Bythotrephes, Cederstromii Schodler, 1877

Korovchinsky, Nikolai M., 2018, Further revision of the genus Bythotrephes Leydig (Crustacea: Cladocera: Onychopoda): redescription of B. brevimanus Lilljeborg, reevaluation of B. cederströmii Schödler, and description of a new species of the genus, Zootaxa 4379 (3), pp. 347-387 : 379-380

Key for species identification of the genus Bythotrephes

1 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 it may be either weekly devel-

oped or absent, in latter case its place is marked only with few or solitary enlarged denticles, in single specimens the bend may be absent at all)............................................................... B. cederströmii Schödler, 1877 - Claws of postabdomen and caudal process are not long and massive with apical end not curved forward. Caudal process always straight without a denticulated bend................................................................. 2

2 Adult specimens have only two pairs of straight claws situated closely on postabdomen and basal part of caudal process. Thoracic limbs of first pair (tl I) and their distal segment are very long (80–132 % of body length and 80–122 % of length of proximal segment of endopodite of tl I, respectively). The apical setae of second segment of endopodite of tl I are reduced or may be absent at all.................................................................. B. longimanus Leydig, 1860

- Adult specimens have two or three pairs of claws. Thoracic limbs of first pair (tl I) and their distal segment are not very long, sometimes comparatively short. The apical setae of second segment of endopodite of tl I normally are well developed..... 3.

3 The 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..................................4.

- The 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................................................................... 5

4 The 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

- The 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

5 Claws of postabdomen and caudal process are usually short (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 sp. nov.