A study on the effects of replacing fishmeal with soybean meal in the feed of Procambarus clarkii : Assessing growth performance, immunity, and gut microbiota

The unstable supply and high cost of fishmeal have led to increased interest in low fish meal feed. As such, the present study aimed to investigate the effects of replacing fishmeal in feeds with soybean meal (SM) on the growth, immunity, and gut microbiota of crayfish. A total of 750 crayfish (4.00 g) were randomly divided into five groups (SS0, SS25, SS50, SS75, and SS100) with three replicates, and were fed diets with different levels of soybean meal substituted for fishmeal for six weeks. The results revealed that crayfish subjected to high SM diets experienced a significant decrease in the final body weight, weight gain rate, and specific growth rate ( p<0.05 ), together with enzyme activities related to nutrient digestion and absorption. Of note, the groups with 25 - 50 % SM replacement exhibited decreased final body weight without significance compared to that in the SS0 group ( p > 0.05 ). Moreover, increasing SM inclusion exhibited the potential to impair the intestine, characterized by the increase in hemolymph LD content, decrease in the levels of TC content, T-AOC, and AKP, along with the observed edema of the lamina propria, and up -regulated mRNA expression levels of nfkb , alf , and tlr . Furthermore, high SM diets resulted in the emergence of new species and a shift in Firmicutes, Bacteroidota, Proteobacteria, and Fusobacteriota. Specifically, the abundance of lower Firmicutes and higher Proteobacteria was observed in the SS100 group, which was closely correlated with genus -level Candidatus Bacilloplasma, and RsaHf231 abundance alterations. MENAs demonstrated that the high SM diets led to a more complex, yet fragile and unstable microbial network. The global network further indicated that the microbes contributing more to crayfish were in module eigen 4, in which 9 OTUs were identified as major biomarkers and mainly classified into Proteobacteria. Taken above, SM could replace 25 % of fishmeal in crayfish diets without negatively affecting growth performance, nutrient utilization, immunity, and antioxidant capacity. Excessive SM levels contributed to gut dysfunction and weakened the immune system of crayfish.