Bythotrephes undetermined

Bythotrephes sp.

(Figs. 5D–F, 5J, 5K)

Material examined. Austria: Lake Wolfgangsee, 28.8.2012, 4 ad, coll. R. Ptáčniková.

Brief description. The general body structure and its parts as in other representatives of the genus.

Thoracic limbs of first pair (tl I) are comparatively not long, their length not surpasses body length (73.9–92.2 %). The first segment of endopodite is long and bears 5–6 anterior lateral setae. Distally, this segment bears shorter anterior seta of the same type and long posterior finely setulated seta (Fig. 5D). The second segment of endopodite is conspicuously shorter and bears two rather long apical setae (Fig. 5E, 5F). The terminal third segment of endopodite is shorter than the proximal first endopodital segment (80.0–86.2% of the latter one) and always bears apically four long roughly spinulated setae, two of them terminally and two subterminally.

Postabdominal claws of moderate size (4.2–7.2% of body length) and directed backwards (Figs. 5G, 5K).

Caudal process is of moderate length (192–253 % of body length), thus exceeding the body length in 1.9–2.5 times. Basally, caudal process bears two pairs of claws similar to those of postabdomen but slightly larger (proximal claws reach 5.9–6.7 % and distal ones 6.7–7.7% of body length, respectively). Pairs of claws sit quite closely (interclaw distance 8.0–11.1% and 8.0–10.6% of body length). Between the pairs of claws, the thickness of the structure is considerable, constituting 7.3–10.0 % and 6.3–7.8 % of body length. Borders separating old molted integuments of caudal process with claws are inconspicuous.

Size. Body length 1.44–1.90 mm.

Remarks. Of all species of the genus, the analyzed specimens seem closest to the representatives of the B. arcticus — B. transcaucasicus species group. They are especially similar to them due to comparatively short tl I, the presence of a long apical setae on the second segment of tl I, and thick base of caudal process always bearing two pairs of claws directed backwards (see Korovchinsky 2016). At the same time, the present specimens differ from both mentioned species in the relatively smaller body size and longer caudal process and from B. arcticus Lilljeborg, 1901 also in the presence of smaller claws of postabdomen and caudal process and smaller distance between their pairs. Regarding the two latter features, the Austrian specimens are closer to B. transcaucasicus.

Due to a small number of specimens and the uncertainty of their diagnostic morphological features, it seems better to leave the problem of a more precise species definition of the specimens under consideration for the future, when more abundant material may be available.