Dietary Application of Synergistically Degraded Low-Molecular-Weight Chitosan to Promote Health and Antioxidant Responses in Pacific White Shrimp (Litopenaeus vannamei)

This study evaluated the potential of synergistically degraded low-molecular-weight chitosan (LMW-CS) as a functional feed additive to promote growth, antioxidant capacity, innate immunity, and disease resistance in Pacific white shrimp (Litopenaeus vannamei). High-molecular-weight chitosan (HMW-CS, approximately 85 kDa) was degraded using γ-irradiation in combination with H2O2 to produce LMW-CS with improved functional properties. Shrimp were fed five experimental diets for 4 weeks: a control diet, HMW-CS0.4 (0.4% w/w), LMW-CS0.1 (0.1% w/w), LMW-CS0.2 (0.2% w/w), and LMW-CS0.4 (0.4% w/w). Growth performance, oxidative stress markers, antioxidant enzyme activities, lysozyme activity, immune-related gene expression, bacterial load, and survival after Vibrio parahaemolyticus challenge were evaluated. The results indicate that dietary LMW-CS supplementation improved growth performance and feed utilization, with LMW-CS0.2 showing significantly higher final weight, total weight gain, and average daily gain than the control group (p < 0.05). Antioxidant assays showed that LMW-CS reduced malondialdehyde levels and increased reduced glutathione, nitric oxide, glutathione reductase, catalase, superoxide dismutase, glutathione peroxidase, and glutathione-S-transferase activities in both plasma and hepatopancreas (p < 0.05). Lysozyme activity was significantly enhanced, particularly in the LMW-CS0.4 and HMW-CS0.4 groups (p < 0.05). Gene expression analysis revealed upregulation of genes associated with growth regulation, antimicrobial defense, pathogen recognition, and prophenoloxidase activation, including igf2, cstn, lgbp, lyz, and propo2. Gut microbiota profiling showed that chitosan supplementation altered bacterial community composition, reduced the relative abundance of some Vibrio-associated taxa, and descriptively lowered predicted pathogenic and stress-tolerant bacterial phenotypes. Following the Vibrio parahaemolyticus challenge, shrimp fed LMW-CS0.4 showed the lowest bacterial load and highest survival rate, indicating improved disease resistance (p < 0.05). Overall, synergistically degraded LMW-CS enhanced growth, redox balance, innate immune competence, gut microbial structure, and resistance to V. parahaemolyticus, supporting its potential as an antibiotic-free functional feed additive for sustainable shrimp aquaculture.