Dense aggregations of Mytilus edulis on the mid and lower shore, on mixed substrata (mainly cobbles and pebbles on fine sediments), on sand, or on sheltered muddy shores. In high densities the mussels bind the substratum and provide a habitat for many infaunal and epifaunal species. The wrack Fucus vesiculosus is often found attached to either the mussels or cobbles and it can be abundant. The mussels are often encrusted with the barnacles Semibalanus balanoides, Elminius modestus or Balanus crenatus. Where boulders are present they can support the limpet Patella vulgata. The winkles Littorina littorea and L. saxatilis and small individuals of the crab Carcinus maenas are common amongst the mussels, whilst areas of sediment may contain the lugworm Arenicola marina, the sand mason Lanice conchilega, the cockle Cerastoderma edule, and other infaunal species. The characterising species list shown below is based on data from epifaunal sampling only. Three sub-biotopes are recognised for this biotope, distinguished principally on the basis of the sediment type associated with the mussel beds. The three types of intertidal mussel beds may be part of a continuum on an axis that is most strongly influenced by the amount of pseudofaeces that accumulate amongst the mussels. The differences may not always be directly connected to the underlying substratum on which the mussel bed may have started a long time ago. It should be noted that there are few data available for the muddy (A2.7213) and sandy (A2.7212) subunits, therefore there are no characterising species lists or comparative tables for these two sub-biotopes. Situation: On more exposed, predominantly rocky shores this biotope can be found below a band of ephemeral green seaweeds (unit A2.821). On sheltered, predominantly rocky shores either a F. vesiculosus dominated biotope or a biotope dominated by the wrack Ascophyllum nodosum (A1.3132; A1.3142) can be found above or the barnacle dominated biotope (A1.1133). On mudflats and sandflats, this biotope may be found alongside Cerastoderma edule beds (A2.242) and other A2.2 and A2.3 biotopes. The intertidal A2.721 biotope can extend seamlessly into the subtidal. Temporal variation: The temporal stability of mussel beds can vary a lot. Some beds are permanent, maintained by recruitment of spat in amongst adults. Other beds are ephemeral, an example of which are beds ocurring at South America Skear where large amounts of spat settle intermittently on a cobble basement. The mussels rapidly build up mud, and are unable to remain attached to the stable cobbles. They are then liable to be washed away during gales. A second example of ephemeral mussel dominated biotopes occurs when mussel spat ("mussel crumble") settles on the superficial shell of cockle beds, such as is known to occur in the Burry Inlet.