high turbulent current, and rocky substratum. As adaptions to strong water currents, O. krongnoensis has dense
epidermal tubercles on the snout tip, a slender, streamlined body with an increased number of predorsal scales
(mostly 17 scales), and a longer caudal peduncle compared to other species of Onychostoma.
Tubercles of cyprinids exhibit sexual dimorphism and ontogenetic variation in the size, shape and distribution
(Wiley & Collette, 1970). In contrast to other species of Onychostoma, in O. krongnoensis the epidermis of the
snout possesses 2−3 irregular transverse rows of 13−43 tubercles in juveniles and adults of both sexes (Fig. 3b).
Variation in the surface of the snout in hill-stream fishes has been suggested to be an adaptation to life in torrential
streams (Hoshiyar et al. 2013). The snout epidermis of O. krongnoensis is subject to frictional stress as it comes
into contact with water current, thus supporting this hypothesis. In contrast, species that inhabit low elevation
streams with slower velocity, such as O. dongnaiensis, O. leptura, and O. uniforme, have fewer smaller tubercles
on the snout (Hoang et al. pers.obs.). Breeding tubercles also occur on the anal-fin rays of males of O.
krongnoensis and other stream cyprinids (Witkowski & Rogowska 1991; Poncin et al. 2011).