Environmental DNA surveys of metazoan diversity across seamounts and abyssal plains in the western Clarion Clipperton Zone

The deep seafloor serves as a reservoir of biodiversity in the global ocean, with > 80% of invertebrates at abyssal depths still undescribed.These diverse and remote deep-sea communities are critically under-sampled and increasingly threatened by anthropogenic impacts, including future polymetallic nodule mining. Using a multi-gene eDNA metabarcoding approach, we characterized metazoan communities sampled from sediments, polymetallic nodules, and seawater in the western Clarion Clipperton Zone (CCZ), to test the hypotheses that deep seamounts (1) are species richness hotspots in the abyss, (2) have structurally distinct communities in comparison to other deep-sea habitats, and (3) that POC flux to the seafloor is positively correlated with diversity, through comparisons across large-scale environmental gradients in the western CCZ. eDNA metabarcoding was effective at capturing the distinct biotas known to occur in association with different substrate types (e.g. nodule-specific fauna), with distinct community composition and very few ASVs and OTUs shared among sample types. We find support for the ‘seamount oasis hypothesis’ as deep seamounts in the western CCZ were ASV/OTU richness hotspots and reservoirs of unique metazoan diversity in comparison to the surrounding abyssal plains. Seamounts were characterized by significantly higher proportions of the community that were unique to a specific seamount or were cosmopolitan across plain and seamount habitats, with a higher proportion of plain fauna widespread in abyssal plain habitats but absent from deep seamounts. Across a gradient of low to moderate POC flux, we find lowest taxon richness and evenness at lowest POC flux, with community composition on nodules also influenced by nodule size. We discuss our results in the context of future deep-seabed mining and the potential importance of biodiversity hotspots to conservation of metazoan communities at the abyssal seafloor.