Patterns of Functional Redundancy and Anaerobic Metabolic Potential Characterise the Skin Microbiome of a Critically Endangered Sawfish.

Using shotgun metagenomics of largetooth sawfish (Pristis pristis) skin and paired pool water samples, we provide the first taxonomic and functional characterisation of a sawfish skin microbiome and assess its ecological distinctiveness, assembly, and resilience relative to the surrounding water column. The skin microbiome was strongly host-associated and compositionally distinct, with lower taxonomic richness than the water column but dominated by Bacillota, particularly spore-forming Bacilli. Functional profiling revealed genes associated with sporulation, dormancy, anaerobic metabolism, and peptide transport, consistent with low-oxygen, low-flow conditions. In contrast, water-column microbiomes were more taxonomically diverse and enriched in phototrophic and polysaccharide-utilisation pathways. Sawfish skin communities exhibited patterns consistent with greater functional redundancy, with gene functions accumulating more rapidly per taxon. Together with greater among-individual taxonomic dispersion, this pattern is consistent with a host-filtered, lottery-like assembly process in which taxonomically variable communities maintain similar functional potential. The enrichment of dormancy and anaerobic pathways suggests the skin microbiome persists through periods of host quiescence and environmental stress while maintaining metabolic potential. Together, these results demonstrate that the sawfish skin supports a resilient, functionally robust microbiome distinct from ambient aquatic communities, highlighting the potential for integrating microbial functional data into conservation strategies for Critically Endangered vertebrates.