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This biotope is characterised by a few ubiquitous robust and/or fast growing ephemeral species which are able to colonise pebbles and unstable cobbles and slates which are regularly moved by wave and tidal action. The main cover organisms tend to be restricted to calcareous tube worms such as Spirobranchus triqueter or S. lamarcki, small barnacles including Balanus crenatus and Balanus balanus, and a few bryozoan and coralline algal crusts. Barnacles may be predominantly observed in shallower variants of the biotope. Scour action from the mobile substratum prevents colonisation by more delicate species. Occasionally, in tide-swept conditions tufts of hydroids such as Sertularia argentea and Hydrallmania falcata are present. Occasional epifauna may include Asterias rubens, Cerianthus lloydii, and Alcyonium digitatum. Bryozoa Parazoanthus anguicomus, Ulva, Porania, and Porifera can also be present. This biotope often grades into SS.SMx.CMx.FluHyd which is characterised by large amounts of the above hydroids on stones also covered in Spirobranchus and barnacles. The main difference here is that SS.SMx.CMx.FluHyd seems to develop on more stable, consolidated cobbles and pebbles or larger stones set in sediment in moderate tides. These stones may be disturbed in the winter and therefore long-lived and fragile species are not found.