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
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
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.
Keystone (structuring) species
Importance of habitat for other species
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 Holt & Shalla (unpublished) found several species of fish on the Modiolus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.
Time for community to reach maturity
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).