Effects of dietary melatonin on hematological immunity, antioxidant defense and antibacterial ability in the Chinese mitten crab, Eriocheir sinensis

The Chinese mitten crab (Eriocheir sinensis) is an important aquatic animal in China. Immunoprotection can reduce the occurrence of disease, which is one of way to promote the healthy culture of Eriocheir sinensis. The aim of this study was to use biochemical analysis, flow cytometry and molecular biology techniques to evaluate the hematological immunity, antioxidant defense, and antibacterial ability of Eriocheir sinensis that had been fed a diet with melatonin. The melatonin (MT) diets were divided into four different concentrations: 0 mg/kg (diet # A), 6.54 ± 0.47 mg/kg (diet # B), 13.06 ± 0.09 mg/kg (diet # C) and 50.80 ± 7.26 mg/kg (diet # D). In this study, we found that the melatonin concentration in the eyestalk of Eriocheir sinensis, was significantly increased at the 14 days in the diet # D group (395.75 ± 89.46 pg/ng) compared with the control group (131.44 ± 1.73 pg/ng). MT in the diet can elevate the hemocyanin content and superoxide dismutase (SOD) activity. Bacterial challenge experiments showed that the alkaline phosphatase (AKP) activity, hemocyte phagocytosis rate, and total hemocyte count (THC) were increased in the groups that received diets with MT. For example, 24 h after the crab had been injected with of Aeromonas hydrophila (A. hydrophila) the blood cell phagocytic rates, as determined by flow cytometry, in the Diet # C (15.35 ± 2.47%) and Diet # D (16.9 ± 3.58%) groups were much higher than those observed in the Diet # A (9.03 ± 1.16%) and Diet # B (11.93 ± 2.08%)groups (P < 0.05). The RT-QPCR results showed that the relative mRNA expression of heat shock protein 90 (HSP90) was significantly upregulated in gill, intestine and hepatopancreas tissue of the diet # B group and diet # C groups 24 h after the injection of the crabs with Aeromonas hydrophila. Therefore, the results of this study indicate that the addition of melatonin to the diet can enhance the antioxidant， immunity capacity and antibacterial ability, and can increase the concentration of melatonin level of Eriocheir sinensis.