Molecular Mechanisms Underpinning Astaxanthin-Induced Body Coloration in the <i>Lutjanus erythropterus</i> Revealed by Phenotypic, Physiological and Transcriptomic Analyses

Astaxanthin has attracted considerable interest, owing to its potent antioxidant and pigmentation properties. To investigate its effects of astaxanthin on body color variation in <i>Lutjanus erythropterus</i>, fish were divided into a control group and a treatment group fed an astaxanthin-supplemented diet. Body color parameters, growth performance, and liver antioxidant enzyme activities were measured at the end of the experiment. Tissues, including skin, intestine, liver, and blood, were subsequently collected for transcriptome sequencing. The results demonstrate that the astaxanthin-treatment group exhibited significantly enhanced body coloration alongside improved body length, body weight, and specific growth rate compared to the control group (<i>p</i> < 0.05). Specifically regarding the red–green value (a*), the treatment group showed significantly higher values on the ventral skin, dorsal skin, and gill cover (<i>p</i> < 0.05). The yellow–blue (b*) and lightness (L*) values were also significantly elevated in the ventral skin and gill cover (<i>p</i> < 0.05), although no significant differences were observed in the dorsal skin (<i>p</i> > 0.05). The skin was identified as the tissue with the highest total carotenoid content. Astaxanthin supplementation enhanced liver antioxidant capacity, evidenced by significantly elevated total superoxide dismutase (T-SOD) activity and significantly reduced malondialdehyde (MDA) levels in the treatment group (<i>p</i> < 0.05). Catalase (CAT) activity did not differ significantly between groups (<i>p</i> > 0.05). Transcriptomic analysis identified several coloration-associated genes, such as bco1, bco2, gstt1, and gstz1. It also revealed significant enrichment in key metabolic pathways (fatty acid, cholesterol, and retinol metabolism) and signaling pathways (PPAR and PI3K-Akt). Furthermore, the expression of multiple solute-carrier family members and apolipoproteins was detected, with notable enrichment in lipid digestion and absorption, cholesterol metabolism, and several key immune-related signaling pathways. These findings provide a theoretical basis for understanding the molecular mechanisms of carotenoid-mediated pigmentation in <i>L. erythropterus</i>.