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The polychaete Capitella capitata (agg.) a widely occurring opportunist species complex that is particularly associated with organically enriched and polluted sediments (Warren 1977; Pearson & Rosenberg 1978) where it may be superabundant. In very polluted/disturbed areas only Capitella, nematodes and occasional Malacoceros fuliginosus may be found whilst in slightly less enriched areas and estuaries species such as Tubificoides, Cirriformia tentaculata, Pygospio elegans and Polydora ciliata may also be found. In some areas, e.g. the Tees Estuary and Firth of Clyde, high numbers of the polychaete Ophryotrocha may also be present. SS.SMu.ISaMu.Cap may become established as a result of anthropogenic activities such as fish farming and sewerage effluent but may also occur with natural enrichment as a result of, for example, coastal bird roosts. This biotope may also occur to some extent in the intertidal and in estuaries and have a varied sediment composition; where shell fragments are present then some epifaunal species may occur.

In circalittoral and deep offshore mud and sandy mud adjacent to oil or gas platforms, organic enrichment from drill cuttings leads to the development of communities dominated by the Capitella capitata, an opportunist especially associated with organically enriched and polluted sediments as described for SS.SMu.ISaMu.Cap (Warren 1977; Pearson & Rosenberg 1978). The bivalves Thyasira flexuosa or T. sarsi may also be found in moderate numbers at some sites. Other taxa may be present in low numbers in areas of less severe enrichment including Pholoe inornata, Lagis koreni, Philine scabra, Anaitides groenlandica, Mediomastus fragilis and Paramphinome jeffreysii.

Reduced or variable salinity muddy sediment characterised by the Capitella capitata species complex with a relatively low species richness. Large numbers of the oligochaetes Tubificoides spp. may be found in conjunction with C. capitata, along with other species such as Marenzellaria sp., Macoma balthica, Arenicola marina and Eteone longa. In some estuaries this biotope may also include high numbers of the polychaete Ophryotrocha. This biotope usually has a moderate organic content and is found away from tidal channels in estuaries. The presence of dense Capitella has classically been associated with organically enriched and physically disturbed habitats in the marine environment (Warren 1977; Pearson & Rosenberg 1978) and areas of higher organic loads in estuaries will typically fall into the biotope SS.SMu.ISaMu.Cap. Where Capitella is less abundant and accompanied by other typical estuarine species the dominance of Capitella may be associated with other natural factors including the occurrence of a competitive refuge for C. capitata in the reduced-salinity environment (Wolff 1973).