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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).

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).

Deep, offshore habitats with cohesive sandy mud (>20% mud) may support communities characterised by infaunal polychaetes and the bivalve Myrtea spinifera. Polychaetes typically include Chaetozone setosa, Paramphinome jeffreysii, Levinsenia gracilis, Aricidea (Acmira) catherinae, and Prionospio malmgreni. The bivalves Thyasira spp. and Abra nitida may also be found as may seapens, such as Pennatula phosphorea. Some examples of the biotope SS.SMu.CSaMu.AfilEten contain Myrtea spinifera (Mackie 1990) in lower numbers but these habitats are generally sandier than those in SS.SMu.OMu.MyrPo.

Deep, offshore cohesive sandy mud communities characterised by the polychaete Paramphinome jeffreysii, bivalves such as Parathyasira equalis and Thyasira gouldi (sometimes in elevated densities) and the brittlestar Amphiura filiformis. Other taxa may include Laonice cirrata, the holothurian Labidoplax buskii and the polychaetes Goniada maculata, Spiophanes kroyeri and Aricidea (Acmira) catherinae. Amphiura chiajei may be occasional in this biotope as may Hermania scabra, Levinsenia gracilis, and Pholoe inornata. In areas of the North Sea, such as the Swallow Sand MCZ, this biotope has been observed in sediments with a coarse material component. This biotope along with SS.SMu.CSaMu.ThyEten, SS.SMuCSaMu.AfilKurAnit, SS.SMu.CSaMu.AfilEten and SS.SSa.OSa.OfusAfil, may comprise the Amphiura dominated components of the 'off-shore muddy sand association' (Jones 1951; Mackie 1990) and the infralittoral etage described by Glemarec (1973) and may exist in a transitional environment with SS.SSa.OSa.OfusAfil.

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.

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.

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).

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.