Distinct Fecal Microbiome Communities and Functional Predictions in Spotted Seals: Age‐Dependent and Dietary Transformations

ABSTRACT The spotted seal ( Phoca largha ), which primarily inhabits the cold waters of the Northwest Pacific Ocean, including China's northern Bohai and Yellow Seas, exhibits age‐related variations in its fecal microbiome that remain poorly understood. This study investigated the fecal microbiome composition and predicted functional capabilities between adult and juvenile spotted seals. Firmicutes was the dominant phylum in both groups, with significant differences in abundances of Firmicutes and Bacteroidota ( p < 0.05). Furthermore, higher taxonomic diversity was found in adults, with six dominant genera compared to three in juveniles. Clostridium sensu stricto 1 and UCG‐005 were identified as core genera, whereas Bacteroides and Peptoclostridium were specific to adults and Eubacterium fissicatena group and Blautia were specific to juveniles. Moreover, bacterial functional prediction revealed a significant enrichment ( p < 0.01) of the Transporters and ABC transporters pathways in juveniles, while the Ribosome pathway was abundant in adults ( p < 0.01). The adults’ fecal microbiome displayed more complex and stable bacterial interactions than that of juveniles. Additionally, phylogenetic analysis revealed that Clostridium perfringens in adults was distributed across marine and terrestrial mammals, whereas Faecalicatena in juveniles demonstrated high host specificity. These findings help to explain how age and diet shape the gut microbiome of marine mammals.