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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.

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.

In shallow fine sand or non-cohesive muddy sand in fully marine conditions (or occasionally in variable salinity) a community characterised by the polychaete Arenicola marina may occur. This biotope appears quite faunally sparse. Taxa present, however, include scavenging crustaceans such as Pagurus bernhardus and Liocarcinus depurator, terebellid polychaetes such as Lanice conchilega and the burrowing anemone Cerianthus lloydii. Occasional Sabella pavonina and frequent Ensis spp. may also be observed in some areas. The majority of records for this biotope are derived from epifaunal surveys and consequently there is little information available for the associated infaunal species. It is possible that this biotope, like SS.SSa.IMuSa.EcorEns (to which it is broadly similar) is an epibiotic overlay on other biotopes from the SSa complex.

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.

In stable, fine, compacted sands and slightly muddy sands in the infralittoral and littoral fringe, communities dominated by venerid bivalves such as Chamelea gallina occur. This biotope may be characterised by a prevalence of Fabulina fabula and Magelona mirabilis or other species of Magelona (e.g. M. filiformis). Other taxa, including the amphipod Bathyporeia spp. and polychaetes such as Chaetozone setosa, Spiophanes bombyx and Nephtys spp. are also commonly recorded. In some areas the bivalve Spisula elliptica may also occur in this biotope in low numbers. The community is relatively stable in its species composition, however, numbers of Magelona and F. fabulina tend to fluctuate. Around the Scilly Isles numbers of F. fabulina in this biotope are uncommonly low whilst these taxa are often found in higher abundances in muddier communities (presumably due to the higher organic content). In deeper, offshore variants of this biotope, although still present, there is a reduction in the component species F. fabula, whilst Magelona filiformis, Bathyporeia spp., annelid and nemertean worms, and Amphiuridae may be more common. Consequently, it may be better to revise this biotope on the basis of less ubiquitous taxa such as key amphipod species (E.I.S. Rees pers. comm. 2002) although more data is required to test this. SS.SSa.IMuSa.FfabMag and SS.SCS.ICS.MoeVen are collectively considered to be the 'shallow Venus community' or 'boreal off-shore sand association' of previous workers (see Petersen 1918; Jones 1950; Thorson 1957). These communities have been shown to correlate well with particular levels of current induced 'bed-stress' (Warwick & Uncles 1980). The 'Arctic Venus Community' and 'Mediterranean Venus Community' described to the north and south of the UK (Thorson 1957) probably occur in the same habitat and appears to be the same biotope described as the Ophelia borealis community in northern France and the central North Sea (K?nitzer et al. 1992). Sites with this biotope may undergo transitions in community composition. The epibiotic biotopes SS.SSa.IMUSa.EcorEns and SS.SSa.IMuSa.AreISa may also overlay this biotope in some areas.