Exposed to moderately exposed mid eulittoral bedrock and boulders are frequently characterised by a mosaic of the barnacle Semibalanus balanoides and the wrack Fucus vesiculosus. The limpet Patella vulgata and the whelk Nucella lapillus are typically present, whilst the anemone Actinia equina and small individuals of the mussel Mytilus edulis are confined to crevices. Underneath the F. vesiculosus is a community of red seaweeds, including Corallina officinalis, Mastocarpus stellatus and Osmundea pinnatifida, usually with the winkles Littorina littorea and Littorina spp. present. Opportunistic seaweeds such as Enteromorpha intestinalis may occur in patches recently cleared on the rock or growing on the M. edulis. Situation: On exposed shores this unit is found below the black lichen Verrucaria maura and sparse barnacles biotope (unit B3.1131) and/or below the Chthamalus spp. and P. vulgata biotopes (A1.1121). It is found above the biotope dominated by the wrack Himanthalia elongata (A1.123) or the red seaweed biotopes (A1 122; A1.2141). This unit forms an intermediate along the wave exposure gradient between the exposed shore barnacle-P. vulgata biotopes (unit A1.1132) and the sheltered shore F. vesiculosus biotope (A1.313). Vertical surfaces tend to be dominated by the barnacle-P. vulgata biotope (A1.112). Temporal variation: On some shores, particularly those, which are moderately exposed to wave action, temporal fluctuations in the abundance of limpets, barnacles and fucoid seaweeds may occur. As a result, over a number of years, a single shore may cycle between the barnacle-P. vulgata dominated biotope (A1.1132), through this mosaic (A1.213) to a F. vesiculosus-dominated biotope (A1.313).