Infauna diversity is reported from the Sabellaria alveolata reefs in the bay of Mont Saint-Michel in France, known as the greatest European reef formation. Polychaetes dominate the fauna, but other species also play a role in terms of reef functioning. Species richness of the associated infauna is much higher than that of the surrounding sediments and is concentrated mainly on the reef surface. Species richness clearly varied according to the three stages of reef evolution: the ‘degraded’ reef stage is different from the ‘ball-shaped structure’ and ‘platform’ stages. Multivariate analyses indicate that three species communities can be distinguished, each corresponding to a different stage of reef development. The number of species is very high, but only a few species are restricted to a particular reef stage. Variations of surface topography and spatial heterogeneity can explain unusual associations of species that make infauna associated with S. alveolata reefs very unique.

Moreover, the high densities of S. alveolata (up to 60,000 ind m−2) raise questions regarding the role of the reefs in the ecosystem of the bay. Anthropogenic influences, notably mechanical disturbances due to fishing activity, show a steady increase and may have a serious impact on the assemblage diversity and the ecological stability of the reef, even though recolonization of S.alveolata is possible in degraded reef areas. An important question that must be addressed is whether human activities could influence the role of S. alveolata reefs in the bay of Mont Saint-Michel.