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Marine Habitat Classification

Description of biotope or habitat type

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   Musculus discors beds on moderately exposed circalittoral rock

Physical habitat description

Salinity Full (30-35 ppt)
Wave exposure Moderately exposed, Sheltered
Tidal streams Moderately strong (1-3 kn)
Substratum Bedrock, boulders, cobbles
Zone Circalittoral - upper
Depth Band 10-20 m
Other Features

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 CR.MCR.CMus.Mdis <I>Musculus discors</I> beds on moderately exposed circalittoral rock

  • 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.


This biotope typically occurs on the upper faces of moderately exposed, moderately tide-swept bedrock, boulders and cobbles in slightly silty conditions. The mussel Musculus discors occurs in dense mats and occasionally completely coats all available surfaces. There is also often a layer of pseudofaeces, forming a thick, silty matrix. A relatively diverse fauna of cushion and branching sponges is often present on rocky outcrops and other hard substratum that is free of mussels. These include Tethya aurantium, Sycon ciliatum, Pachymatisma johnstonia, Dysidea fragilis, Cliona celata and Stelligera stuposa. There may be isolated clumps of silt-tolerant bryozoans such as Flustra foliacea and Crisularia plumosa. Various species may be observed on top of the mussels, including Asterias rubens, Crossaster papposus and the brittlestar Ophiura albida. Occasional Alcyonium digitatum and clumps of the hydroid Nemertesia antennina are found attached to rocky outcrops and boulders whilst the anemone Urticina felina may be seen in crevices in the rock or on gravely patches between boulders. Colonial ascidians such as Clavelina lepadiformis and didemnids may occasionally be present. A wide range of seaweeds may be present, including Dictyota dichotoma, Plocamium cartliagineum, Dictyopteris polypodioides, Cryptopleura ramosa and Heterosiphonia plumosa. The crab Cancer pagurus may be observed in crevices. The majority of the records for this biotope are from the Lleyn Peninsula.


No situation data available.

Temporal variation

Not known.

Characterising species

Taxon Relative importance of taxon for defining this community (%) Typical abundance - SACFOR scale % of core records where taxon was recorded
Musculus discors 30 Super abundant 81-100%
Asterias rubens 7 Frequent 61-80%
Flustra foliacea 5 Occasional 61-80%
Tethya aurantium 5 Occasional 61-80%
Crossaster papposus 4 Occasional 61-80%
Sycon ciliatum 4 Occasional 61-80%
Dictyota dichotoma 3 Frequent 41-60%
Alcyonium digitatum 2 Occasional 41-60%
Crisularia plumosa 2 Frequent 41-60%
Cancer pagurus 2 Occasional 41-60%
Clavelina lepadiformis 2 Occasional 41-60%
Cliona celata 2 Rare 41-60%
Dysidea fragilis 2 Occasional 41-60%
Pachymatisma johnstonia 2 Occasional 41-60%
Plocamium cartilagineum 2 Occasional 41-60%
Urticina felina 2 Occasional 41-60%
Cryptopleura ramosa 1 Occasional 21-40%
Dictyopteris polypodioides 1 Occasional 41-60%
Didemnidae 1 Occasional 21-40%
Heterosiphonia plumosa 1 Occasional 21-40%
Nemertesia antennina 1 Occasional 41-60%
Ophiura albida 1 Frequent 21-40%
Stelligera stuposa 1 Occasional 41-60%

Similar biotopes or habitat types

M. discors may be present in this biotope, but at nowhere near the density present in CR.MCR.CMus.Mdis.

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