information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Phymatolithon calcareum maerl beds in infralittoral clean gravel or coarse sand



UK and Ireland classification

UK and Ireland classification


Maerl beds characterised by Phymatolithon calcareum in gravels and sands. Associated epiphytes may include red algae such as Dictyota dichotoma, Halarachnion ligulatum, Callophyllis laciniata, Cryptopleura ramosa, Brongniartella 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, Kefersteinia cirrata, Mediomastus fragilis, Chone duneri, Parametaphoxus fultoni and Grania may be present. Gastropods such as Gibbula cineraria, Gibbula magus, Calyptraea chinensis, Dikoleps pusilla 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 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 SMp.Mrl.Pcal occurs, there may be significant patches of maerl gravel containing the rare burrowing anemone Halcampoides elongatus; this may be a separate biotope, but insufficient data exists at present. Northern maerl beds in the UK do not appear to contain Lithothamnion corallioides but in south-west England and Ireland, Lithothamnion corallioides may occur to some extent in SMp.Mrl.Pcal as well as SMp.Mrl.Lcor, where it dominates. This biotope (SMp.Mrl.Pcal) includes two sub-biotopes. SMp.Mrl.Pcal.R is a shallower sub-biotope with red seaweeds, while SMp.Mrl.Pcal.Nmix is a deeper sub-biotope with less epiphytic seaweeds. (Information from Connor et al., 2004).

Depth range

0-5 m, 5-10 m, 10-20 m

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Sensitivity reviewHow is sensitivity assessed?



Species indicative of sensitivity


Physical Pressures

No sensitivity data available

Chemical Pressures

No sensitivity data available

Biological Pressures

No sensitivity data available

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This review can be cited as:

Perry, F., 2018. [Phymatolithon calcareum] maerl beds in infralittoral clean gravel or coarse sand. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 19-06-2018]. Available from:

Last Updated: 27/03/2018