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Physically very stable muds, occasionally with small stones, with a high proportion of fine material (typically greater than 80 %) may contain the opisthobranch Philine aperta and the seapen Virgularia mirabilis. These muds typically occur in shallow water down to about 12-15 m where significant seasonal variation in temperature is presumed to occur. This habitat is restricted to the most sheltered basins in, for example, sea lochs. Although most records suggest full salinity conditions are prevalent, some sites may be subject to variable salinity. Philine aperta is the most characteristic species of this habitat, occurring in high densities at many sites, whilst Virgularia mirabilis, a species found more widely in muddy sediments, appears to reach its highest densities in this shallow mud but may not be present in all examples of this biotope. Other conspicuous species found in this shallow muddy habitat include Cerianthus lloydii, Pagurus bernhardus, Sagartiogeton spp. and Hydractinia echinata. Burrowing crustacean megafauna, characteristic of deeper mud, are rare or absent from this shallow sediment although Nephrops norvegicus may sometimes be recorded. This biotope has been primarily recorded on the basis of its epifauna and a few conspicuous infauna. Little data exists on the infaunal component of this biotope but it may include Nephtys spp., spionid polychaetes, Ampelisca spp. and the bivalves Nucula spp., Thyasira flexuosa, Kurtiella bidentata and Abra spp. In the south of Great Britain, the polychaete Sternaspis scutata is also characteristic of this biotope. This polychaete is rare in Great Britain (Sanderson 1996). Indeed, this southern variant of the biotope is very restricted in the UK to Portland Harbour but is known to occur further south in the Gulf of Gascony and the Mediterranean (Glemarec 1973; Dauvin et al. 1994).

Dense aggregations of Ocnus planci on various substrata, typically muddy but occasionally with stones or shells, in sheltered conditions such as sea lochs. Philine aperta also characterises this biotope but is present in lower abundances than in SS.SMu.IFiMu.PhiVir. Other associated species vary but are typical of very sheltered muddy habitats and include the ophiuroids Ophiura spp. and Ophiothrix fragilis. Melanella alba, which parasitises holothurians, has been found in large numbers at one site.

Sheltered sublittoral mud or sandy mud in shallow water with relatively few conspicuous species may be characterised by the anemone Cylista undata in low numbers and the tunicate Ascidiella aspersa. Other taxa may include Carcinus maenas, Pagurus bernhardus and terebellid polychaetes. The burrowing anemones Cerianthus lloydii may also be found occasionally. The status of this biotope is uncertain at present as it is not known whether it is an impoverished, disturbed or epifaunal variant of other sheltered, shallow mud biotopes such as SS.SMu.IFiMu.PhiVir or if the areas in which it has been recorded have been incompletely surveyed.

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.

Plains of fine mud at depths greater than about 15 m may be heavily bioturbated by burrowing megafauna; burrows and mounds may form a prominent feature of the sediment surface with conspicuous populations of seapens, typically Virgularia mirabilis and Pennatula phosphorea. The burrowing crustacea present typically include Nephrops norvegicus, which is frequently recorded from surface observations although grab sampling may fail to sample this species. Indeed, some forms of sampling may also fail to indicate seapens as characterising species. This biotope also seems to occur in deep offshore waters in the North Sea and the Irish Sea. The burrowing anemone Cerianthus lloydii and the ubiquitous epibenthic scavengers Asterias rubens, Pagurus bernhardus and Liocarcinus depurator are present in low numbers in this biotope whilst the brittlestars Ophiura albida and Ophiura ophiura are sometimes present, but are much more common in slightly coarser sediments. Low numbers of the anemone Pachycerianthus multiplicatus may also be found, and this species, which is scarce in the UK, appears to be restricted to this habitat (Plaza & Sanderson 1997). The infauna may contain significant populations of the polychaetes Pholoe spp., Glycera spp., Nephtys spp., spionids, Pectinaria belgica and Terebellides stroemii, the bivalves Nucula sulcata, Varicorbula gibba and Thyasira flexuosa, and the echinoderm Brissopsis lyrifera. The biotope may include a range of the component fauna, potentially including burrowing megafauna and seapens, burrowing megafauna without seapens or seapens without burrowing megafauna depending on local environmental conditions. This biotope is closely related to the biotope SS.SMu.CFiMu.MegMax and SS.SMu.CFiMu.SpnMeg.Fun and may be have infaunal communities similar to the biotopes SS.SMu.CFiMu.BlyrAchi or SS.SMu.IFiMu.PhiVir, depending on environmental factors.