Exposed to moderately exposed bedrock and boulders in the eastern basin of the Irish Sea (and as far south as Cornwall) characterised by reefs of the polychaete Sabellaria alveolata. The sand based tubes formed by S. alveolata form large reef-like hummocks, which serve to stabilise the boulders and cobbles. Other species in this biotope include the barnacles Semibalanus balanoides and Elminius modestus and the limpet Patella vulgata, the winkle Littorina littorea, the mussel Mytilus edulis and the whelk Nucella lapillus. The anemone Actinia equina and the crab Carcinus maenas can be present in cracks and crevices on the reef. Low abundance of seaweeds tend to occur in areas of eroded reef. The seaweed diversity can be high and may include the foliose red seaweeds Palmaria palmata, Mastocarpus stellatus, Osmundea pinnatifida, Chondrus crispus and some filamentous species e.g. Polysiphonia spp. and Ceramium spp. Coralline crusts can occur in patches. Wracks such as Fucus vesiculosus, Fucus serratus and the brown seaweed Cladostephus spongiosus may occur along with the ephemeral green seaweeds Enteromorpha intestinalis and Ulva lactuca. On exposed surf beaches in the south-west S. alveolata forms a crust on the rocks, rather than the classic honeycomb reef form, and may be accompanied by the barnacle Balanus perforatus (typically common to abundant). On wave-exposed shores in Ireland, the wrack Himanthalia elongata can also occur. Situation: Above this unit are biotopes dominated either by ephemeral seaweeds, such as Enteromorpha spp. and Porphyra spp. or the perennial wrack Fucus vesiculosus on mixed substrata (units A1.213; A1.3132; A2.821; A1.452). Rockpool biotopes dominated by the red seaweed Corallina officinalis (unit A1.411), by wracks such as Fucus spp. or by kelp such as Laminaria spp. (A1.412) can usually be found above this biotope. Beneath this biotope is a community consisting of mixed scour-tolerant like the kelp Laminaria digitata and opportunistic foliose red seaweeds such as Polyides rotundus and Ahnfeltia plicata (units A3.2111; A3.125; A1.45; A3.127). In adjacent sediment areas Lanice conchilega may dominante (A2.245). Temporal variation: These reefs may be susceptible to storm damage in the winter, although they can regenerate remarkably quickly in a season as long as some adults are left as they facilitate the larval settlement. S. alveolata is tolerant to burial under sand for several weeks. Changes in desiccation over a period of time can cause part of the population to die.