Effects of Different Light Spectra on Growth Performance, Body Coloration, and Pigmentation-Related Gene Expression in Koi Carp (Cyprinus carpio var. koi)

Light is a critical environmental factor in aquaculture, directly or indirectly regulating the growth, reproduction, body coloration, and biological rhythms of cultured animals. However, in koi farming, spectral composition is often overlooked or arbitrarily selected. This study aimed to evaluate the effects of different LED spectra (red light “RL”, blue light “BL”, yellow light “YL”, white light “WL”) and natural light (NL) on the growth performance, body coloration, and underlying molecular regulatory mechanisms of all-red koi (Cyprinus carpio var. koi). The results showed that WL treatment significantly suppressed the weight gain rate and specific growth rate of koi. After 60 days of BL exposure, the condition factor significantly increased, whereas YL, WL, and RL led to varying degrees of reduction. In terms of body color, BL significantly increased skin lightness (L* value) and promoted the deposition of lutein and β-carotene in the skin, while upregulating the expression of pigmentation-related genes such as gch (GTP cyclohydrolase), scarb (Scavenger receptor class B), oca2 (Oculocutaneous albinism II), and xdh (Xanthine dehydrogenase). Meanwhile, the calcium signaling, MAPK signaling, and TGF-β signaling pathways were activated, synergistically mediating light adaptation, metabolic regulation, and pigmentation. However, chronic or excessive BL irradiation exacerbated dermal loosening and vacuolation, decreased a* and b* values, and was thus unfavorable for color maintenance. In contrast, long-term exposure to RL and WL tended to inhibit growth and cause dermal tissue loosening, whereas YL was relatively beneficial to fish health but had limited color-enhancing effects. Overall, BL is the optimal spectrum for short-term (30–60 days) color enhancement, and long-term exposure to RL and WL should be avoided. In practical production, a cyclic lighting regimen is recommended, i.e., short-term BL for color enhancement followed by recovery under NL or YL. This study provides an important theoretical basis for optimizing the light environment for koi culture, improving the quality of ornamental koi, and enhancing aquaculture production efficiency.