Daily rhythms of digestive enzyme activities and gut microbiota composition in two bivalve species: <i>Meretrix meretrix</i> and <i>Mactra veneriformis</i>

Daily rhythms regulate many physiological processes in aquatic animals, but their relationship with digestive physiology and gut microbiota in bivalves remains poorly understood. This study investigated daily rhythms in digestive enzyme activities and gut microbial composition in two filter-feeding bivalves, <i>Mactra veneriformis</i> and <i>Meretrix meretrix</i>, under a 12-h light/12-h dark (LD 12:12 h) photoperiod in February. Samples were collected at four time points (6:00, 12:00, 18:00 and 24:00), and digestive enzyme activities were measured alongside 16S rRNA-based gut microbiome profiling. Both species exhibited significant daily fluctuations in digestive enzyme activities, accompanied by temporal variation in gut microbial diversity and composition, with <i>M. meretrix</i> exhibited relatively stable microbial composition across sampling times, while <i>M. veneriformis</i> exhibited greater temporal variability. Key genera, including <i>Colwellia</i>, <i>Sulfitobacter</i> and <i>Arcobacter</i>, showed time-specific enrichment, which coincided with fluctuations in enzyme activities. Significant positive correlations were observed between specific bacterial taxa (e.g. <i>Verrucomicrobium</i> and <i>Perlucidibaca</i>) and digestive enzyme activities. These results highlight the temporal coordination between digestive physiology and gut microbiota in bivalves and provide insights for improving aquaculture management.