Dietary Melatonin Supplementation Improved Intestinal Health and Immune Function of Pacific White Shrimp (Litopenaeus vannamei) Under High Alkali Stress

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The intestinal tract serves as a critical immune regulator in aquatic species, maintaining homeostasis and environmental stress resistance. This study evaluates the protective effects of melatonin (MT) on Litopenaeus vannamei (L. vannamei) under acute alkaline stress through a comprehensive analysis of intestinal morphology, antioxidant responses, apoptosis regulation, and microbial community dynamics. A total of six groups of melatonin treatment groups were designed. After another 2 months of breeding, a 96 h acute alkalinity stress experiment was conducted. Experimental supplementation revealed dose-dependent outcomes: 82.7 mg/kg MT significantly improved survival rates without affecting growth parameters, while higher concentrations (329.2 mg/kg) induced elevated apoptosis (p < 0.05). Histological examination demonstrated mitigated intestinal structural damage in MT-treated groups compared to non-supplemented controls under alkaline stress. Antioxidant capacity initially increased and then stabilized at optimal MT doses (82.7–165.1 mg/kg), accompanied by enhanced immune marker expression (p < 0.05). Microbial profiling indicated MT-mediated enrichment of commensal bacteria associated with polysaccharide metabolism, energy utilization, and intestinal immunity. This study establishes that melatonin exerts dose-dependent protection in L. vannamei under alkaline stress, balancing antioxidant enhancement, apoptosis modulation, and microbiome regulation to fortify intestinal health, with 82.7–165.1 mg/kg identified as the optimal therapeutic range for mitigating environmental stress without compromising physiological homeostasis. The results of this study establish an empirical framework for optimizing MT application in crustacean aquaculture, particularly highlighting its role in maintaining intestinal barrier integrity and microbial homeostasis under alkaline environmental challenges.