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Background: The brittle star Ophiothela mirabilis has recently invaded and is expanding its range in the southwest Atlantic.

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Aim:Our aim was to uncover patterns of distribution of marine subtidal rocky reef communities across six taxonomic groups and decompose the relative roles of species loss and turnover in total community variation. Additionally, we propose an easily calculated index that can be used to highlight areas with unique species composition for conservation planning. We estimated the strengths of associations between environmental factors and species richness and rarity.Location: Ilha Grande Bay, Brazil, covering about 150,000 ha harbouring different marine habitats. Methods:We used the Marine Rapid Assessment Protocol at 42 sites to gather information on environmental variables and species in six subtidal marine groups. We determined "singular" sites as the regions harbouring higher numbers of rare species.Then, we estimated the roles of species loss and turnover on the observed total variation among sites. We used Generalized Linear Model to partition the relative importance of the selected environmental factors in driving variation in species richness and singularity. Results:The singularity index and richness showed that the bay could be divided into three subregions for subtidal communities. Richness and rarity were structured at different spatial scales and associated with environmental variables related to water productivity and nutrients but varied among taxonomic groups. Community variation over space was largely associated with turnover of species.Main conclusions: Higher singularity and richness on the western side of the bay and around the main island suggested that these regions should be conservation priorities, but high species turnover across the whole bay indicated that portions of the central channel should be included in conservation strategies. This draws attention to the importance of community variation rather than just species numbers in conservation and management planning. The high species turnover indicated that these

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