Long-term crowding stress disrupts intestinal homeostasis in largemouth bass (Micropterus salmoides)

As an inevitable stressor in high-density aquaculture environment, crowding stress has attracted extensive attention due to its detrimental impact on fish growth and immunity. However, the physiological indicators of crowding stress onset and the intricate mechanisms underlying this stressor remain unclear. In the present study, the impact of crowding stress on growth performance, intestinal microbiota composition, intestinal structure and function in largemouth bass (Micropterus salmoides) have been explored. Growth indices showed that high stocking density suppressed fish growth. The intestinal microbiota composition results showed a significant increase in the abundance of bacterial genera such as Muribaculaceae, Bacteroides, and Enterococcus after crowding stress. Functional predictions of microbiota indicated a weakening of metabolism-related pathways. Additionally, intestinal tissue damage and disrupted transcriptional profile of intestinal cells were observed in the high-density group. Specifically, genes significantly upregulated in "endopeptidase activity" and "intestinal immune network for IgA production" pathways, while significantly downregulated in "oxidative phosphorylation" and "cardiac muscle contraction" pathways. Changes in expression of genes linked to stress response, intestinal barrier integrity, anaerobic glycolysis, and inflammation processes were further validated. Notably, a strong correlation was found between intestinal microbiota and these differentially expressed genes in intestine, particularly pathogenic Enterococcus and probiotic Kocuria. Overall, these results suggest that intestinal microbiota dysbiosis and intestinal cells dysfunction are likely the direct factors contributing to intestinal structural damage and hindered growth of largemouth bass, which offers valuable insights into developing indicators for crowding stress occurrence, as well as strategies to alleviate its detrimental effects by maintaining intestinal homeostasis.