and are equipped to degrade a cascade of polysaccharides along the hindgut of the herbivorous fish .

The gut microbiota of the marine herbivorous fish are thought to play an important role in host nutrition by supplying short-chain fatty acids (SCFAs) through fermentation of dietary red and brown macroalgae. Here, using 645 metagenome-assembled genomes (MAGs) from wild fish, we determined the capacity of different bacterial taxa to degrade seaweed carbohydrates along the gut. Most bacteria (99%) were unclassified at the species level. Gut communities and CAZyme-related transcriptional activity were dominated by and . Both classes possess genes CAZymes acting on internal polysaccharide bonds, suggesting their role initiating glycan depolymerization, followed by rarer and . Results indicate that utilize substrates in both brown and red algae, whereas other taxa, namely, , , and , utilize mainly brown algae. had the highest CAZyme gene densities overall, and were especially enriched in CAZyme gene clusters ( = 73 versus just 62 distributed across all other taxa), pointing to an enhanced capacity for macroalgal polysaccharide utilization (e.g., alginate, laminarin, and sulfated polysaccharides). Pairwise correlations of MAG relative abundances and encoded CAZyme compositions provide evidence of potential inter-species collaborations. Co-abundant MAGs exhibited complementary degradative capacities for specific substrates, and flexibility in their capacity to source carbon (e.g., glucose- or galactose-rich glycans), possibly facilitating coexistence via niche partitioning. Results indicate the potential for collaborative microbial carbohydrate metabolism in the gut, that a greater variety of taxa contribute to the breakdown of brown versus red dietary algae, and that encompass specialized macroalgae degraders.