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Marine Habitat Classification

Description of biotope or habitat type

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   Phymatolithon calcareum maerl beds in infralittoral clean gravel or coarse sand

Physical habitat description

Salinity Full (30-35 ppt)
Wave exposure Moderately exposed, Sheltered, Extremely sheltered
Tidal streams Moderately strong (1-3 kn), Weak (>1 kn)
Substratum Maerl gravel and sand
Zone Infralittoral
Depth Band 0-5 m, 5-10 m, 10-20 m
Other Features

Biotope origin

Derived using data from Various
Faunal group Epifauna

Download comparative physical and biological data. The comparative tables enable a rapid comparison of the species composition and principal physical characteristics between a given set of biotopes.

Distribution of habitat SS.SMp.Mrl.Pcal <I>Phymatolithon calcareum</I> maerl beds in infralittoral clean gravel or coarse sand

  • Records used to define the biotope (core records)
  • Other records assigned to this biotope, marked as 'certain'
  • Other records assigned to this biotope, marked as 'uncertain'
  • Predicted extent of the level 3 (for sublittoral rock & deep sea) or 4 (for sublittoral sediment) habitat

Point data based on records in the UK Marine Recorder Snapshot. Predicted habitat extent is from UKSeaMap.


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.


No situation data available.

Temporal variation

No temporal data available.

Characterising species

Taxon Relative importance of taxon for defining this community (%) Typical abundance - SACFOR scale % of core records where taxon was recorded
Phymatolithon calcareum 36 81-100%
Nematoda 9 Common 81-100%
Asterias rubens 7 Occasional 41-60%
Metaphoxus fultoni 7 Common 81-100%
Grania 6 Abundant 81-100%
Corallinaceae 5 21-40%
Chone duneri 5 Common 81-100%
Escharoides coccinea 5 21-40%
Amphipholis squamata 4 Super abundant 61-80%
Callionymus lyra 4 Frequent 21-40%
Psamathe fusca 4 Super abundant 81-100%
Mediomastus fragilis 4 Frequent 81-100%
Socarnes erythrophthalmus 4 Abundant 61-80%
Animoceradocus semiserratus 3 Common 61-80%
Corallina officinalis 3 21-40%
Cumella (Cumella) pygmaea 3 Common 61-80%
Steromphala cineraria 3 Occasional 21-40%
Leptocheirus pectinatus 3 Abundant 61-80%
Onoba aculeus 3 Super abundant 61-80%
Polycirrus 3 Common 81-100%
Alcyonidium diaphanum 2 Occasional 21-40%
Cellepora pumicosa 2 Occasional 21-40%
Clavelina lepadiformis 2 Occasional 21-40%
Cymodoce truncata 2 81-100%
Didemnidae 2 Rare 21-40%
Dikoleps nitens 2 Abundant 61-80%
Megamphopus cornutus 2 Frequent 81-100%
Hildenbrandia rubra 2 21-40%
Leptocheirus hirsutimanus 2 Frequent 61-80%
Liocarcinus depurator 2 Occasional 21-40%
Nannastacus unguiculatus 2 Common 61-80%
Nemertea 2 81-100%
Obelia dichotoma 2 Common 21-40%
Sphaerosyllis taylori 2 Frequent 61-80%
Vaunthompsonia cristata 2 81-100%
Porifera 1 21-40%
Aonides paucibranchiata 1 61-80%
Austrosyrrhoe fimbriatus 1 Common 41-60%
Crisia 1 21-40%
Eurysyllis tuberculata 1 Common 61-80%
Harmothoe impar 1 Common 61-80%
Saccharina latissima 1 Occasional 21-40%
Lithothamnion glaciale 1 21-40%
Microporella ciliata 1 21-40%
Porifera indet crusts 1 Frequent 21-40%
Cradoscrupocellaria reptans 1 21-40%
Pseudosyllis brevipennis 1 Frequent 61-80%

Similar biotopes or habitat types

SS.SMp.Mrl.Pcal.R is a shallower subtype with red seaweeds.

SS.SMp.Mrl.Pcal.Nmix is a deeper subtype with notably less epiphytic seaweeds.

Classification history of this biotope or habitat type

Classification version Code Habitat name
2015 (15.03) SS.SMp.Mrl.Pcal Phymatolithon calcareum maerl beds in infralittoral clean gravel or coarse sand
2004 (04.05) SS.SMp.Mrl.Pcal Phymatolithon calcareum maerl beds in infralittoral clean gravel or coarse sand
1997 (97.06) SS.IGS.Mrl.Phy Phymatolithon calcareum maerl beds in infralittoral clean gravel or coarse sand

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