Documenting the diversity of marine life is challenging because many species are cryptic, small, and rare, and belong to poorly known groups. New sequencing technologies, especially when combined with standardized sampling, promise to make comprehensive biodiversity assessments and monitoring feasible on a large scale. We used this approach to characterize patterns of diversity on oyster reefs across a range of geographic scales comprising a temperate location [Virginia (VA)] and a subtropical location [Florida (FL)]. Eukaryotic organisms that colonized multilayered settlement surfaces (autonomous reef monitoring structures) over a 6-mo period were identified by cytochrome c oxidase subunit I barcoding (>2-mm mobile organisms) and metabarcoding (sessile and smaller mobile organisms). In a total area of ∼15.64 m 2 and volume of ∼0.09 m 3 , 2,179 operational taxonomic units (OTUs) were recorded from 983,056 sequences. However, only 10.9% could be matched to reference barcodes in public databases, with only 8.2% matching barcodes with both genus and species names. Taxonomic coverage was broad, particularly for animals (22 phyla recorded), but 35.6% of OTUs detected via metabarcoding could not be confidently assigned to a taxonomic group. The smallest size fraction (500 to 106 μm) was the most diverse (more than two-thirds of OTUs). There was little taxonomic overlap between VA and FL, and samples separated by ∼2 m were significantly more similar than samples separated by ∼100 m. Ground-truthing with independent assessments of taxonomic composition indicated that both presence-absence information and relative abundance information are captured by metabarcoding data, suggesting considerable potential for ecological studies and environmental monitoring. U nderstanding the diversity of life in the sea continues to challenge marine scientists because samples typically contain many rare species, most of them small and difficult to identify (1). Moreover, recent estimates suggest that between 33% and 91% of all marine species have never been named (2, 3). These constraints have limited our ability to investigate patterns of diversity beyond a few indicator groups (4), most often conspicuous macroinvertebrates and fish. For this reason, molecular methods, particularly high-throughput sequencing (HTS) approaches, hold considerable promise not only for fundamental understanding of diversity but also for biodiversity monitoring in the context of global change (5).Molecular methods are particularly powerful when combined with standardized sampling, allowing for direct comparisons across space and through time. In the ocean, analyzing standard volumes of readily sampled material (e.g., seawater, sediments) has a long tradition, and, increasingly, HTS approaches are being applied to these samples (6). Complex hard substrates provide greater challenges for consistent sampling, which can be met either by collecting approximately standard volumes (e.g., of rubble) or by deploying settlement structures (e.g., ref. 7).Here, we combin...