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  • It is widely recognised that M. modiolus reefs represent biodiversity ‘hotspots’ (e.g. Rees et al., 2008). In the Craigbrain and Burial Island Modiolus reefs, diversity indices from samples containing more than 9 live Modiolus per m2 showed numbers of taxa per sample ranging from 31 to 161, Shannon-Weiner diversity indices of between 2.0 and 4.3, and Pielou’s evenness index ranging from 0.52 to 0.95, which are all comparable to figures from similar open coast Modiolus reefs such as those of North Llyn and sites in Loch Alsh (see Fariñas-Franco et al., 2014). The diversity indices did not differ markedly from the surrounding area which is characterised by muddy sandy gravels with high shell content (mostly dead Modiolus shell), however the infaunal communities were significantly different between “Modiolus reef” and “surrounding sediments” (as determined through multivariate community analysis, including cluster analysis and ANOSIM routines). Due to inconsistencies in recording of epifaunal species from the grab samples, the epifaunal communities could not be statistically examined; however it was clear that there is a marked increase in the number of epifaunal species over the reef areas. Density of living Modiolus ranged from zero to 530 per m2 . These numbers are similar to those in other open coast reefs, such as the North Llyn beds (Fariñas-Franco et al., 2014), and the variation in numbers per m2 is indicative of the patchy nature of the reefs. The Craigbrain site appears to show the highest numbers of living Modiolus per m2 , however due to the limited number of samples and small areal coverage of grab samples it cannot be concluded that this reef site is markedly different from the Burial Island reef site, and the infaunal community analysis does not show statistically significant differences between the Craigbrain and Burial Island reef sites.