Abstract: Brachymystax  tsinlingensis and Brachymystax lenok are phylogenetically related species, both of which are national class II key protected wild animals in China. The comparison of geometric morphological characteristics of current-year juveniles in the two species can provide important basis for species identification, protection, proliferation and release. To explore the morphological differences of current-year juveniles between those two species, a thin-plate spline model was established by using the Tps software. We compared the differences of those two species with respect to 24 relative geometric morphology characteristics and also by principal component analysis. The results showed that the background grids of their overall structural framework exhibited different degrees of curvature in the head, back, tail, and abdomen. The head of B. lenok was more pointed. There were 13 relative geometric morphological features with differences between B.  tsinlingensis and B. lenok (all P<0.05). The length of dorsal fin base (C4/C1), the length of anal fin base (C13/C1), eye diameter (C20/C1) and head height (C25/C1) of B.  tsinlingensis was larger than that of B. lenok (P<0.05). However, the distance from pectoral fin base to snout (C16/C1) was shorter than that of B. lenok (P<0.05). Results of principal component analysis showed that only a few samples of the two species overlapped at the PC1 and PC2, and that the samples of each species were relatively concentrated. In conclusion, our results suggested that there were significant interspecific differences in the geometric morphological features of current-year juveniles of B.  tsinlingensis and B. lenok, especially for the features of head and tail.

Key words: Brachymystax , tsinlingensis, Brachymystax lenok, geometric morphology, phylogenetically related species, interspecific difference.