Modulation of Growth, Intestinal Microbiota, and Immune Responses in Cherax quadricarinatus by Mannan Oligosaccharides

This study evaluated the effects of mannan oligosaccharides (MOSs) on Cherax quadricarinatus and explored the underlying mechanisms. Crayfish were randomly assigned to six dietary treatments containing 0 (M0), 0.2 (M1), 0.4 (M2), 0.6 (M3), 0.8 (M4), and 1.0 (M5) g·kg−1 MOSs for an 8-week feeding trial. The results demonstrated that MOS supplementation significantly improved growth performance, with weight gain rate and specific growth rate in the M3 group being higher than those in the control (p < 0.05). Survival rate did not differ significantly among dietary treatments, although the M2 group showed the highest numerical value. MOSs significantly reduced muscle lipid content (p < 0.05) without affecting moisture, protein, or ash levels. Microbiota analysis revealed an increase in Firmicutes and a decrease in Proteobacteria in the M3 group (p < 0.05). Non-specific immune parameters, including alkaline phosphatase, acid phosphatase, total antioxidant capacity, superoxide dismutase, and glutathione peroxidase, were significantly elevated in the M2 and M3 groups, while malondialdehyde was reduced (p < 0.05). Immune-related genes, including glutathione peroxidase (GSH-Px), superoxide dismutase (SOD), thioredoxin (Trx), lysozyme (LZM), and anti-lipopolysaccharide factor (ALF), were upregulated in MOS-fed crayfish (p < 0.05). Following Aeromonas hydrophila challenge, survival rates were significantly higher in all MOS-treated groups (p < 0.05). These results indicate that MOS supplementation enhances growth performance, modulates gut microbiota composition, and strengthens immune responses and disease resistance in C. quadricarinatus.