​Diet and digestive traits associated with crab digestion in the crab-eating frog (Fejervarya cancrivora)

Despite their mechanically resistant, chitin-rich exoskeletons, how anurans with limited chewing capacity utilize crustacean prey remains poorly understood. The crab-eating frog ( Fejervarya cancrivora ), which inhabits mangrove and brackish-water environments and feeds on crustaceans, provides a suitable model system for addressing this question. However, how its natural diet relates to host digestive traits and gut microbial functional potential remains unclear. To address this, we combined stomach-content analysis via stomach flushing with gastrointestinal histology, digestive enzyme assays, and gut metagenomics to examine crustacean consumption in relation to host digestive characteristics and microbial functional potential. The results showed that decapods dominated stomach contents by volume, accounting for 83.50% in males and 92.83% in females. The stomach exhibited prominent longitudinal folds and circular, oblique and longitudinal muscle bundles, consistent with food accommodation, mixing and propulsion. Chitinase activity was detected in the stomach, small intestine and large intestine, indicating chitin-hydrolysing capacity at multiple gastrointestinal sites. Gut metagenomes harboured genes spanning chitin polymer cleavage, chitooligosaccharide and chitobiose processing, N-acetylglucosamine (GlcNAc) transport, and intracellular metabolism, with distinct candidate source genera at each stage: Plesiomonas and Aeromonas (polymer cleavage), Candidatus Avirikenella and Ruthenibacterium (chitooligosaccharide and chitobiose processing), Cetobacterium (GlcNAc transport), and Mucinivorans , Parabacteroides , and Alistipes (intracellular metabolism). These annotations indicate that genes associated with different stages of chitin utilization are distributed across multiple bacterial genera. Together, these findings link crustacean consumption in F. cancrivora to gastrointestinal structural traits, chitin-hydrolysing capacity, and gut microbial functional potential, offering an ecophysiological perspective on how anurans process and utilize chitin-rich animal prey.