Modiolus has a strong structuring influence on the sediments in which reef areas usually occur. The communities associated with Modiolus are known generally to be extremely rich and diverse. There are clearly variations in composition of associated species. Sponges, ascidians, Alcyonium digitatum, Chlamys varia, Aequipecten opercularis, hydroids and Ophiothrix fragilis are all very abundant in some, but not all, Modiolus communities. Urchins, starfish and whelks are numerous on most.

Apart from the infauna, the Modiolus community in Strangford Lough (Northern Ireland) has been described as consisting of mainly three components (Magorrian et al. 1995): Very dense aggregations of living and dead Modiolus shells which form the framework in single or multiple layers; a rich community of free living and sessile epifauna and predators; a rich and diverse community which seeks shelter in the crevices between the Modiolus shells and byssus threads and flourishes on its rich sediment. In the Gulf of Maine it has been found that the diversity of other benthic species increased as Modiolus clump size and number increased (Ojeda & Dearborn 1989). From limited data plus subjective observations it seems likely that this would be the case in British waters and moreover that the reef areas would have a more diverse fauna than non-reef areas.

The possible role of Modiolus reef communities in providing a nursery refuge for other species has not been investigated. Dense growths of bushy hydroids and bryozoans could conceivably provide an important settling area for spats of bivalves such as Pecten maximus and Aequipecten opercularis, adults of which are often abundant in nearby areas. Established Modiolus beds are also very important for the recruitment of juveniles as it is suspected that their survival is greatly enhanced by settling within the mass of adults byssus threads where predators cannot easily attack them.

Predators are significant mainly in young Modiolus. In the early years predation is probably largely by crabs and starfish, which are very numerous. In shallower areas red seaweeds such as Phycodrys rubens and corallines may be present on Modiolus beds. Holt & Shalla (unpublished) found several species of fish on the Modiolus reef areas to the north-east of the Isle of Man.

There are very few temporal changes as Modiolus beds are slow-growing ,long-lived and static communities.

Rates of development of reefs are not known. There would appear to be some potential for spread of existing bioherms where these take the form of very dense raised beds, as off the Lleyn Peninsula, Wales as a result of clumps of mussels dropping off from the edges, which are often quite discrete. This would undoubtedly be a very slow process taking probably many years per meter of spread. Spread or recovery of more infaunal types of reefs would presumably be slower still, although this is purely speculative. Individual mussels are long lived with ages up to 35 years occasionally being reported. Ages in excess of 25 years are very frequent with maximum ages likely to be well in excess of 50 years (Anwar et al. 1990).

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