Philine aperta and Virgularia mirabilis in soft stable infralittoral mud
|Within Vocab||Marine Habitat Classification for Britain and Ireland Version 97.06|
|Definition||Physically very stable muds with a high proportion of fine material (greater than 80 %) may contain the seapen Virgularia mirabilis. These muds typically occur in shallow water 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, sealochs. Although most records suggest full salinity conditions are prevalent, some sites may be subject to variable salinity. The opisthobranch Philine aperta is the most characteristic species of this habitat, occurring in high densities at many sites. The seapen Virgularia mirabilis, a species found more widely in muddy sediments, appears to reach its highest densities in this shallow mud. Other conspicuous species found in this shallow muddy habitat include Cerianthus lloydii, Sagartiogeton spp., Ascidiella aspersa and Myxicola infundibulum. Amphiura chiajei and Amphiura filiformis may also be present at some sites. Burrowing crustacean megafauna, characteristic of deeper mud, are rare or absent from this shallow sediment. Of these burrowers Nephrops norvegicus may sometimes be recorded. The bivalves Nucula sp., Thyasira flexuosa and Corbula gibba may be other conspicuous infaunal species. The sediment may be covered by a diatom film. 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 (Glémarec 1973; Dauvin et al. 1994). Similar but deeper more stable muds to IMU.PhiVir are characterised by burrowing megafauna (CMU.SpMeg). IMU.PhiVir has a lot of similarity with CMU.BriAchi, possibly differing on account of low disturbance or linkage with enriched overlying waters, however, these hypotheses are untested. IMU.PhiVir may also be closely allied to CMS.AbrNucCor, showing some of the infaunal elements of this biotope.|
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