Comparative Analyses of the Exopalaemon carinicauda Gut Bacterial Community and Digestive and Immune Enzyme Activity during a 24-Hour Cycle

The change in life activities throughout a cycle of approximately 24 h is called the circadian rhythm. Circadian rhythm has an important impact on biological metabolism, digestion, immunity, and other physiological activities, but the circadian rhythm of crustaceans has rarely been studied. In this study, the activity of digestive enzymes (α-amylase, trypsin, and lipase) and immune enzymes (superoxide dismutase, lysozyme, and catalase), as well as the circadian rhythm of the intestinal bacterial community of Exopalaemon carinicauda, were studied. The results showed that the digestive and immune enzyme activities of E. carinicauda changed significantly (p < 0.05) at four time points throughout the day by one-way ANOVA analysis, with the highest value at 24:00 and the lowest value at 12:00. The highest values of alpha diversity and richness were observed in the 24:00 samples, which were significantly higher than those in the other groups (p < 0.05). The principal coordinate analysis (PCoA) results obviously showed that the samples from the same sampling time had higher similarity in the bacterial community structure. Candidatus hepatoplasma had the highest abundance among the intestinal microorganisms at 24:00, and Marinomonas had the highest abundance at 12:00. This study contributed to the understanding of digestive enzyme activity, immune enzyme activity, and the circadian rhythm of the intestinal bacterial community structure in E. carinicauda. It will play an important role in optimizing feeding times and improving digestion and nutrient utilization for E. carinicauda. The results of this study provide a basis for further study on the physiological mechanism of diurnal variation of intestinal flora in crustaceans.