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Deep-sea communities are becoming increasingly vulnerable to anthropogenic disturbances, as fishing, hydrocarbon exploration and extraction, and mining activities extend into deeper water. Negative impacts from such activities were recently documented in the Gulf of Mexico (GoM), where the Deepwater Horizon oil spill caused substantial damage to a deep-water octocoral community. Although a faunal checklist and numerous museum records are currently available for the entire GoM, local-scale diversity and assemblage structure of octocoral communities remains unknown, particularly in deep water. On a series of recent cruises (2008–2011) using remotely operated vehicles, 435 octocorals were collected from 33 deep-water sites (250–2500 m) in the northern GoM. To elucidate species boundaries, the extended mitochondrial barcode (COI+igr1+msh) was successfully amplified and sequenced for 422 of these specimens, yielding a total of 64 haplotypes representing at least 52 species. Further, at least 29% of the species collected were either previously not known to occur in the GoM (12 species) or represent new species (at least three species). Overall, species richness at each site was fairly low (1–12 spp.). The greatest species richness occurred at the shallowest (<325 m: GC140, n=8 spp.) and the deepest (2100–2500 m: DC673, n=12 spp., DC583, n=10 spp.) sites, and minimum taxonomic and phylogenetic (Faith's Index) diversity was evident at 600–950 m. This pattern is the opposite of the typical pattern of deep-sea diversity in the GoM, which normally peaks at mid-slope depths. Sorensen's Index of taxonomic β-diversity indicated that six distinct (65–95% dissimilarity) species assemblages corresponded with five depth breaks at ~325, 425, 600, 1100, and 2100 m. Further assemblage structure was observed within certain depth zones. Of note, within the 425–600 m depth range, species assemblages at the West Florida Slope differed from the other sites, corresponding to an established biogeographic barrier. The phylogenetic approach used in this study provided important insights into the species boundaries of many taxa while demonstrating that evolutionary history plays a critical role in community structure of deep-sea octocorals. [ABSTRACT FROM AUTHOR]