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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).

In cohesive and non-cohesive sandy mud, off moderately exposed coasts in deep water dense populations of Amphiura filiformis with the bivalve Ennucula tenuis may occur. This biotope together with SS.SMu.CSaMu.AfilKurAnit, SS.SMu.CSaMu.ThyEten and SS.SSa.OSa.OfusAfil may be part of the Amphiura filiformis dominated infralittoral etage described by Glemarec (1973) and part of the 'off-shore muddy sand association' described by other workers (Jones 1951; Mackie 1990). Other species characteristic of this biotope may include the echinoderms Ophiura albida and Echinocardium flavescens and the bivalve Kurtiella bidentata. Phaxas pellucidus, Owenia fusiformis and Virgularia mirabilismay also be present. At the sediment surface the hydroid Sertularia argentea may be present although only at very low abundances. Variations of this biotope exist in the northern North Sea (SS.SMu.CSaMu.AfilKurAnit) and it is possible that more than one entity exists for this biotope. 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).

Areas of slightly muddy sand (generally <20% mud) in offshore waters may be characterised by high numbers of the tube building oweniid polychaete Owenia fusiformis and Galathowenia sp., often with the brittlestar Amphiura filiformis. Whilst O. fusiformis is also found in other circalittoral or offshore biotopes it usually occurs in lower abundances than in SS.SSa.OSa.OfusAfil. Other species found in this community are the polychaetes Goniada maculata, Pholoe inornata, Diplocirrus glaucus, Chaetozone setosa and Spiophanes kroyeri with occasional bivalves such as Timoclea ovata and Thyasira equalis. The sea cucumber Labidoplax buski and the cumacean Eudorella truncatula are also commonly often found in this biotope. This biotope along with SS.SMu.CSaMu.ThyEten, SS.SMu.CSaMu.AfilKurAnit, SS.SMu.CSaMu.AfilEten and SS.SMu.OMu.PjefThyAfil, may comprise the Amphiura dominated components of the 'offshore muddy sand association' (Jones 1951; Mackie 1990) and the infralittoral etage described by Glemarec (1973). Variants of the biotope may contain the characteristic high numbers of Owenia fusiformis and Amphiura filiformis, but may also include Arctica islandica and Ennucula tenuis. Where these occur, the biotope may be considered a transitionary variant between SS.SSa.OSa.OfusAfil and SS.SMu.CSaMu.AfilKurAnit.

In circalittoral stable mud distinctive populations of megafauna may be found with a range of component fauna. This biotope may include the decapod crustaceans Nephrops norvegicus, Munida rugosa, Calocaris macandreae and Callianassa subterranea, the seapens Pennatula phosphorea and Virgularia mirabilis (although in reduced numbers than in SS.SMu.CFiMu.SpnMeg) and the echiuran Maxmuelleria lankesteri sometimes present in large mounds. Whilst this biotope is primarily identified from epifauna, the infaunal species present may include Nephtys hystricis, Chaetozone setosa, Amphiura chiajei and Abra alba. This biotope is closely related to the biotope SS.SMu.CFiMu.SpnMeg and may have infaunal communities similar to the biotopes SS.SMu.CSaMu.AfilKurAnit or SS.SMu.CSaMu.ThyEten, depending on environmental factors and/or the sampling gear used to describe the record.

Dense stands of Ampharete falcata tubes which protrude from muddy sediments, appearing as a turf or meadow in localised areas. These areas seem to occur on a crucial point on a depositional gradient between areas of tide-swept mobile sands and quiescent stratifying muds. Dense populations of the small bivalve Parvicardium pinnulatum occur in the superficial sediment. Other infauna in this diverse biotope include Lumbrineris scopa, Levinsenia sp., Prionospio steenstrupi, Diplocirrus glaucus and Praxillella affinis, although a wide variety of other infaunal species may also be found. Both the brittlestars Amphiura filiformis and Amphiura chiajei may be present together with Nephrops norvegicus in higher abundance than the SS.SMu.CFiMu.BlyrAchi or SS.SMu.CSaMu.AfilKurAnit biotopes. Substantial populations of mobile epifauna such as Pandalus montagui and smaller fish also occur, together with those that can cling to the tubes, such as Macropodia spp. A similar turf of worm tubes formed by the maldanid polychaete Melinna cristata has been recorded from Northumberland (Buchanan 1963). Nephrops trawling may severely damage this biotope and it is possible that such activity has destroyed examples of this biotope in the Irish Sea (E.I.S. Rees pers. comm. 2002).

Circalittoral fine sandy mud with shell gravel and notable quantities of shells or small stones scattered over the sediment surface. These sediments, like SS.SMu.CSaMu.VirOphPmax, may contain Virgularia mirabilis, Pecten maximus and Ophiura spp. but shells and small stones scattered over the sediment surface provided sufficient stable substrata for a variety of sessile epifaunal species to occur. Of these the hydroids Kirchenpaueria pinnata, Nemertesia antennina and Nemertesia ramosa are most common with solitary ascidians such as Corella parallelogramma and Ascidia mentula also present. The anemone Cerianthus lloydii is often found in the sediment together with occasional Lanice conchilega. The serpulids Protula tubularia, Serpula vermicularis and Spirobranchus triqueter and the barnacles Balanus balanus and Balanus crenatus are also often present on pebbles and shells. Munida rugosa are occasionally found under larger stones. All these species are typical of more rocky habitats in such sheltered conditions. As with SS.SMu.CSaMu.VirOphPmax this biotope is primarily identified on the basis of its epifauna and may be an epibiotic overlay over other closely related biotopes such as SS.SMu.CSaMu.AfilKurAnit and SS.SMu.CSaMu.AfilEten.

In infralittoral cohesive sandy mud, in sheltered marine inlets, and occasionally variable salinity environments or coarser sediments, dense populations of the polychaete Melinna palmata may occur, often with high numbers of Magelona spp. and the bivalve Thyasira flexuosa (although the latter may be present in variable abundances or absent in some cases). Other important taxa may include Chaetozone gibber, Nephtys hombergii, Galathowenia oculata, Euclymene oerstedii, Ampelisca tenuicornis, Ampharete lindstroemi, Abra alba, and Phoronis spp. In addition the polychaete Aphelochaeta spp. and the gastropod Turritellinella tricarinata may be common or abundant in some areas. At the sediment surface visible taxa may include occasional Virgularia mirabilis, and mobile epifauna such as Pagurus bernhardus. This biotope is characteristic in many southern UK marine inlets and in some areas, e.g. Plymouth Sound, during high levels of recruitment when M. palmata often occurs in abundances between 500 to 1000 per m2 moderate numbers of the species often 'overspill' into adjacent biotopes. This biotope may also exist in a continuum with SS.SMu.CSaMu.AfilKurAnit.

Circalittoral cohesive sandy muds with small quantities of gravel, off sheltered or moderately exposed coasts may support populations characterised by Thyasira spp., in particular Thyasira flexuosa. Other characteristic taxa may include Ennucula tenuis, Goniada maculate and in some areas Rhodine gracilior. Kurtiella bidentata, Abra alba, Harpinia antennaria and Amphiura filiformis may be abundant in some examples of this biotope. Whilst moderately diverse, animal abundances are often low and it is possible that the biotope is the result of sedimentary disturbance e.g. from trawling and is possibly an impoverished version of SS.SMu.CSaMu.AfilEten. 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).

Expanses of clean or muddy fine sand and sandy mud in shallow water and on the lower shore (typically to about 5 m depth) can have dense stands of Zostera marina/angustifolia [Note: the taxonomic status of Z. angustifolia is currently under consideration]. In SS.Smp.SSgr.Zmar the community composition may be dominated by these Zostera species and therefore characterised by the associated biota. Other biota present can be closely related to that of areas of sediment not containing Zostera marina, for example, Saccharina latissima, Chorda filum and infaunal species such as Ensis spp. and Echinocardium cordatum (e.g. Bamber 1993). From the available data it would appear that a number of sub-biotopes may be found within this biotope dependant on the nature of the substratum and it should be noted that sparse beds of Zostera marina may be more readily characterised by their infaunal community. For example, coarse marine sands with seagrass have associated communities similar to SS.SCS.ICS.MoeVen, SS.SCS.ICS.SLan or SS.SCS.ICS.Glap whilst muddy sands may have infaunal populations related to SS.SSa.IMuSa.EcorEns, SS.SMu.IMuSa.AreISa and SS.SSa.IMuSa.FfabMag. Muddy examples of this biotope may show similarities to SS.SMu.ISaMu.CundAasp, SS.SMu.IFiMu.PhiVir, SS.SMu.IFiMu.Are or SS.SMu.CSaMu.AfilKurAnit. At present the data does not permit a detailed description of these sub-biotopes but it is likely that with further study the relationships between these assemblages will be clarified. Furthermore, whilst the Zostera biotope may be considered an epibiotic overlay of established sedimentary communities it is likely that the presence of Zostera will modify the underlying community to some extent. For example, beds of this biotope in the south-west of Britain may contain conspicuous and distinctive assemblages of Lusitanian fauna such as Laomedea angulata, Hippocampus spp. and Stauromedusae. In addition, it is known that seagrass beds play an important role in the trophic status of marine and estuarine waters, acting as an important conduit or sink for nutrients and consequently some examples of Zostera marina beds have markedly anoxic sediments associated with them.

Circalittoral fine sandy mud may contain Virgularia mirabilis and Ophiura spp. A variety of species may occur, and species composition at a particular site may relate, to some extent, to the proportions of the major sediment size fractions. Several species are common to most sites including Virgularia mirabilis which is present in moderate numbers, Ophiura albida and Ophiura ophiura which are often quite common, and Pecten maximus which is usually only present in low numbers. Virgularia mirabilis is usually accompanied by occasional Cerianthus lloydii, Liocarcinus depurator and Pagurus bernhardus. Amphiura chiajei and Amphiura filiformis may occur in some examples of this biotope. Polychaetes and bivalves are generally the main components of the infauna, although the nemerteans, Edwardsia claparedii, Phoronis muelleri and Labidoplax buski may also be widespread. Of the polychaetes Goniada maculata, Nephtys incisa, Prionospio cirrifera, Chaetozone setosa, Notomastus latericeus and Owenia fusiformis are often the most widespread species whilst Myrtea spinifera, Lucinoma borealis, Kurtiella bidentata, Abra alba and Varicorbula gibba are typical bivalves in this biotope. This biotope is primarily identified on the basis of its epifauna and may be an epibiotic overlay over other closely related biotopes such as SS.SMu.CFiMu.SpnMeg, SS.SMu.CSaMu.AfilKurAnit and SS.SMu.CSaMu.AfilEten.