Symbiotic microalgae (Symbiodinium spp.) strongly influence the performance and stresstolerance of their coral hosts, making the analysis of Symbiodinium communities in corals (and metacommunities on reefs) advantageous for many aspects of coral reef research.High-throughput sequencing of ITS2 nrDNA offers unprecedented scale in describing these communities, yet high intragenomic variability at this locus complicates the resolution of biologically meaningful diversity. Here, we demonstrate that generating operational taxonomic units by clustering ITS2 sequences at 97% similarity within, but not across, samples collapses sequence diversity that is more likely to be intragenomic, while preserving diversity that is more likely interspecific. We utilize this 'within-sample clustering' to analyze Symbiodinium from ten host taxa on shallow reefs on the north and south shores of St. John, US Virgin Islands. While Symbiodinium communities did not differ between shores, metacommunity network analysis of host-symbiont associations revealed Symbiodinium lineages occupying 'dominant' and 'background' niches, and coral hosts that are more 'flexible' or 'specific' in their associations with Symbiodinium. These methods shed new light on important questions in coral symbiosis ecology, and demonstrate how application-specific bioinformatic pipelines can improve the analysis of metabarcoding data in microbial metacommunity studies. 63 antillogorgium, and potentially other species (Parkinson et al., 2015). Moreover, the 64 position of ITS2 within the tandemly-repeating ribosomal DNA array creates multiple ITS2 65 sequence variants within a single genome (Thornhill et al., 2007) that evolve through 66 concerted evolution (e.g. (Dover, 1986)). In fact, concerted evolution may mask species 68 dominants in multiple derived lineages ( Thornhill et al., 2013). Together, these features of 69 ITS2 complicate the interpretation of intragenomic versus interspecific variation, and 70 preclude its use as a true species-level marker for Symbiodinium. Nevertheless, numerically 71 dominant intragenomic variants of ITS2 are still phylogenetically informative across the 72 genus (LaJeunesse, 2001), and resolve diversity at a functionally and ecologically important 73 level. Moreover, due to the large quantity of existing sequence data for comparative 74 analysis (e.g., (Franklin et al., 2012;Tonk et al., 2013)), and the relative ease of amplifying 75 and sequencing ITS2, it remains an essential and powerful marker for Symbiodinium.76 Therefore, it is important to develop best practices in the bioinformatic analysis and 77 interpretation of ITS2 metabarcoding surveys of Symbiodinium.78 Here, we describe a Symbiodinium metacommunity associated with scleractinians and a 79 Millepora hydrocoral in St. John, U.S. Virgin Islands, with the objectives of: 1) developing an 80 appropriate bioinformatic approach for Symbiodinium ITS2 metabarcoding surveys, and 2) 81 exploring network analysis and statistical applications for such datasets that can advance 8...