1 Biogenic habitats, such as coral reefs, facilitate diverse communities. In aquatic systems, 2 aggregations of mobile benthic species may play a similar ecological role to that of typically 3 sessile biogenic habitats, however, this has rarely been considered. We quantified the abundance 4 of sessile mussels (Modiolus modiolus) and aggregating brittle stars (Ophiotrix fragilis) and 5 tested for correlations between the density of mussels (live and dead) and brittle stars each with: 6 1)

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... ce, biomass, diversity and assemblage structure of associated benthic macrofauna; 7 and 2) percent organic matter of the sediment. We found that the abundance of live M. modiolus 8 was positively associated with the abundance and biomass of macrofauna. The positive 9 association between M. modiolus and macrofauna abundance was further amplified with an 10 increase in brittle stars and a decrease in dead mussel shells. Macrofauna biomass was lower 11 with more dead mussel shells and macrofauna diversity increased with more live M. modiolus 12 and brittle stars. Sediment organic matter was positively related with brittle star density, but not 13 with the abundance of live or dead mussels. The positive relationship between brittle stars and 14 sediment organic matter suggests that brittle stars could enhance rates of benthic-pelagic 15 coupling. Given the importance of understanding the functional role of threatened habitats, it is 16 essential that the underlying community patterns be understood through robust observational 17 studies to then derive testable hypotheses to determine drivers. These findings provide novel 18 insight into the ecological role of aggregations of mobile species, which is essential to prioritize 19 conservation and restoration strategies. 20 Running header: Aggregations of mussels and brittle stars