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In moderately exposed or sheltered, circalittoral muddy sands and gravels a community characterised by the bivalves Thyasira spp. (often Thyasira flexuosa), Kurtiella bidentata and Prionospio fallax may develop. Infaunal polychaetes such as Hilbigneris gracilis, Chaetozone setosa and Scoloplos armiger are also common in this community whilst amphipods such as Ampelisca spp. and the cumacean Eudorella truncatula may also be found in some areas. The brittlestar Amphiura filiformis may also be abundant at some sites. Conspicuous epifauna on larger pebbles or shell gravel may include hydroids, encrusting bryozoans Escharella spp. particularly Escharella immersa, Disporella hispida, and, in shallower waters, maerl (Phymatolithon calcareum), although at very low abundances and not forming maerl beds. In some sheltered areas, organic enrichment of this biotope increases the occurrence of species such as Ophryotrochasp., Scoloplos sp., Mediomastus fragilis, Lumbrineris sp., Capitellids and Tubificoides pseudogaster.

Cohesive sandy mud, sometimes with a small quantity of shell in shallow water may contain the bivalves Kurtiella bidentata and Abra spp. (typically A. alba and A. nitida). Other characteristic taxa may include Scoloplos armiger, Mya sp., and Thyasira flexuosa. Tube building amphipods are also characteristic of this biotope in particular Ampelisca spp. and Aoridae such as Microprotopus maculatus. This biotope may also be compared with similar biotopes such as SS.SSa.CMuSa.AalbNuc, SS.SMx.CMx.KurThyMx or SS.SMu.ISaMu.MelMagThy.

Mixed (heterogeneous) sediment habitats in the circalittoral zone (generally below 15-20 m) including well mixed muddy gravelly sands or very poorly sorted mosaics of shell, cobbles and pebbles embedded in or lying upon mud, sand or gravel. Due to the variable nature of the seabed a variety of communities can develop which are often very diverse. A wide range of infaunal polychaetes, bivalves, echinoderms and burrowing anemones such as Cerianthus lloydii are often present in such habitats and the presence of hard substrata (shells and stones) on the surface enables epifaunal species to become established, particularly hydroids such as Nemertesia spp. and Hydrallmania falcata. The combination of epifauna and infauna can lead to species rich communities. Coarser mixed sediment communities may show a strong resemblance, in terms of infauna, to biotopes within the SS.SCS complex. However, infaunal data for this biotope complex is limited to that described under the biotope SS.SMx.CMx.KurThyMx, and so are not representative of the infaunal component of this biotope complex.

Sublittoral marine sand in moderately exposed or sheltered inlets and voes in shallow water may support large populations of semi-permanent tube-building amphipods and polychaetes. Typically dominated by Crassicorophium crassicorne with other tube building amphipods such as Ampelisca spp. also being common. Other taxa include typical shallow sand fauna such as Spiophanes bombyx, Urothoe elegans, Bathyporeia spp. along with various polychaetes including Parexogone hebes, and Lanice conchilega. Polydora ciliata may also be abundant in some areas. At the sediment surface, Arenicola marina worm casts may be visible and occasional seaweeds such as Saccharina latissima may be present. As many of the sites featuring this biotope are situated near fish farms, it is possible that it may have developed as the result of moderate nutrient enrichment, which may lead the biotope to transition into an organically enriched variant, related to SS.SMx.CMx.KurThyMx. The distribution of this biotope is poorly known and like the muddier SS.SMu.ISaMu.AmpPlon, to which it is related, appears to have a patchy distribution.

Cohesive sandy mud off wave exposed coasts with weak tidal streams can be characterised by super-abundant Amphiura filiformis with Kurtiella bidentata and Abra nitida. This community occurs in muddy sands in moderately deep water (Hiscock 1984; Picton et al. 1994) and may be related to the 'offshore muddy sand association' described by other workers (Jones 1951; Thorson 1957; Mackie 1990). This community is also characterised by the sipunculid Thysanocardia procera and the polychaetes Nephtys incisa, Phoronis sp. and Pholoe sp., with cirratulids, such as Notomastus latericeus or Mediomastus fragilis, and terebellids, such as Polycirrus plumosus or Diplocirrus glaucus, also common in some areas. Other taxa such as Nephtys hombergii, Echinocardium cordatum, Nucula nitidosa, Callianassa subterranea and Eudorella truncatula may also occur in offshore examples of this biotope. Additionally, several variants of this biotope can be described in transitionary environments between biotopes such as SS.SMx.CMx.KurThyMx where coarser material is present, SS.SSa.OSa.OfusAfil in sandier environments offshore or SS.SMu.ISaMu.MelMagThy in shallower waters. Collectively the biotopes SS.SMu.CSaMu.ThyEten, SS.SMu.CSaMu.AfilKurAnit, SS.SMu.CSaMu.AfilEten, SS.SMu.OMu.PjefThyAfil, and SS.SSa.OSa.OfusAfil, may form the Amphiura dominated components of the 'off-shore muddy sand association' described by other workers (Jones 1951; Thorson 1957; Mackie 1990) and the infralittoral etage described by Glemarec (1973).