Effects of dietary mannan oligosaccharides on growth, non-specific immunity, and intestinal health in juveniles of the Japanese sea cucumber (Apostichopus japonicus)

We investigated the beneficial dietary effects of mannan oligosaccharide (MOS) on sea cucumber (Apostichopus japonicus) and the optimum dietary level of MOS. Juvenile sea cucumbers (initial mean weight of 1.50 +/- 0.00 g, N = 225) were fed once daily for 60 days in tanks with aerated seawater (3 tanks/diet; 15 inds/tank) on feeds supplemented with 0 g/kg (M0), 4 g/kg (M1), 8 g/kg (M2), 12 g/kg (M3), and 16 g/kg (M4) MOS. The results showed that 4 g/kg of MOS in feeds significantly improved the growth performance of sea cucumber. There was a positive linear and quadratic correlation between the total superoxide dismutase (T-SOD) activity and dietary MOS level (linear, P = 0.043, R-2 = 0.279; quadratic, P = 0.009, R-2 = 0.545). Compared with the control group, 8 g/kg MOS significantly increased the protease (PRO) and amylase (AMS) activities (P < 0.05). Quadratic regression was observed between PRO and dietary MOS level (P = 0.022, R-2 = 0.470). Moreover, M2 diets significantly increased the muscular thickness and width of mucosal folds of sea cucumber than in the M0 group (P < 0.05). Base on the quadratic regression between muscular thickness and dietary MOS level was also observed , the optimal level of MOS for intestinal muscular thickness 8 g/kg. High-throughput sequencing was used to analyze the diversity and relative abundance of microorganisms. Proteobacteria, Firmicutes, and Bacteroidetes represented the most abundant phyla in each sample. There was a positive correlation between the expression of heat shock protein 70 (HSP70) (P = 0.015, R-2 = 0.503) and heat shock protein 90 (Hsp90) (HSP90a: P = 0.016, R-2 = 0.496; HSP90b (linear, P = 0.049, R-2 = 0.266; quadratic, P = 0.015, R-2 = 0.505)) with dietary MOS levels. However, a negative correlation was observed for the expression of major yolk protein (MYP) and proprotein convertase subtilisin kexin 9 (PCSK9) and dietary MOS levels. The present results showed that dietary MOS supplementation enhanced the antioxidant capacity and immunity, increased the abundance and diversity of intestinal microbiota, improved intestinal morphology, and enhanced thermal stress tolerance of sea cucumber. The optimal dietary MOS concentration was 4-8 g/kg.