Abstract
Cyprinidae is the largest freshwater fish family, and their habitats are diverse from clear to turbid water. Several specific genera from this group exhibit unique nuptial coloration, although most of them are drab in their coloration. This study was aimed to test two hypotheses. First, differences in nuptial coloration among colorful cyprinids could reflect differences in color vision and opsin gene sequences. Second, photic environments of habitats could affect the visual abilities and the opsin genes of cyprinids. To test the first hypothesis, genes encoding the visual pigments of three colorful cyprinids (Opsariichthys evolans, Opsariichthys pachycephalus and Candidia barbatus) were cloned and sequenced, the λmax of cone photoreceptor absorption spectra and the reflectance spectra of their body coloration were measured. It was found that the differentiation of spectral sensitivities and unique nuptial coloration of the colorful cyprinids might have evolved through sexual selection. The results also indicated that the spectral shift among colorful cyprinids could result from differential expression of opsin genes and amino acid substitutions. At the same time, other four dull-colored cyprinid (Acrossocheilus paradoxus, Hemibarbus labeo, Pseudorasbora parva and Onychostoma barbatula) lived in different habitats were tested to test the second hypothesis. The photic environments of habitats where the cyprinids lived could affect their visual abilities in blue-, green-, and red-sensitive photoreceptors. The cyprinids inhabited clear water use photoreceptors with shorter wavelength opsins, yet the species lived in turbid water possess photoreceptors with longer wavelength opsins. The alternative chromophore usage and accumulation of the complex interactive substitutions could dominate the obvious spectral differences of rhodopsin, Rh2, and LWS between the cyprinids inhabited two distinct photic environments. The amino acid substitutions may be responsible for differences in the absorbance of SWS1 and SWS2. These results support our hypotheses that nuptial coloration and photic environment of habitats could be tied to visual sensitivities and opsin genes.