Metagenomic analysis reveals hidden links between gut microbes and habitat adaptation among cave and surface dwelling Sinocyclocheilus species.

Intestinal microbes are closely related to vital host functions such as digestion and nutrient absorption, which play important roles in enhancing host adaptability. As a natural "laboratory", caves provide an outstanding model for understanding the significance of gut microbes and feeding habits in the habitat adaptability of hosts. However, research on the relationship between gut microbes, feeding habits, and the adaptability of troglobites remains insufficient. In this study, we compared the characteristics of the intestinal microbes of cavefish and surface fish and further established the relationship between intestinal and habitat microbes. Furthermore, we conducted environmental DNA (eDNA) (metabarcoding) analysis of environmental samples to clarify the composition of potential food resources in the habitats of the cavefish and surface fish. Results showed that the structure of the gut microbes was more related to ecological type (habitat type) than phylogenetic relationships. While horizontal transfer of habitat microbes was a source of gut microbes, hosts also showed strong selection for inherent microbes as dominant microorganisms. Differences in the composition and structure of gut microbes, especially dominant microbes, may enhance the adaptability of the two fish types from the perspectives of food intake, nutrient utilization, and harmful substance metabolism, suggesting that food resources, predation patterns, intestinal flora, digestive and absorptive capacity, and feeding habits and preferences are linked to habitat adaptability. These results should facilitate our understanding of the significance of fish gut microbes to habitat adaptation and provide a new perspective for studying the adaptive mechanisms of cavefish.