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Dense loose-lying beds of the 'Trailliella' phase of Bonnemaisonia hamifera may occur in extremely sheltered shallow muddy environments. Beds of this alga are often 10 cm thick but may reach 100 cm at some sites. Other loose-lying algae may also occur such as Audouinella floridula, Phyllophora crispa and species of Derbesia. Often the mud is gravelly or with some cobbles and may be black and anoxic close to the sediment surface. This biotope is widely distributed in lagoons, sea lochs and voes but should only be described as SS.SMp.KSwSS.Tra when a continuous mat is found. It is likely that the infaunal component of this biotope may be considerably modified by the overwhelming quantity of loose-lying algae.

Infralittoral muddy sand and sandy mud, sometimes with some shells or pebbles, and a dense, loose-lying cover of Phyllophora crispa. This biotope occurs in very sheltered conditions such as those found in sea lochs and voes. SS.SMp.KSwSS.Pcri is similar to other biotopes described with dense, loose-lying algae but has been less frequently recorded, and from the few records available, appears to occur in slightly deeper infralittoral waters primarily between 10 m to 30 m and typically in fully saline waters. The seaweeds in this biotope may be epiphytised by ascidians such as Ascidiella aspera. Kelp such as Saccharina latissima and red seaweeds including Plocamium cartilagineum may be present in some areas. The scallops Pecten maximus and Aequipecten opercularis may also be found occasionally in this biotope and Trailliella/Bonnemaisonia hamifera may also be present but not at the levels found in SS.SMp.KSwSS.Tra.

Slightly deeper kelp community in the lower infralittoral, found on sandy gravelly mud, in sheltered and very sheltered conditions, with very weak tidal currents. The community is characterised by occasional Saccharina latissima with an undergrowth of red and brown algae. Characteristic red seaweeds, as with other SS.SMp.KSwSS.SlatR biotopes include Plocamium cartilagineum and Phycodrys rubens. However, the sheltered conditions of this biotope allow the 'Trailliella' phase of Bonnemaisonia hamifera to develop (although not to the extent of forming distinct mats as in SS.SMp.KSwSS.Tra), and the related species Bonnemaisonia asparagoides. Brown algal species present, as with other SS.SMp.KSwSS.SlatR biotopes, include Desmarestia spp at low abundance. The ubiquitous green seaweed Ulva sp. may also be present. The muddy substrate is home to a variety of typical mud dwelling fauna including the burrowing anemone Cerianthus lloydii. The gravelly component of this biotope provides a substrate for encrusting species such as the polychaete Spirobranchus triqueter and coralline encrusting algae.