seta” according to Scourfield (1896)).

Swimming antennae. Comparatively long, with elongated cylindrical basipodite ( Figs. 1A, 1C View FIGURE 1 ). Of two antennal branches, the lower three-segmented one (endopodite), sitting on the apical basipodital prominence, is slightly longer than upper branch. The upper branch is four-segmented ( Fig. 1C View FIGURE 1 ). Proximalmost segment of the upper branch is rudimentary and clearly visible only externally; all other segments of both branches are much more developed. Small proximalmost segment of upper branch lacks setae, while other segments possess two-segmented swimming setae of more or less similar size except distalmost of them which are shorter. All setae bilaterally armed with rows of uniform thin setulae. General formula of antennal setae: 0–1–2–5/1–1–5 (see Korovchinsky (2015) for more details).

Mouth parts. They are represented by upper lip (labrum), mandibles, and maxillules (maxilla I). The upper lip is composed of two parts: the posterior thick and slightly flattened triangular lobe and anterior large proboscislike outgrowth. The latter is separated from the former one by a deep indention of the cuticle. The triangular lobe bears numerous papillae along its posterior-interior (oral) margin while the outgrowth is armed with tiny spinules. Mandibles are bilobed and adapted for biting ( Fig. 3A View FIGURE 3 ), with a toothed, blade-like posterior lobe and small anterior lobe (mandibular process) armored with a cluster of about 20 long outgrowths, slightly differing in size and bearing some prominences distally ( Figs. 3B, 3C View FIGURE 3 ). Posterior lobe is strongly sclerotized and divided in two tooth-shaped parts, the larger (posterior) of which has a small additional tooth about midway along its border ( Fig. 3A View FIGURE 3 ). Anterior side of large sclerotized lobe possesses a group of minute spinules which often might be broken ( Fig. 3A View FIGURE 3 , arrowed). Maxillules (mx I) look like two cylindrical structures situated posterior to mandibles. Distally, they bear short central seta and some spinules near it. Maxillae (mx II) are absent; the opening of maxillar glands are situated near the bases of tl I laterally.

Carapace. It looks like a bag-like structure, strongly modified into closed brood pouch ( Fig. 1A View FIGURE 1 ) widely connected in its base with dorsal side of thorax. It may be often well developed and massive being filled by large embryos and clearly inclined anteriorly over the head. Its length can reach 75 % of body length.

Thoracic limbs. Four pairs of strongly chitinized, stenopodous limbs are densely situated along the muscular ventral side of thorax and directed antero-ventrally ( Fig. 1A View FIGURE 1 ). All of them have complex and variously setaceous armament along their inner side. Limbs of three anterior pairs are five-segmented and those of the last fourth pair are three-segmented. Protopodites of all of them, covered by comparatively softer cuticle, are inconspicuously delimited into two parts (segments), coxa and basis, while the endopodites of limbs of three anterior pairs are composed of three well developed segments, being those ones of the fourth pair unisegmented ( Figs. 1E, 1J, 1K, 1L View FIGURE 1 ).

Trunk limb I (tl I) especially long and strong, its length rarely exceed body length (up to 107.0 % of its length) but usually shorter (71.0–97.0 % of body length) ( Table 1). Terminally, the inner side of its protopodite bears a small triangular lobe, pseudognathobasic process (see the explanation of the term in Korovchinsky (2015)), armed laterally and distally with two outgrowths with apical setae and numerous spinules ( Fig. 1F View FIGURE 1 ). The external part of protopodite is longer than internal one and bears apically a small conical outgrowth. The first segment of endopodite is long and bears 5–6 anterior lateral setae (their number on limbs of one individual can vary) with two rows of posterior rough incurved spines ( Fig. 2I View FIGURE 2 , 4A View FIGURE 4 ) and an anterior row of fine setulae. Distally, this segment bears shorter anterior seta of the same type and long posterior finely setulated seta ( Fig. 1E View FIGURE 1 ). The second segment of endopodite is conspicuously shorter and bears only two apical setae similar to those on the end of previous segment but shorter, its size varies; sometimes longer posterior seta may be either rather short or absent ( Fig. 1E, 1H, 1I View FIGURE 1 , 2F View FIGURE 2 ). The terminal third segment of endopodite varies in length, being mostly shorter than proximal first endopodital segment (usually 65.0–87.0 % of the latter one but sometimes it may be longer being either almost equal or even slightly surpassing the first segment (90.0–103.0 %)), and always bears apically four long roughly spinulated setae, two of them terminally and two subterminally ( Fig. 1A View FIGURE 1 ). Basally, these setae are armed by a row of smaller spines, while distally by larger lanceolate spines situated in two rows and directed terminally.

Trunk limb II (tl II) is considerably shorter, its protopodite, again externally, is conspicuously longer and bears a small conical outgrowth ( Figs. 1J, 1 View FIGURE 1 D-left, 3D-upper). The first, basal segment of its endopodite, bears a row of 5–6 rather long anterior lateral setae (their number also can vary in one individual) ( Figs. 1J View FIGURE 1 : as, 2H). The terminal setae of the segment are different; the anterior one is shorter and roughly armored, while posterior one is longer and finely setulated. Internally, this segment bears stout cylindrical pseudognathobasic process, possessing some prominences of different size, one small, thin seta, and a pore apically ( Fig. 3E View FIGURE 3 ). The second segment of endopodite is short with only two setae, the anterior of which ( Fig. 2I View FIGURE 2 ) is similar to anterior terminal seta of previous segment, while the posterior seta is longer and finely setulated. The distal, third segment of endopodite of the limb bears four setae, two terminal setae and two subterminal ones ( Fig. 1J View FIGURE 1 ). Of the latter, the anterior seta is comparatively short, thick and armed with a number of thin lateral denticles ( Figs. 2K View FIGURE 2 , 4C View FIGURE 4 ), while its distal end is naked and slightly hooked apically. Its neighboring posterior subterminal seta ( Fig. 2M View FIGURE 2 ) is considerably longer and similar to subterminal and terminal setae of tl I, having similar spine armament and sharp apex. The anterior terminal seta is thick and comparatively short with longitudinal ribs, few thin lateral denticles, and slightly hooked apical end ( Fig. 2J View FIGURE 2 ). The posterior terminal seta is similar to the neighboring anterior one but longer, it has few lateral denticles and slightly hooked apical end ( Fig. 2L View FIGURE 2 ).

Trunk limb III (tl III) is generally similar to the previous ones, differing in some details. The external outgrowth of its protopodite is conspicuously larger ( Figs. 1 View FIGURE 1 D– right, 3D-lower) and lateral anterior setae of first segment of endopodite are fewer (3–4) ( Fig. 1K View FIGURE 1 , 4B View FIGURE 4 ). Distal setae of the segment are similar to other ones. The pseudognathobasic process is also similar to that one of tl II ( Fig. 3F View FIGURE 3 ). Of setae of the second segment, the anterior one is similar to the respective one of tl II. Terminal and subterminal setae of third segment are similar to those of tl II but slightly shorter and bear fewer denticles.

Trunk limb IV (tl IV) is considerably reduced; its protopodite bears slightly spinulated seta sited on a short cylindrical base ( Figs. 1L View FIGURE 1 , 4D View FIGURE 4 ). The only segment of endopodite has two rows of comparatively short spine-like setae. The external row (group) ( Figs. 1L View FIGURE 1 , 4D View FIGURE 4 : es) always consists of two setae, and the internal row of 5–7 setae, which differ in their appearance and armament. Almost the whole internal part of the endopodital segment is occupied by the reduced pseudognathobasic process, also having a pore and armed by some denticles and thin seta ( Fig. 4D View FIGURE 4 ).

Abdomen (metasoma) ( Fig. 1A View FIGURE 1 ) is often deformed. It is inconspicuously delimited in two segments, short proximal and long distal, sometimes with prominent fold more or less in the middle dorsal side.

“ Postabdomen”, actually consisting of two parts; the last small abdominal segment and postabdomen per se (see Korovchinsky (2015)), is comparatively small, and the anal opening is situated between postabdominal claws. The latter ones are of moderate size (4.7–9.7 % of body length), usually straight and directed slightly backwards, sometimes with apex curved slightly forward ( Figs. 1A View FIGURE 1 , 2A, 2B, 2C, 2D, 2E View FIGURE 2 ).

Caudal process is directly connected with postabdomen and proceeds as a very long, proximally rather thick, then thin, straight spine-like structure ( Figs. 1A, 1M View FIGURE 1 ) variable in its length (148.0–253.0 % of body length), thus surpassing the body length in one and a half—two and a half times. Generally, the caudal process is strongly chitinized and its surface is covered by numerous minute spinules. Basally, caudal process always bears one pair of claws similar to those of postabdomen but on average slightly larger (4.7–10.6 % of body length), and apically two minute setae arose from common base ( Fig. 2G View FIGURE 2 ). Pairs of claws usually sit rather closely (12,0–22.6 % of body length). Between them, the thickness of the structure constitutes 3.2–6.4 % of body length. Borders separating old molted integuments of caudal process with claws normally are conspicuous. Body length 1,70–2,30 mm.

Gamogenetic females and males have not been found.

Intra- and interpopulation variability. According to data of Table 1, the individuals from Lake Kadysh are smaller than those from Lake Er-Kara-Hol. Also, the former ones have shorter caudal process, smaller interclaw distance, and longer distal segment of tl I. The length of claws, interclaw distance, and interclaw thickness are the most variable parameters. Body structure parameters of juvenile specimens do not differ from those of adults but apical setae of segments of their tl I are shorter.

Remarks. In the neighboring Tyva lakes, Borzu-Khol’ and Todzha (Azas), the interspecific hybrid form B. centralasiaticus x B. cederströmii (“ Bythotrephes sp. x B. cederströmii ”) was previously found (see Korovchinsky 2019).

Differential diagnosis. Generally, the representatives of the new species are especially close to those of B. longimanus having long tl I, straight caudal process, and only two pairs of claws, one on postabdomen and one on caudal process. Adult females of all other species of the genus possess either two or three pairs of such claws. At the same time, the former are on average smaller (body length 1.70–2.30 mm v. 1.62–3.18 mm in B. longimanus ) having shorter caudal process (148–263 % of body length v. 186–301 % in B. longimanus ). The main difference concerns apical setae of second endopodital segment of tl I which are fairly long in the new species ( Fig. 1 View FIGURE 1 E–H) and rudimentary or even absent in B. longimanus . The representatives of the hybrid form B. centralasiaticus x B. cederströmii differ from those of a new species by presence of caudal process with a denticulated bend in some of them.