Abstract:Brachymystax tsinlingensis (Li 1966) is a landlocked Salmonidae fish endemic to China. Based on long-term artificial breeding and resource conservation studies, it was found that B. tsinlingensis has obvious photophobic behavior and light-sensitive characteristics. To further understand the visual characteristics of B. tsinlingensis in different habitats, this study studied the second filial generation population (SFGP) [body weight: (51.50±10.86) g, body length: (15.40±0.75) g] and wild population (WP) [weight: (85.68±31.81) g, body length: (18.17±2.34) cm] of B. tsinlingensis under artificial breeding and natural environment. Traditional histological methods have been used to study the structural characteristics of the retina, and fluorescence quantitative PCR technology has been used to identify five key opsin genes that play a crucial role in visual formation: rhodopsin (RH1), rhodopsin-like pigments (RH2), short wavelength-sensitive pigments (SWS1), SWS1-like pigments (SWS2), and long wavelength-sensitive pigments (LWS). The results showed that there was no significant difference in the retina between SFGP and WP. The number of cones (C.) and outer nuclear layer (O.N.) in the retina of SFGP was significantly higher than that of WP (P<0.01), and the number of ganglion cells (G.) was significantly higher than that of WP (P<0.05). The relative expression level of RH1 in WP was significantly lower than that in SFGP (P<0.01), and RH1 had the highest expression level among the five opsin genes in both populations. In cone opsin genes RH2, SWS1, SWS2, and LWS were significantly different; no significant difference in relative expression of RH2 was observed. The transcription levels of SWS1, SWS2, and LWS genes were significantly higher than those of WP (P<0.01). The expression ratio of cone opsins in the SFGP was LWS>SWS1>SWS2>RH2 from high to low, whereas that in the WP was LWS>SWS2>RH2>SWS1. The expression of LWS was dominant in both populations, accounting for more than 60% of total expression. Five opsin mRNA were specifically expressed in the retinas of the two populations. This study indicated that both the SFGP and WP of B. tsinlingensis have strong photosensitive characteristics that can regulate the expression profile of opsin genes to trigger spectral sensitivity changes to cope with different habitats, and the performance characteristics of the SFGP are more significant. The exhibited photophobic behavior may be an adaptive strategy for different environments, and photosensitivity indicates that behavior depends largely on visual communication. The results of the present study provide a vital reference for the proliferation and maintenance of B. tsinlingensis.