The effects of ammonia-N stress on immune parameters, antioxidant capacity, digestive function, and intestinal microflora of Chinese mitten crab, Eriocheir sinensis, and the protective effect of dietary supplement of melatonin

Ammonia nitrogen pollution seriously affects the economic benefits of Chinese mitten crab (Eriocheir sinensis) farming. In this study, we first evaluated the protective effects of melatonin (MT) on immune parameters, antioxidant capacity, and digestive enzymes of E. sinensis under acute ammonia nitrogen stress. The results showed that ammonia-N stress significantly decreased the antibacterial ability of crabs, nevertheless MT could significantly improve it under ammonia-N stress (P < 0.05). Ammonia-N group hemolymph antioxidant capacity indicators (T-AOC, T-SOD, GSH-Px) were significantly decreased than control (p < 0.05), while the MT ammonia-N group hemolymph T-SOD activity significantly increased than ammonia-N group (p < 0.05). For hepatopancreas, ammonia-N group GSH-PX activity significantly decreased than control group, but MT ammonia-N group was significant increased than ammonia-N (p < 0.05). Ammonia-N stress has significantly increased the content of MDA in hemolymph and hepatopancreas (p < 0.05), but MT ammonia-N treatment significantly decreased than ammonia-N group (p < 0.05). Compared with the control group, ammonia-N significantly reduced the activities of Trypsin in the intestine and hepatopancreas (p < 0.05), while MT ammonia-N group can significantly improve the intestinal trypsin activity than ammonia-N (p < 0.05). The intestinal microbiota of E. sinensis results showed that ammonia-N stress significantly decreased the relative abundance of Bacteroidetes (p < 0.05). Ammonia-N stress significantly decreased the Dysgonomonas and Rubellimicrobium, and the Citrobacter significantly increased. In summary, melatonin has a protective effect on E. sinensis under ammonia-N stress. Acute ammonia-N stress may lead to the decrease of probiotics and the increase of pathogenic bacteria, which may be closely related to the impairment of digestive function and immune function.