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Muddy gravelly sand with pebbles off shallow, sheltered or moderately exposed coasts or embayments may support dense populations of the peacock worm Sabella pavonina. This community may also support populations of sponges such as Amphilectus fucorum, Haliclona oculata and Halichondria panicea and anemones such as Cylista elegans, Cerianthus lloydii and Urticina felina. Hydroids such as Hydrallmania falcata and the encrusting polychaete Spirobranchus triqueter are also important. This biotope may have an extremely diverse epifaunal community. Less is known about its infaunal component, although it is likely to include polychaetes such as Nephtys spp., Harmothoe spp., Glycera spp., syllid and cirratulid polychaetes, bivalves such as Abra spp., Aoridae amphipods and brittlestars such as Amphipholis squamata.

Shallow mixed (heterogeneous) sediments in fully marine or near fully marine conditions, supporting various animal-dominated communities, with relatively low proportions of seaweeds. This habitat may include well mixed muddy gravelly sands or very poorly sorted mosaics of shell, cobbles and pebbles embedded in mud, sand or gravel. Due to the quite variable nature of the sediment type, a widely variable array of communities may be found, including those characterised by bivalves (SS.SMx.IMx.VcorAsquAps, SS.SMx.IMx.CreAsAn, and SS.SMx.IMx.Ost), polychaetes (SS.SMx.IMx.SpavSpAn) and file shells (SS.SMx.IMx.Lim). This has resulted in many species being described as characteristic of this biotope complex all contributing only a small percentage to the overall similarity (see below). This biotope complex may also include a newly proposed Chaetopterus biotope (Rees pers com.) recently found in the eastern English Channel. This biotope is characterised by an undescribed Chaetopterus sp. and small Lanice conchilega. Further sampling is need in order to assess and fully characterise this potential biotope. As a result, the Chaetopterus biotope has not been included in this revision. Infaunal data for this biotope complex are limited to that described in the biotope SS.SMx.IMx.VcorAsquAps and so are not representative of the infaunal component of the whole biotope complex.