Abstract:A 9-week feeding trial was performed to investigate the effects of substituting fish meal (FM) with defatted mealworm Tenebrio molitor meal (TM) on the growth, body composition, serum immune index, as well as histology, barrier functions, digestive enzymatic activities, and microbial communities of the intestine of spotted seabass (Lateolabrax maculatus). In this study, the basal diet was formulated to contain 30% FM, and five experimental diets were formulated by replacing FM with TM at different levels: 0 (TM 0), 5% (5% TM), 10% (10% TM), 15% (15% TM), and 20% (20% TM). Juvenile spotted seabass (2.83±0.02) g were randomly assigned to five treatments with three replicates and 20 fish per replicate. The results showed that the weight gain rate, specific growth rate, and protein productive value of spotted seabass first increased and then decreased with an increase in TM. Among the treatments, there were no difference in the feed efficiency, feeding rate, survival, hepatosomatic index, abdominal fat ratio, or body composition (P>0.05), but the viscerosomatic index was higher in the 5% TM treatment than that in the 20% TM treatment (P<0.05). The serum lysozyme activity was induced in the 5% TM treatment compared to that in other TM treatments (P>0.05). Intestinal histomorphology of spotted seabass was altered with increased dietary TM levels. Compared with the FM treatment, the intestinal villus width, villus height, and muscular thickness were increased significantly in the 5% TM treatment, while all three indices were decreased significantly in the 20% TM treatment (P<0.05). Meanwhile, the expression of the pro-inflammatory gene IL-1β was significantly down-regulated in 5%–15% TM treatments compared to that in the FM and 20% TM treatments (P<0.05). A similar pattern was observed in the expression of the anti-inflammatory gene IL-4. The transcripts of genes associated with barrier functions (ZO-1 and Ocln) were significantly up-regulated in the 5% TM treatment (P<0.05). However, the activity of digestive enzymes (protease and lipase) was not different among all treatments (P>0.05). Furthermore, the alteration of intestinal microbial communities was observed with increasing dietary TM levels. Higher genus abundance of Bacillus was observed in the TM treatments compared to that in the FM treatment (P<0.05), and the relative abundance of Plesiomonas tended to decrease in the 5% TM and 10% TM treatments. In conclusion, substituting fish meal with 5% TM can improve the growth and intestinal health of spotted seabass, while 15% TM had no negative effects on fish. However, excess dietary TM (20%) inhibits growth, causes histopathological damage, and alters the composition of intestinal microbial community in L. maculatus. According to the results of the quadratic regression model, the level of fishmeal substitution by TM in the diet of spotted seabass should not be greater than 7.31%.
