Redescription of Cottus koshewnikowi (Cottidae) and its morphological variability in tributaries of the upper Volga

Abstract

Three samples of the genus Cottus have been studied: type specimens from the upper Volga system (tributaries of the second and third orders and the Lake Seliger) and also individuals from the upper stream of the Oka River (near Kaluga) and from the middle Kama River (near Perm). They are morphologically related to one species Cottus koshewnikowi Gratzianow, 1907. The following diagnostic characters are determined: usually, trunk partially covered by short bone spickles; small skin papillae present on dorsal side of head; caudal peduncle short, its length 6–11% (on average 8.3%) of SL; head dorsoventrally flattened, its depth near occiput 52.2% of head length, on average; interorbital space narrow, 1.6–2.1 (1.9) times less than eye diameter; mouth comparatively small, length of upper jaw 28–35 (30.5)% of head length; lateral canal partial, reaches 10th-16th ray of second dorsal fin and with 22–27 pores, terminal 1–4 autonomous segments with pores present.