Bythotrephes cf. brevimanus Lilljeborg, 1901

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 : 88

Bythotrephes cf. brevimanus Lilljeborg, 1901

( Fig. 5A–C, G–I )

B. styriacus Ischreyt, 1939: 126–128 , Abb. 7.

B. longimanus longimanus Leydig, 1860 : Flössner, 1972: 405–406.

B. longimanus styriacus Ischreyt, 1939 : Flössner, 2000: 375–376.

Material examined. Austria (Upper Austria): Halstättersee , 29.8.2012, 10 ad, 7 juv., coll. R. Ptáčniková .

Data on body and body parts measurements are presented in Table 1 .

Description. Female. The general body structure and its parts as in other representatives of the genus.

Thoracic limbs of first pair (tl I) are especially long and strong, their length often surpasses body length (93.7–114.8 %, av. 104.7% of its length) ( Table 1 ). The first segment of endopodite is long and bears 5–6 anterior lateral setae (their number on limbs of one individual can vary) with a posterior row of rough incurved spines and anterior and lateral rows of fine setules. Distally, this segment bears shorter anterior seta of the same type and long posterior finely setulated seta ( Fig. 5A ). The second segment of endopodite is conspicuously shorter and bears only two apical shorter setae similar to those on the end of previous segment ( Fig. 5B, 5C ). The terminal third segment of endopodite is shorter than proximal first endopodital segment (81.1–95.6% of the latter one) and always bears apically four long roughly spinulated setae, two of them terminally and two subterminally.

The structure and armament of limbs of the second-fourth pairs as in the representatives of the species described earlier (see Korovchinsky, 2018).

The “ postabdomen” is actually consisting, as usual, of two parts—the last small abdominal segment and postabdomen per se (see Korovchinsky (2015) for more details), is comparatively small, The postabdominal claws are comparatively small (2.5–4.8% of body length), may be directed either almost down or backwards, sometimes with apex curved slightly forward ( Figs. 5G–I ).

Caudal process is directly connected with postabdomen and proceeds as a very long, proximally rather thick and curved, then straight spine-like structure variable in its length (180–248 % of body length), thus surpassing the body length in 1.8–2.5 times. Basally, caudal process bears one or two pairs of claws similar to those of postabdomen but usually larger (e.g., proximal claws reach 3.2–6.4 % and distal ones 4.1–6.3% of body length, respectively). Pairs of claws sit more or less closely (distance between postabdominal claws and proximal claws of caudal process (interclaw distance) usually constitutes 9.3–19.2 % of body length). Between the latter, the thickness of the structure constitutes 5.1–7.2% of body length. Borders separating old molted integuments of caudal process with claws are moderately conspicuous.

Size. Body length 1.66–2.53 mm.

Gamogenetic females and males have not been detected.

Interpopulation variability. Despite the presence of a small set of Austrian specimens (adult females) of the species (n =10), it is possible to make some comparisons with those of B. brevimanus previously studied ( Korovchinsky (2018). The specimens from Lake Halstättersee are comparatively larger, having unusually long thoracic limbs of the first pair (tl I) with rather long endopodital distal segment which, however, is always shorter than proximal one. Then, their postabdominal claws are smaller and all their pairs sit more densely than in most other studied populations of the species. Also, the interclaw thickness is larger and borders separating old molted integument of caudal process with claws are less noticeable than usual.

Remarks. Ischreyt (1936, 1937) suggested the specificity of the representatives of Bythotrephes from Lake Halstättersee and soon later described them as new species B. styriacus ( Ischreyt 1939) but without real evidence presenting doubtful diagnostic traits. Moreover, he considered this species occurring in some other adjacent lakes (Wolfgangsee, Mondsee, Grundlsee, Altaussee) where other taxa, B. inexpectatus sp. nov. and B. l. austriacus ssp. nov., occur, which makes the situation more confusing.

Generally, Ischreyt brings his new species closer to B. brevimanus (= B. balticus Ischreyt ) but noted the comparatively larger body size of its specimens which matches the data of the present study. Based on the above described morphological specificities of studied specimens and their unusual for B. brevimanus occurrence in mountain lake it is possible to suspect that they really represent a separate taxon, but this statement requires additional material and evidence.

Figures

FIGURE 5. Bythotrephes cf. brevimanus Lilljeborg, 1901, females, Lake Halstättersee (A–C, G– I). Bythotrephes sp., females, Lake Wolfgangsee (D–F, J, K). A, D, setae of distal part of first endopodital segment of tl I. B, C, E, F, setae of distal part of second endopodital segment of tl I. G–K, claws of postabdomen and caudal process.