Bythotrephes inexpectatus, Korovchinsky, 2023

Korovchinsky, Nikolai M., 2023, Unexpected high species richness of Bythotrephes Leydig, 1860 (Branchiopoda: Cladocera: Cercopagididae) in subalpine Austrian lakes, with the description of new taxa, Zootaxa 5264 (1), pp. 77-93 : 79-84

publication ID	https://doi.org/ 10.11646/zootaxa.5264.1.5
publication LSID	lsid:zoobank.org:pub:DA96D94F-C359-4D1B-B8D5-AAC811FC4917
DOI	https://doi.org/10.5281/zenodo.7838112
persistent identifier	https://treatment.plazi.org/id/AB2A87A3-FFBF-FFF5-FF03-FD4A74EAFEC7
treatment provided by	Plazi
scientific name	Bythotrephes inexpectatus
status	sp. nov.

Treatment

Bythotrephes inexpectatus sp. nov.

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

Etymology. The name of the species denotes the surprise of its discovery in Austrian lakes of Central Europe.

Type locality. Lake Altaussee , Styria, Austria (47°36′36′′N; 13°46′58′′E), 712 m asl GoogleMaps .

Type material. Holotype. A female with body length 2.69 mm deposited in Zoological Museum of Moscow State University (№ MGU ML 262 ).

Paratypes. Two females from the same sample deposited in the same museum (№ MGU ML 263 ). All other paratypes are deposited in author’s collection in the Institute of Ecology and Evolution.

Material examined. Austria, Styria: 1) Lake Altaussee, 29.8.2012, 12 ad, 7 juv, coll. R. Ptáčniková; 2) Lake Toplitzsee , 29.8.2012, 9 ad, 4 males, 6 juv, coll. R. Ptáčniková. Data on body and body parts measurements of specimens are presented in Table 1 View 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, 1G View FIGURE 1 ). The longitudinal axis is conspicuously incurved when the head is located at almost right angle to the thorax. Highly movable abdomen can be either in a 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 bears 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 sizes directed antero-ventrally. Dorsally, thorax bears a sack-like carapace transformed into a brood pouch, sometimes reaching large size. Abdomen (metasome) is elongated, cylindrical, inconspicuously three-segmented (see Korovchinsky 2015 for details) and flexible, connected with a small postabdomen, bearing ventrally a pair of claws and posteriorly a very long caudal process with a pair of similar claws proximally. General body length of females (without caudal process) may reach 2.8 mm or slightly more (in the examined specimens it ranges from 1.73 to 2.88 mm) while the length of caudal process may exceed the body length by 1.4–2.4 times (on average, about 1.8 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. The 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 of two and three, and one shorter and thinner aesthetasc-like structure, situated in a group with two regular aesthetascs, and having slightly expanded apical end (“accessory simple seta” according to Scourfield (1896).

Swimming antennae. Comparatively long, with elongated cylindrical basipodite ( Fig. 1A View FIGURE 1 ). Of the two antennal branches, the lower three-segmented one, sitting on the apical basipodital prominence, is slightly longer than the upper branch. The upper branch (exopodite) is four-segmented and lower branch (endopodite) is three-segmented. Proximalmost segment of the upper branch is rudimentary and clearly visible only externally; all other segments of both branches are much more developed. Small spine with a row of neighboring minute prominences at the end of the distal segment of the superior antennal branch ( Fig. 1F View FIGURE 1 ). 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 setules. 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 proboscis-like outgrowth. Mandibles are bilobed and adapted for biting, with a toothed, blade-like posterior lobe and small anterior lobe (mandibular process). 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. Maxillules (mx I) resemble two cylindrical structures situated posterior to the mandibles. Distally, they bear short central seta and some spinules near it. For details see description of mouth parts in other species of the genus which have the same structure ( Korovchinsky 2015, 2018).

Carapace. It resembles a bag-like structure, strongly modified into a closed brood pouch ( Fig. 1A View FIGURE 1 ) widely connected in its base with the dorsal surface of the thorax. It may be often well developed and massive, when filled with large embryos.

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 the 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 the three anterior pairs are composed of three well developed segments and those ones of the fourth pair are single-segmented ( Figs. 1C View FIGURE 1 , 2A, 2B, 2C View FIGURE 2 ).

First pair of limbs (tl I) are especially long and strong ( Fig. 1C View FIGURE 1 ), though comparatively they are more or less short (63.8–92.6 %, av. 79.4% of body length). Terminally, the inner side of their protopodite bears a small triangular lobe, pseudognathobasic process (see the explanation of the term in Korovchinsky (2015)). The first segment of the endopodite is long and bears 5–6 anterior lateral setae with rows of spines and fine setules. Distally, this segment bears a shorter anterior seta of the same type and long posterior finely setulated seta ( Figs. 1C, 1D View FIGURE 1 ). The second segment of the endopodite is conspicuously shorter and bears only two apical setae similar to those on the end of previous segment but shorter ( Fig. 1E View FIGURE 1 ). The terminal third segment of the endopodite slightly varies in length, being shorter than the proximal first endopodital segment (72.2–95.4 %, av. 81% of the latter one) and always bearing apically four long roughly spinulated setae, two of them terminally and two subterminally.

Second pair of limbs (tl II) are considerably shorter, their protopodite, again externally, is conspicuously longer and bears a conical outgrowth. The first basal segment of their endopodite bears a row of 5–7 rather long anterior lateral setae (their number can vary in one individual) ( Fig. 2A View FIGURE 2 : as). There are 1–2 posterior lateral setae (ps) on this segment. The terminal setae of the segment differ in that the anterior one is shorter and roughly armored, whereas the posterior one is longer and finely setulated. Internally, this segment bears a stout cylindrical pseudognathobasic process, possessing some prominences of different size and one small, thin seta ( Fig. 2A View FIGURE 2 ). The second segment of the endopodite is short with only two setae, the anterior one of which is similar to the anterior terminal seta of previous segment, whereas the posterior seta is longer and finely setulated. The distal, third segment of endopodite of the limb bears four setae, two terminal and two subterminal ones ( Fig. 2A View FIGURE 2 ).

Third pair of limbs (tl III) are generally similar to those of the previous ones, differing in some details. The external outgrowth of their protopodite is conspicuously larger and lateral anterior and posterior setae (if present) of first segment of endopodite are fewer (4–5 and 1, respectively) ( Fig. 2B View FIGURE 2 ). Distal setae of the segment are similar to other ones. The pseudognathobasic process is also similar to that of tl II. Of setae of the second segment, the anterior one is similar to the respective one of tl II. Terminal and subterminal setae of the third segment are similar to those of tl II but slightly shorter and bear fewer denticles.

Fourth pair of limbs (tl IV) are considerably reduced ( Fig. 2C View FIGURE 2 ); their protopodite bears slightly spinulated seta sited on a short cylindrical base. The only segment of the endopodite has two rows of comparatively short spine-like setae. The external row (group) ( Fig. 2C View FIGURE 2 : as) always consists of two setae, and the internal row of 6–7 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 armed by some denticles and thin seta.

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

“ Postabdomen” actually consists of two parts: the last small abdominal segment and the postabdomen per se (see Korovchinsky (2015) for more details), which is comparatively small, with the anal opening situated between the postabdominal claws. These claws are comparatively small (3.4–6.8 %, av. 4.7% of body length), more or less straight or slightly curved and directed slightly backwards ( Figs. 2D–I View FIGURE 2 ).

Caudal process is directly connected with the postabdomen and proceeds as a very long, proximally rather thick and curved, then straight spine-like structure ( Figs. 1A, 1G View FIGURE 1 ), variable in its length (138.0–239.0 %, av. 184% of body length), thus exceeding the body length by about 1.4-2.4 times. Generally, the caudal process is strongly chitinized and its surface covered by numerous minute spinules. Basally, the caudal process bears one pair of claws similar to those of the postabdomen but usually larger (4.0–8.5%, av. 5.9% of body length), and apically two minute setae arise from a common base ( Fig. 1H View FIGURE 1 ). Pairs of claws of postabdomen and caudal process sit closely, interclaw distance usually constitutes 5.8–8.4% (av. 7.0%). Between the claws, the thickness of the structure constitutes 4.4–7.9 % (av. 6.0%). Borders separating old molted integuments of caudal process with claws always are quite conspicuous.

Gamogenetic females have not been detected.

Males. The thoracic limbs of the first pair (tl I) are comparatively short (59.0–73.0 % of body length) as well as each segment of them, especially the distal one, which is slightly swollen proximally and bears on its inner side a small strongly chitinized hook with two inner denticles ( Figs. 1I, 1J View FIGURE 1 ). The copulatory appendages are small and armed with numerous minute spinules terminally ( Fig. 1K View FIGURE 1 ).

Size. Adult females— 1.73–2.88 mm. in length. Males, on average, are smaller than females (body length— 1.86–2.08 mm.).

Intrapopulation variability of females. According to Table 1 View TABLE 1 , the length of claws of the postabdomen and caudal process is especially variable (CV = 21.3–22.5, respectively), the interclaw distance and interclaw thickness follow them in this respect (CV = 11.3–11.8). At the same time, the claw shape is more or less stable. Of other structures, the comparative length of thoracic limbs of the first pair (tl I) seems also rather variable (CV = 8.4).

Differential diagnosis. Bythotrephes inexpectatus sp. nov. is close to B. longimanus s.l. and B. centralasiaticus Korovchinsky, 2020 in having, in adult specimens, only one pair of claws on caudal process. At the same time, it differs from the former species ( Table 2 View TABLE 2 ) in a unique combination of morphological features: relatively smaller body length; presence of very closely situated pairs of claws, the borders of which separating old molted integument always are quite conspicuous; presence of shorter thoracic limbs of first pair (tl I) and shorter caudal process (for all parameters p <0.001). Also, tl I of the new species bears long seta (vs. short or rudimental in B. longimanus s.l.) at the apical end of the second endopodital segment of tl I. Compared to B. centralasiaticus , the specimens of Bythotrephes inexpectatus sp. nov. have on average larger body length, shorter caudal process, smaller claws of the postabdomen and caudal process, larger thickness of caudal process between pairs of claws, and conspicuously smaller interclaw distance (for all parameters p <0.001).

The adult females of all other species of the genus Bythotrephes and those of widely distributed interspecies hybrids B. brevimanus x B. cederströmii possess one-two pairs of claws on caudal process. Besides that, the relatively closely distributed B. brevimanus s.str., possesses, compared to the new species, smaller claws of postabdomen and caudal process that sit more distant from each other (p <0.001).

Geographical distribution. So far, representatives of a new species are known to occur only in two deep subalpine lakes in the central part of Austria.