Mussel beds (Mytilus edulis) on Atlantic sublittoral sediment

Sublittoral mussel beds of the common mussel Mytilus edulis may be sublittoral extensions of littoral reefs or exist independently. They beds are found in a variety of situations ranging from sheltered estuaries and marine inlets to open coasts and offshore areas, in fully marine or sometimes variable salinity conditions in the outer regions of estuaries. They may occupy a range of substrata, although due to the accumulating and stabilising effect such communities have on the substratum muddy mixed sediments are typical. There are three distinct  habitat components: the interstices withn the mussel matrix; the biodeposits beneath the bed; and the substratum afforded by the mussel shells themselves. 

All three components often contain a diverse range of epibiota and infauna. The mussel matrix may support sea cucumbers, anemones, boring clionid sponges, ascidians, crabs, nemerteans, errant polychaetes and flatworms. The biodeposits attract infauna such as sediment dwelling sipunculids, oligochaetes, and polychaetes while epizoans may use the mussels shells themselves as substrata.

Indicators of Quality:

Both biotic and abiotic indicators have been used to describe marine habitat quality. These include: the presence of characteristic species as well as those which are sensitive to the pressures the habitat may face; water quality parameters; levels of exposure to particular pressure, and more integrated indices which describe habitat structure and function, such as trophic index, or successional stages of development in habitats that have a natural cycle of change over time.

There are no commonly agreed indicators of quality for this habitat, although particular parameters may have been set in certain situations e.g. protected features within Natura 2000 sites, where reference values have been determined and applied on a location-specific basis.

The overall quality and continued occurrence of this habitat is, however, largely dependent on the presence of Mytilus edulis which creates the biogenic structural complexity on which the characteristic associated communities depend. The density and the maintenance of a viable population of this species is a key indicator of habitat quality, together with the visual evidence of presence or absence of physical damage. Monitoring programmes may include measures of biomass, coverage, length frequency distribution, a condition index for the mussels (a ratio between biomass versus shell length) and descriptions of the structure of a bed including vertical height profile, thickness and type of accumulated sediment, coverage and biomass of macroalgae.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats