Gut Microbial Characteristics and Sex‐Specific Profiles Across Different Regions of the Digestive Tract in the Critically Endangered Hucho bleekeri

The structure and function of the intestinal microbiota play a significant role in the growth of fish and are closely related to physiological processes such as metabolism, immune defence and the maintenance of intestinal homeostasis. Hucho bleekeri is a critically endangered species that plays the role of a top carnivorous predator in the Yangtze River system. In this study, we investigated the enzyme activities and microbiome community of multiple regions of the digestive tract (pyloric caecum, duodenum, hindgut and rectum) in H. bleekeri . The activities of trypsin (TPS), lipase (LPS) and α‐amylase (AMS) were the highest in the pyloric caecum, while cellulase (CL) has the highest activity in the hindgut. The activity of antioxidant enzyme superoxide dismutase (SOD) was the highest in the hindgut. 16s RNA sequencing of microbiota in different digestive tracts generated a total of 2,348,738 raw tags. At the phylum level, Pseudomonadota accounted for the most abundant microorganisms in the gut of H. bleekeri , followed by Bacillota, Cyanobacteriota and Actinobacteriota. The results of alpha diversity analysis showed that the abundance of microbiota in the duodenum and pyloric caecum were higher than that in the hindgut and rectum, especially in male fish. Dependentiae and Deinococcota accounted for the main significant differential microorganisms between different segments of digestive tracts. Different regions of digestive tracts had different microbial compositions, with the enrichment of bacterial communities associated with nutrient absorption in pyloric caecum, while hindgut was enriched with microorganisms related to immune function. In addition, significant differences were also observed in microorganisms between different genders. Sporosarcina and Enterococcus exhibited higher abundances in females compared to males, whereas Clostridium sensu stricto 1, Flavobacterium , Aeromonas and Streptococcus were more abundant in males. The results of the present study elucidate the structure and function of the intestinal microbial community of H. bleekeri and provide basic information for further studies on the nutritional regulation and successful feeding transition.