## 3 Discussion
Simultaneously, we observed that the number of neutral mucous cells in the intestinal mucosa layer of PRS samples also significantly increased (Table 2). The mucus layer of the host gut can adhere to and protect the microorganisms inhabiting the gut, subsequently making them viscoelastic in the intestinal environment [40; 41]. Furthermore, neutral mucin, as one of the components of the mucous layer, can be degraded by some microorganisms to provide carbon sources. The digestive enzyme activity of PRS groups also sharply increased compared to the RS groups, verifying the recovery of fish intestinal digestive function [19]. Complex interactions between changes in physiological function and fish gut microbes play important roles in stabilizing and maintaining the ability of the microbiota to respond to perturbations. In addition, our investigation of pathogenic bacteria among three different groups showed that _Aeronomas veronii_ and _Strptococcus dysgalactiae_ were enriched in the RS gut content samples (Supplementary Figure S1). Meanwhile, our results showed that the number of acidic mucous cells and eosinophilic granulocyte saw an obvious increase in the RS group, and the antioxidant enzyme activity of the host gut was also significantly up-regulated. These physiological changes may be considered as feedback from the significant increase in the abundance of intestinal pathogens [42]. Secreting acidic mucus in the intestinal epithelium can reduce the pH in the intestinal cavity to inhibit the growth of harmful bacteria, thereby protecting the intestine from invasion by pH-sensitive pathogens [29; 41]. During the reproductive process of _M. terminali_, harmful bacteria invaded the host and caused the host immune response. Additionally, we observed great KEGG and CAZy function divergence between the RS and NRS and PRS groups. Our results also showed a greater functional variety and an increase in functional genes related to carbohydrate, lipid, amino acid, cofactors, and vitamin metabolic pathways in the NRS and PRS group. The results indicated that variations in the reproductive state of a host may alter the natural composition and functional capabilities of its gut microbiome. This enables animals to cultivate distinct gut microbiomes to adjust to significant changes in surroundings or diets during their reproductive life cycle. In the present research, we further focused on six important functional pathways. The genes encoding the putative enzymes involved in fatty acid metabolism and the biosynthesis of unsaturated fatty acids were enriched in the NRS samples (Figure 6). Before spawning, _M. terminali_ tends to consume foods of high-nutrient foods (e.g., high protein and fat) in order to store enough energy in the body for reproductive behavior [12; 22]. It is noteworthy that an enrichment of genes encoding putative enzymes implicated in the metabolism of taurine and hypotaurine was observed in the RS samples. Taurine and hypotaurine have been documented to perform significant functions in animals, including the promotion of motor ability and mitochondrial health [43; 44]. Therefore, we deduced that the gut microbiota putatively responds to the host's fast-flowing environment. In our study, _Cetobacterium_ and _Aeronmias_ played an essential role in taurine and hypotaurine metabolism in the gut of the RS population. The genes responsible for enzymes related to starch, sucrose, and galactose metabolism in the PRS group were significantly increased, indicating a potential energy compensation mechanism in _M. terminali_ following breeding. _Cetobacterium_ and _Clostridium_ are of great importance in starch and sucrose and galactose metabolism in the gut of the PRS population. The PRS group has greater microbiome diversity and composition compared to the RS group, indicating a varied and complex community of microbial fermenters capable of breaking down a wide range of feed for digestion. In this study, the interaction between the gut microbiome and _M.