Effects of Fermented Longan Peel (Dimocarpus longan) on Growth Performance, Digestive Enzyme Activity, Intestinal Microstructure, Immune Response, and Gene Expression of Nile Tilapia (Oreochromis niloticus) Raised Under Biofloc System

The valorization of agricultural by-products as functional feed additives represents a promising strategy for sustainable aquaculture. This study evaluated the effects of dietary fermented longan peel (FLP), produced through enzymatic hydrolysis and <i>Lactiplantibacillus plantarum</i> fermentation, on growth performance, digestive physiology, gut morphology, innate immunity, and gene expression in Nile tilapia (<i>Oreochromis niloticus</i>) cultured under a biofloc system. Five experimental diets were formulated with graded FLP levels (0, 5, 10, 20, and 40 g kg^-1) and fed to fish for eight weeks. Growth indices, including final weight, weight gain, and specific growth rate, improved significantly in fish receiving 20 g kg^-1 FLP, following a strong quadratic response pattern. <i>In vitro</i> digestibility assays showed enhanced carbohydrate and protein digestibility, coinciding with increased intestinal amylase and protease activities. Histological analysis indicated that moderate FLP inclusion (10-20 g kg^-1) promoted villus height, crypt depth, and epithelial organization. Innate immune parameters, including lysozyme, peroxidase, and alternative complement activity, were markedly elevated in serum and mucus, particularly at 20-40 g kg^-1 after eight weeks. Gene expression profiling revealed significant up-regulation of growth-related (<i>IGF-1</i>, <i>GH</i>, <i>NPY-α</i>, <i>Galanin</i>), immune-related (<i>TLR-7</i>, <i>TNF-α</i>, <i>NFκB</i>), and antioxidant-related (<i>hsp70</i>, <i>Keap-1</i>, <i>nrf-2</i>, <i>GST-α</i>) genes in fish fed higher FLP levels, with responses plateauing beyond 20 g kg^-1. Overall, FLP supplementation at 20 g kg^-1 optimally enhanced growth, digestive efficiency, intestinal health, and innate immune status. These findings demonstrate the potential of fermented longan peel as a cost-effective, bioactive, and sustainable functional feed ingredient for tilapia and other warm-water aquaculture species.