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Live maerl beds in sheltered, silty conditions which are dominated by Lithothamnion corallioides with a variety of foliose and filamentous seaweeds. Live maerl is at least common but there may be noticeable amounts of dead maerl gravel and pebbles. Other species of maerl, such as Phymatolithon calcareum and Phymatolithon purpureum, may also occur as a less abundant component. Species of seaweed such as Dictyota dichotoma, Halarachnion ligulatum. and Ulva spp. are often present, although are not restricted to this biotope, whereas Dudresnaya verticillata tends not to occur on other types of maerl beds. The anemones Anemonia viridis and Cerianthus lloydii, the polychaetes Notomastus latericeus and Caulleriella alata, the isopod Janira maculosa and the bivalve Hiatella arctica are typically found in SS.SMp.Mrl.Lcor where as Echinus esculentus tends to occur more in other types of maerl. The seaweeds Saccharina latissima and Chorda filum may also be present in some habitats. SS.SMp.Mrl.Lcor has a south-western distribution in Britain and Ireland. Sheltered, stable, fully saline maerl beds in the north of Great Britain (where L. corallioides has not been confirmed to occur) may need to be described as an analogous biotope to SS.SMp.Mrl.Lcor (see SS.SMp.Mrl.Pcal).

Maerl beds characterised by Phymatolithon calcareum in gravels and sands. Associated epiphytes may include algae such as Dictyota dichotoma, Halarachnion ligulatum, Metacallophyllis laciniata, Cryptopleura ramosa, Vertebrata byssoides and Plocamium cartilagineum. Algal species may be anchored to the maerl or to dead bivalve shells amongst the maerl. Polychaetes, such as Chaetopterus variopedatus, Lanice conchilega, Psamathe fusca, Mediomastus fragilis, Chone duneri, Metaphoxus fultoni, and Grania may be present. Gastropods such as Steromphala cineraria, Gibbula magus, Calyptraea chinensis, Dikoleps nitens and Onoba aculeus may also be present. Liocarcinus depurator and Liocarcinus corrugatus are often present, although they may be under-recorded; it would seem likely that robust infaunal bivalves such as Circomphalus casina, Mya truncata, Dosinia exoleta and other venerid bivalves are more widespread than available data currently suggests. It seems likely that stable wave-sheltered maerl beds with low currents may be separable from SS.SMp.Mrl.Pcal; having a generally thinner layer of maerl overlying a sandy/muddy substratum with a diverse cover of epiphytes (e.g. Bosence 1976; Blunden et al. 1977; 1981; Davies & Hall-Spencer 1996) but insufficient data currently exists on a national scale. Wave and current-exposed maerl beds, where thicker depths of maerl accumulate, frequently occur as waves and ridge/furrows arrangements (see Bosence 1976; Blunden et al. 1977; 1981; Irvine & Chamberlain 1994; Hall-Spencer 1995). At some sites where SS.SMp.Mrl.Pcal occurs, there may be significant patches of maerl gravel containing the rare burrowing anemone Halcampoides purpureus; this may be a separate biotope, but insufficient data exists at present. Northern maerl beds in the UK do not appear to contain L. corallioides but in south-west England and Ireland L. corallioides may occur to some extent in SS.SMp.Mrl.Pcal as well as SS.SMp.Mrl.Lcor, where it dominates.