Identification, evolution, and expression analysis of opsin in large yellow croaker

Opsins are light-sensitive G protein–coupled receptors that mediate photoreception and phototransduction in fish and play essential roles in visual development, light-dependent behavior, and adaptation to aquatic environments, and in teleosts the expansion and diversification of the opsin gene family are considered key molecular mechanisms underlying spectral tuning to complex underwater light conditions; however, comprehensive characterization of opsins in the economically important large yellow croaker (Larimichthys crocea) remains limited. The present study aimed to systematically identify the opsin gene family in large yellow croaker, elucidate its structural and evolutionary features, and clarify developmental and light spectrum–dependent expression patterns. Genome-wide bioinformatics analyses were performed to identify opsin genes, followed by characterization of physicochemical properties, conserved motifs, transmembrane domains, secondary structure composition, subcellular localization, and phylogenetic relationships, while transcriptomic data and quantitative real-time PCR (qRT-PCR) were used to examine expression dynamics during early development (1–35 days post-hatching, dph) and transcriptional responses under different monochromatic light treatments. A total of 25 opsin genes were identified, encoding proteins of 334–540 amino acids with predicted molecular weights of 37.1–62.3 kDa and isoelectric points ranging from 5.95 to 9.57, all of which were predicted to localize to the cell membrane and contained seven highly conserved motifs and the canonical seven-transmembrane (7tm_1) domain characteristic of Class A G protein–coupled receptors. Secondary structure prediction revealed that α-helices (30.07%–40.41%) and random coils (44.54%–55.93%) were the predominant structural elements. Phylogenetic analysis classified these genes into visual opsins, including rho, opn1lw1, opn1sw2, and rh2a, and non-visual opsins, including opn3, opn4, opn5, and related subtypes. Transcriptome profiling showed that most opsin genes exhibited relatively low expression levels throughout early development, whereas rho and opn1lw1 displayed pronounced upregulation at 15 dph, suggesting a critical window for rapid visual system maturation. Furthermore, light spectrum experiments demonstrated that blue light exposure significantly upregulated the expression of rh2a and opn1sw2 compared with other light treatments (P < 0.05), indicating high sensitivity of these opsins to short-wavelength light and their involvement in spectral regulation. Collectively, these results demonstrate that the opsin gene family of large yellow croaker is structurally conserved yet exhibits distinct stage-specific and spectrum-dependent expression patterns, providing molecular evidence for visual development and spectral adaptation and offering practical implications for optimizing light environments, feeding strategies, and larval rearing protocols in aquaculture production.