This habitat type occurs on moderately wave-exposed upper circalittoral bedrock subject to strong or moderately strong tidal streams. This complex is characterised by dense aggregations of the mussels Mytilus edulis or Musculus discors carpeting the underlying substrata. Sponges that may be recorded in this complex are Scypha ciliata, Tethya aurantium, Pachymatisma johnstonia, Dysidea fragilis and Cliona celata. A sparse hydroid/bryozoan turf composed primarily of Nemertesia antennina, Alcyonidium diaphanum and Flustra foliacea is often recorded. Anemones present are Urticina felina and Sagartia elegans. Other species recorded are the crabs Cancer pagurus, Carcinus maenas and Necora puber, the starfish Crossaster papposus and Asterias rubens, and Alcyonium digitatum and in this upper circalittoral complex, algae species such as Dictyota dichotoma, Cryptopleura ramosa and Plocamium cartilagineum.