Supplementation with Enterococcus faecium enhances growth performance, intestinal health and immunity of big-belly seahorses (Hippocampus abdominalis) during diet conversion

Diet conversion is a key stage of seahorse aquaculture. Herein, we set up five groups to evaluate the probiotic effects of Enterococcus faecium supplementation on the growth, development, intestinal health, and immunity of big-belly seahorses (Hippocampus abdominalis) during this stage, including control (C) (no E. faecium), H2 (1 ×107 CFU mL−1, every 2 d), H4 (1 ×107 CFU mL−1, every 4 d), L2 (1 ×105 CFU mL−1, every 2 d), and L4 (1 × 105 CFU mL−1, every 4 d) groups. After 48 d culture, concentration and frequency of E. faecium supplementation, as well as their interaction, were confirmed to have significant effects on growth performance of seahorses (P < 0.05). Body weight, specific growth rate, body weight increase, standard length and standard length increase of H4 and L4 groups were significantly higher (P < 0.05); Ovary weight but not testis weight and brood pouch length significantly increased in H4 group (P < 0.05); Feed intake increased (P < 0.05), while feed conversion ratio significantly reduced in H4 group (P < 0.05); Together with the increase of intestinal absorption area (P < 0.05) and digestive enzymes in H4 group, it suggests that the increased growth performance of big-belly seahorses may be attributed to the integrated improvements in food intake, digestion, and absorption capacity. Additionally, significant increase of expression of intestinal immune-related genes, and remodeling of both intestinal microbiota and their functions endowed seahorses with healthier intestinal environment, which resulted in the increase of survival rate by 50% against pathogenic Edwardsiella tarda challenge. Constructing comprehensive evaluation systems of both growth performance and probiotic effects for the first time could help us determine the best application strategy of E. faecium during the diet conversion period of big-belly seahorse aquaculture. This finding may provide a novel reference for the application of E. faecium in seahorse aquaculture.