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This biotope represents part of a transition between sand-scoured circalittoral rock where the epifauna is conspicuous enough to be considered as a biotope and a sediment biotope where an infaunal sample is required to characterise it and is possibly best considered an epibiotic overlay. Flustra foliacea and the hydroid Hydrallmania falcata characterise this biotope; lesser amounts of other hydroids such as Sertularia argentea, Nemertesia antennina and occasionally Nemertesia ramosa, occur where suitably stable hard substrata is found. The anemone Urticina felina and the soft coral Alcyonium digitatum may also characterise this biotope. Barnacles Balanus crenatus and tube worms Spirobranchus triqueter may be present and the robust bryozoans Alcyonidium diaphanum and Vesicularia spinosa appear amongst the hydroids at a few sites. Sabella pavonina and Lanice conchilega may be occasionally found in the coarse sediment around the stones. In shallower (i.e. upper circalittoral) examples of this biotope scour-tolerant robust red algae such as Polysiphonia nigrescens, Calliblepharis spp. and Gracilaria gracilis are found. In offshore areas, such as in the Greater Gabbard North Sea Area, where there is circalottoral mixed sediment, with pebbles and gravels, the biotope may further support rich encrusting fauna, including bryozoans, Spirobranchus lamarcki, and the barnacle Verruca stroemia, and occasionally Sabellaria spinulosa. Alongside these encrusting fauna, infauna such as Lumbrinerids (Hilbigneris gracilis), Glycera lapidum, Echinocyamus pusillus, Amphipholis squamata, Caulleriella alata may be present, and may represent a transitionary form between SS.SMx.CMx.FluHyd and SS.SCS.CCS.MedLumVen.

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.

Medium-coarse sands with gravel, shells, pebbles and cobbles on moderately exposed coasts may support populations of the slipper limpet Crepidula fornicata with ascidians and anemones. C. fornicata is common in this biotope though not as abundant as in the muddier estuarine biotope SS.SMx.SMxVS.CreMed to which this is related. Anemones such as Urticina felina and Alcyonium digitatum and ascidians such as Styela clava are typically found in this biotope. Bryozoans such as Flustra foliacea are also found along with polychaetes such as Lanice conchilega. Little information is available with regard to the infauna of this biotope but given the nature of the sediment the infaunal communities are liable to resemble those in biotopes from the SS.SCS habitat complex. As with SS.SMx.CMx.FluHyd this biotope could be considered a superficial or epibiotic overlay but more data is required to support this.

Circalittoral gravels, coarse to medium sands, and shell gravels, sometimes with a small amount of silt and generally in relatively deep water (generally over 15-20 m), may be characterised by polychaetes such as Mediomastus fragilis, Lumbrineris spp., Glycera lapidum with the pea urchin Echinocyamus pusillus. Other taxa may include Nemertea spp., Protodorvillea kefersteini, Owenia fusiformis, Spiophanes bombyx and Amphipholis squamata along with amphipods such as Ampelisca spinipes. This biotope may also be characterised by the presence of conspicuous venerid bivalves, particularly Timoclea ovata. Other robust bivalve species such as Moerella spp., Glycymeris glycymeris and Astarte sulcata may also be found in this biotope. Spatangus purpureus may be present especially where the interstices of the gravel are filled by finer particles, in which case, Gari tellinella may also be prevalent (Glemarec 1973). Venerid bivalves are often under-sampled in benthic grab surveys and as such may not be conspicuous in many infaunal datasets. Such communities in gravelly sediments may be relatively species-rich and they may also contain epifauna such as Hydroides norvegicus and Spirobranchus lamarcki. In sand wave areas this biotope may also contain elements of the SS.SSa.IMuSa.FfabMag biotope, particularly Magelona species. This biotope has previously been described as the 'Deep Venus Community' and the 'Boreal Off-Shore Gravel Association' (Ford 1923; Jones 1950) and may also be part of the Venus community described by Thorson (1957) and in the infralittoral stage described by Glemarec (1973). SS.SCS.CCS.MedLumVen may be quite variable over time and in fact may be closer to a biotope complex in which a number of biotopes or sub-biotopes may yet be defined. For example, Ford (1923) describes a 'Series A' and a 'Series B' characterised by Echinocardium cordatum-Chamelea gallina and Spatangus purpurea-Clausinella fasciata. Furthermore, mosaics of cobble and lag gravel often contain ridges of coarse gravelly sand and these localised patches are also characterised by robust veneriid and similar bivalves including Arcopagia crassa, Laevicardium crassum and others including Glycymeris glycymeris (E.I.S. Rees pers. comm., 2002). In the presence of pebbles, cobbles or shell, in coarse sandy gravel sediment, the biotope may support encrusting fauna such as hydroids, Sertularia cupressina and Hydrallmania falcata, bryozoa including Disporella hispida, Schizomavella spp., and Escharella immersa and encrusting polychaetes, Spirobranchus triqueter and instances of Sabellaria spinulosa. In the presence of these encrusting forms, and with the transition of sediment types to more tidally swept circalittoral mixed sediment, the biotope may form a transition to SS.SMx.CMx.FluHyd. Other variants in gravel, sands and stones in circalittoral waters, from records in the east English Channel, show this biotope may support high densities of polychaetes and copepods, Nematoda and Nemertea. The biotope may be represented in moderately exposed, shallower areas, with muddy mixed gravel or sand with shell sediments and maerl (Hapalidiaceae), supporting the characteristic fauna of Mediomastus and Hilbigneris gracilis, but absence of venerid bivalves. Furthermore, in impoverished variants of the biotopes, there may be a reduced component of Mediomastus and Hilbigneris gracilis.