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Non-cohesive muddy sands or slightly shelly/gravelly muddy sand characterised by the bivalves Abra alba and Nucula nitidosa. Other important taxa include Nephtys spp., Chaetozone setosa and Spiophanes bombyx with Fabulina fabula also common in many areas. The echinoderms Ophiura albida and Asterias rubens may also be present. The epibiotic biotope SS.SSa.IMuSa.EcorEns may overlap this biotope. This biotope is part of the Abra community defined by Thorson (1957) and the infralittoral etage described by Glemarec (1973). In organically enriched variants of this biotope, there may be higher occurrences of amphipods, such as Bathyporeia tenuipes, Perioculodes longimanus, and Urothoe elegans.

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.

In shallow non-cohesive muddy sands, in fully marine conditions, a community characterised by the bivalve Spisula subtruncata and the polychaete Nephtys hombergii may occur. The sediments in which this community is found may vary with regard to silt content but they generally have less than 20% silt/clay and in some areas may contain a degree of shell debris. This biotope falls somewhere between SS.SSa.IMuSa.FfabMag and SS.SSa.CMuSa.AalbNuc with regard to sediment type (i.e. somewhat muddier than SS.SSa.IMuSa.FfabMag and less muddy than SS.SSa.CMuSa.AalbNuc) and may have species in common with both. As a result, other important species in this community include Abra alba, Fabulina fabula and Kurtiella bidentata. In addition, Diastylis rathkei/typical, Philine aperta (in muddier sediments), Ampelisca spp., Ophiura albida, Phaxas pellucidus and occasionally Bathyporeia spp, may also be important, although this is not clear from the data available. In areas of slightly coarser, less muddy sediment S. solida or S. elliptica may appear occasionally in this biotope. Abundances of Spisula subtruncata in this biotope are often very high and distinguish it from other closely related biotopes. Extensive areas of this community to the north east of the Dogger Bank were recorded in the 1950s, but these seem to have declined since then (Kroncke 1990). More information is required with regard to the status of this biotope.

Dense beds of Lanice conchilega occur in coarse to medium fine gravelly sand in the shallow sublittoral, where there are strong tidal streams or wave action. Several other species of polychaete also occur as infauna e.g. Spiophanes bombyx, Scoloplos armiger, Chaetozone setosa and Magelona mirabilis. Lanice beds are found in a wide range of habitats including muddier mixed sediment. The dense Lanice biotope (LS.LSa.MuSa.Lan) on certain lower shores may be a littoral extension of the current biotope. The presence of L. conchilega in high numbersmay, over time, stabilise the sediment to the extent where a more diverse community may develop. Possibly, as a result of this, there is a high level of variation with regard the infauna found in SS.SCS.ICS.SLan. It is likely that a number of sub-biotopes may subsequently be identified for this biotope. Offshore from the Wash and the North Norfolk coast Lanice beds are often found intermixed with Sabellaria spinulosa beds in muddier mixed sediment, particularly in the channels between the shallow sandbanks, which are prevalent in this area. It is possible that the presence of Lanice has stabilised the habitat sufficiently to allow the deposition of finer material, which has subsequently assisted the development of S. spinulosa. It may be more accurate to define SS.SCS.ICS.SLan as an epibiotic biotope which overlays a variety of infaunal biotopes (e.g. SS.SSa.IFiSa.NcirBat in finer sands and SS.SSa.CMuSa.AalbNuc or SS.SSa.IMuSa.FfabMag in slightly muddier areas).

Sheltered lower shore and shallow sublittoral sediments of sand or muddy fine sand in fully marine conditions, support populations of the urchin Echinocardium cordatum and the razor shell Ensis siliqua or Ensis ensis. Other notable taxa within this biotope include occasional Lanice conchilega, Pagurus, Liocarcinus spp. and Asterias rubens. This biotope has primarily been recorded by epifaunal dive, video or trawl surveys where the presence of relatively conspicuous taxa such as E. cordatum and Ensis spp. have been recorded as characteristic of the community. However, these species, particularly E. cordatum, have a wide distribution and are not necessarily the best choice for a characteristic taxa (Thorson, 1957). Furthermore, detailed quantitative infaunal data for this biotope is rather scarce, possibly as a result of survey method as remote grab sampling is likely to under-estimate deep-burrowing species such as Ensis sp. (Warwick & Davis 1977). Consequently, it may be better to treat this biotope as an epibiotic overlay which is likely to overlap with a number of other biotopes such as SS.SSa.IMuSa.FfabMag, SS.SSa.IFiSa.NcirBat and SS.SSa.CMuSa.AalbNuc with infaunal components of these biotopes occurring within SS.SSa.IMuSa.EcorEns. The precise nature of this infaunal community will be related to the nature of the substratum, in particular the quantity of silt/clay present. Infaunal species may include the polychaetes Spiophanes bombyx, Magelona mirabilis, Nephtys cirrosa and Chaetozone setosa and the amphipod Bathyporeia spp. This biotope is currently broadly defined and needs further consideration as to whether it should be placed at biotope or biotope complex level. SS.SSa.IMuSa.AreISa is another biotope based primarily on epibiotic data. It is likely that this biotope and SS.SSa.IMuSa.EcorEns form a wider epibiotic sand /muddy sand community with SS.SSa.IMuSa.EcorEns biased towards sandier areas and SS.SSa.IMuSa.AreISa towards slightly muddier areas.