Adaptation to Fasting in Crucian Carp (Carassius auratus): Gut Microbiota and Its Correlative Relationship with Immune Function.

Fasting influences the overall physiology of fish, and the knowledge how the gut microbiota, growth performances, and immune function in response to intermittent and long-term fasting is still insufficient. Here, we characterized the effects of fasting on the host-gut microbiota in crucian carp, which would enhance our insight into physiological adaptation to fasting. To achieve this, we investigated the gut microbial communities of crucian carp with different fasting stress, and corresponding immune and growth parameters. The gut microbial communities were structured into four clusters according to different fasting stress, namely one control group (feed regularly), two intermittent fasting groups (fasting period and re-feeding period, respectively), and one long-term fasting group. Intermittent fasting significantly improved the activity of superoxide dismutase (SOD) and lysozyme (LZM) (ANOVA, p < 0.05) and significantly increased alpha diversity and ecosystem stability of gut microbiota (ANOVA, p < 0.05). Gut length (GL) and condition factor (CF) showed no significant difference between the control group (CG) and intermittent fasting group under re-feeding period (RIF) (ANOVA, p = 0.11), but relative gut length (RGL) in group RIF was higher than that in the CG (ANOVA, p = 0.00). The bacterial genera Bacteroides, Akkermansia, and Erysipelotrichaceae were enriched in fishes under intermittent fasting. Two Bacteroides OTUs (OTU50 and OTU1292) correlated positively with immune (SOD, complement, and LZM) and growth (GL and RGL) parameters. These results highlight the possible interplay between growth performances, immune function, and gut microbiota in response to fasting.