Effects of Spirulina replacement on growth performance, antioxidant capacity, immunity, liver and intestinal tissue and intestinal flora of juvenile Monopterus albus

Exploring the effects of replacing fish meal in feed with spirulina on Monopterus albus, with the aim of determining the optimal inclusion level of spirulina in M. albus feed. In this experiment, a basal diet containing 50% fish meal served as the control. Spirulina was used to isoproteically replace 5%, 10%, 15%, 20%, 25%, and 30% of the fish meal in the basal diet. Juvenile M. albus with an initial body weight of (20.72±0.04) g were fed these experimental diets for 60 days. Growth performance, antioxidant capacity, immune response, hepatointestinal histology, and intestinal microbiota of the juvenile M. albus were analyzed. ① With the increase of the proportion of spirulina replacing fish meal, the final weight, weight gain rate and specific growth rate of Juvenile M. albus all showed a trend of increasing first and then decreasing, with the highest value in the group replacing 10%, significantly higher than that in the group replacing 25% and 30%. The protein deposition rate showed a decreasing trend, 25% and 30% fish meal was significantly lower than the control and 5% groups; the feed coefficient showed an upward trend, and the 25% and 30% groups were significantly higher than the control and 5% group. The regression analysis with weight gain rate as the evaluation index showed that the appropriate replacement ratio of spirulina to fish meal was 10.49%. ② With the increase of the proportion of fish meal as a substitute, the activity of T-AOC and SOD in the liver of juvenile M. albus showed an increasing trend. The activity of T-AOC and SOD in the groups with 20%, 25% and 30% substitution was significantly higher than that in the control group; except for the group with 5% substitution, the MDA content in the Juvenile M. albus was significantly lower than that in the control group. ③ With the increase of replacement ratio, the contents of LZM, ACP, AKP, IgM and C3 in serum of juvenile M. albus showed a trend of rising first and then decreasing, which were the highest in the replacement group 15%, and significantly higher than that in the control group.④ No effect of spirulina replacing fish meal on the liver tissue of juvenile M. albus. Both the 25% and 30% replacement groups showed significantly lower intestinal villus height, lamina propria thickness, and villus height to crypt depth ratio in Juvenile M. albus compared to the control group. ⑤ At the phylum level, with the proportion of alternative fish meal, the relative abundance of intestinal desulfurobacteria and Proteobacteria decreased, while the relative abundance of Firmicutes, Fusobacterium and Bacteroidtes increased; at the genus level, the relative abundance of Acinetobacter, Plesiomonas and Sphingomonas decreased with the proportion of alternative fish meal , and the relative abundance of Cetobacterium increased with the proportion of substitution. The replacement of some fish meal by spirulina can improve the antioxidant capacity and immunity of M. albus, improve the richness of intestinal beneficial bacteria, benefit intestinal health, and reduce the risk of juvenile M. albus. Taking the weight gain rate as the evaluation index, the suitable proportion of spirulina to replace fish meal was 10.49%