Bythotrephes, Leydig, 1860

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 : 454-455

Key for species and a hybrid form identification of the genus Bythotrephes

1 Claws of postabdomen and caudal process are both uniformly long and massive with apical end curved forward, or claws are of different shape, size and direction. Similarly, caudal process can be uniformly long with usually prominent denticulated bend, or this bend is weakly developed and rarely lacking altogether.................................................. 2

- Claws of postabdomen and caudal process are of another shape, not long and massive with apical end not curved forward. Caudal processes always straight, without a denticulated bend.................................................. 3

2 Claws of postabdomen and caudal process are normally long and massive (rarely comparatively small) with apical end curved forward. Caudal process long and usually possesses a prominent denticulated bend (sometimes this bend may be either weakly developed or absent; in latter case its place is marked only with few or solitary enlarged denticles, in some specimens the bend and enlarged denticles may be absent altogether).................................... B. cederströmii Schödler, 1877

- Claws of postabdomen and caudal process are of very variable shape, both in one individual and in different individuals of any population: either large, medium or small, curved or straight, directed forward, down or backwards, sitting more or less closely or distantly to each other. Denticulated bend can be either well or weakly developed, marked by a group or only single large denticles, or the latter can be absent altogether. In addition to these individuals with highly variable morphological traits, those ones typical for the parental species, B. cederströmii and B. brevimanus , are usually present........................................................................................... hybrid form B. brevimanus x B. cederströmii

3 Adult specimens always have only two pairs of claws situated on postabdomen and basal part of caudal process.......... 4

- Adult specimens have two or three pairs of claws............................................................ 5

4 Adult specimens with two pairs of claws situated closely (interclaw distance 7.5–15.0 % of body length) on postabdomen and basal part of caudal process. The apical setae of second segment of endopodite of tl I are small, reduced or rarely may be absent altogether..................................................................... B. longimanus Leydig, 1860

- Adult specimens with two pairs of claws situated comparatively sparsely (interclaw distance 12.0–23.0 % of body length) on postabdomen and basal part of caudal process. The apical setae of second segment of endopodite of tl I are fairly large..................................................................................... B. centralasiaticus sp.nov.

5 Body length of adult specimens is large (2.3–6.1 mm), the first segment of endopodite of tl I bears 6–10 anterior lateral setae. Caudal process comparatively short (80–216 % of body length) and thick at its base. Claws of postabdomen and caudal process are long and stout, inserted rather closely and normally curved backwards........................................ 6

- Body length of adult specimens is comparatively small (1.2–2.6 mm). The first segment of endopodite of tl I bears 4–6 anterior lateral setae. Caudal process comparatively long (120–306 % of body length) and thin at its base. Claws of postabdomen and caudal process are comparatively short (sometimes rudimentary), inserted rather sparsely and normally straight, being directed either backwards or downwards.......................................................................... 7

6 Thickness of caudal process at its base is large (5.5–14.8 % of body length). Claws of postabdomen and caudal process are long (4.5–19.3 % of body length) and inserted comparatively sparsely (interclaw distance 8.0–17.0 % of body length)........................................................................................ B. arcticus Lilljeborg, 1901

- Thickness of caudal process at its base is moderate (4.4–7.2 % of body length). Claws of postabdomen and caudal process are comparatively short (5.0–11.0 % of body length) and inserted comparatively densely (interclaw distance 7.6–10.0 % of body length).................................................................... B. transcaucasicus Behning, 1941

7 Claws of postabdomen and caudal process are usually short, sometimes rudimentary (0.8–8.3 % of body length) and directed backwards................................................................... B. brevimanus Lilljeborg, 1901

- Claws of postabdomen and caudal process are comparatively long (5.4–12.1 % of body length) and directed downwards............................................................................. B. lilljeborgi Korovchinsky, 2018