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In infralittoral cohesive sandy mud, in sheltered marine inlets, and occasionally variable salinity environments or coarser sediments, dense populations of the polychaete Melinna palmata may occur, often with high numbers of Magelona spp. and the bivalve Thyasira flexuosa (although the latter may be present in variable abundances or absent in some cases). Other important taxa may include Chaetozone gibber, Nephtys hombergii, Galathowenia oculata, Euclymene oerstedii, Ampelisca tenuicornis, Ampharete lindstroemi, Abra alba, and Phoronis spp. In addition the polychaete Aphelochaeta spp. and the gastropod Turritellinella tricarinata may be common or abundant in some areas. At the sediment surface visible taxa may include occasional Virgularia mirabilis, and mobile epifauna such as Pagurus bernhardus. This biotope is characteristic in many southern UK marine inlets and in some areas, e.g. Plymouth Sound, during high levels of recruitment when M. palmata often occurs in abundances between 500 to 1000 per m2 moderate numbers of the species often 'overspill' into adjacent biotopes. This biotope may also exist in a continuum with SS.SMu.CSaMu.AfilKurAnit.

Cohesive sandy mud, sometimes with a small quantity of shell in shallow water may contain the bivalves Kurtiella bidentata and Abra spp. (typically A. alba and A. nitida). Other characteristic taxa may include Scoloplos armiger, Mya sp., and Thyasira flexuosa. Tube building amphipods are also characteristic of this biotope in particular Ampelisca spp. and Aoridae such as Microprotopus maculatus. This biotope may also be compared with similar biotopes such as SS.SSa.CMuSa.AalbNuc, SS.SMx.CMx.KurThyMx or SS.SMu.ISaMu.MelMagThy.

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).