Regulatory effects of light intensity and lutein on body coloration, growth performance, and intestinal microbiota in the lined seahorse (Hippocampus erectus)

The lined seahorse ( Hippocampus erectus ) exhibits diverse and vibrant body coloration in natural habitats, yet abnormal coloration frequently occurs under intensive aquaculture, severely hindering the sustainable development of its commercial farming industry. In this study, 6-month-old H. erectus were exposed to two light intensities (ambient indoor light: 0–220 lx; strong light: 450 ± 20 lx) and two dietary lutein levels (0 and 1000 mg/kg), to investigate their individual and combined effects on body color traits, chromatophore distribution, skin pigment content, growth performance, and intestinal microbiota. The results demonstrated that either strong light or dietary lutein alone significantly increased skin lightness ( L* ), redness ( a* ), and yellowness ( b* ), facilitated cutaneous carotenoid deposition and xanthophore dispersion, and decreased melanin content. Conversely, their combined treatment triggered a significant antagonistic interaction ( P < 0.05), with the magnitude of yellowing, carotenoid accumulation, and chromatophore dispersion in the combined group significantly lower than in single-treatment groups. For growth performance, the ambient indoor light group exhibited optimal results. Strong light exerted stress on growth and reproduction, whereas lutein alone did not improve growth metrics. Intestinal microbiota analysis revealed that lutein was the primary driver of microbial community structural shifts. Lutein alone increased the relative abundance of Vibrio to 91.26% (significantly higher than other groups), upregulated bacterial pathogenicity-associated pathways, and downregulated nutrient catabolic pathways. When paired with strong light, Vibrio abundance dropped to 52.42%, and normal nutrient metabolic functions were restored. In conclusion, strong light and lutein exert antagonistic effects on body color regulation and intestinal microecological homeostasis in H. erectus , thus their combined use is not recommended in aquaculture practices. To enhance body coloration in cultured seahorses, priority should instead be given to optimizing light management strategies. These findings provide a scientific basis for the precise regulation of body color and health management in lined seahorse farming.