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Marine Habitat Classification

Description of biotope or habitat type

To understand more about what this page is describing, see How to use the classification. See also How to cite.

   Flustra foliacea and colonial ascidians on tide-swept exposed circalittoral mixed substrata

Physical habitat description

Salinity Full (30-35 ppt)
Wave exposure Extremely exposed, Very exposed, Exposed, Moderately exposed
Tidal streams Strong (3-6 kn), Moderately strong (1-3 kn), Weak (>1 kn)
Substratum Boulder; cobble; pebble
Zone Circalittoral - lower, Circalittoral - upper
Depth Band 5-10 m, 10-20 m, 20-30 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.HCR.XFa.FluCoAs.X <I>Flustra foliacea</I> and colonial ascidians on tide-swept exposed circalittoral mixed substrata

  • 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 variant is typically found on very exposed to moderately exposed, circalittoral mixed substrata subject to moderately strong tidal streams. It most frequently occurs between 10m and 20m water depth. This variant is characterised by a dense hydroid and Flustra foliacea turf, along with other scour-tolerant species, growing on the more stable boulders and cobbles which overlie coarse muddy sand and gravel. Although Nemertesia antennina is the dominant species within the hydroid turf, other species such as Halecium halecinum, Nemertesia ramosa and Hydrallmania falcata may also be present. Other bryozoans found amongst the hydroid and Flustra turf include Cellepora pumicosa, Bugulina flabellata, Bugulina turbinata, and a crisiid turf. Encrusting red algae, the polychaete Spirobranchus triqueter and barnacles such as Balanus crenatus may be found on the smaller cobbles and pebbles, which may become mobile during extreme storms. Echinoderms such as Asterias rubens and Ophiothrix fragilis may be present on the boulders, or the coarse sediment in between. On the larger, more stable boulders, isolated sponge communities may develop, with species such as Sycon ciliatum, Dysidea fragilis, Hemimycale columella, Amphilectus fucorum and Stelligera montagui. In addition, small Alcyonium digitatum, various ascidians (Clavelina lepadiformis, Botryllus schlosseri), Pododesmus patelliformis and top shells (Calliostoma zizyphinum, Steromphala cineraria) may colonise the upper faces and vertical sides of larger boulders. At some shallower sites, the foliose red algae Hypoglossum hypoglossoides may be found on the tops of larger boulders. Within the coarse sediment underlying these boulders and cobbles, anemones such as Cerianthus lloydii and Urticina felina may be recorded. Under-boulder fauna typically consists of terebellid worms, and crabs such as Pisidia longicornis and Cancer pagurus.


This variant is found in wave-exposed locations, resulting in kelp forest in the infralittoral zone being dominated by Alaria esculentus and Laminaria hyperborea. When the substratum becomes rockier, this biotope will graduate into the variant FluCoAs.SmAs.

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
Asterias rubens 6 Frequent 81-100%
Nemertesia antennina 6 Frequent 81-100%
Flustra foliacea 5 Frequent 81-100%
Spirobranchus triqueter 4 Frequent 81-100%
Alcyonium digitatum 3 Occasional 81-100%
Calliostoma zizyphinum 3 Occasional 81-100%
Clavelina lepadiformis 3 Frequent 61-80%
Pisidia longicornis 3 Common 61-80%
Corallinaceae 2 Occasional 61-80%
Balanus crenatus 2 Occasional 61-80%
Botryllus schlosseri 2 Occasional 61-80%
Bugulina flabellata 2 Occasional 61-80%
Cellepora pumicosa 2 Occasional 61-80%
Dysidea fragilis 2 Occasional 61-80%
Steromphala cineraria 2 Occasional 61-80%
Halecium halecinum 2 Frequent 61-80%
Hemimycale columella 2 Occasional 61-80%
Hypoglossum hypoglossoides 2 Occasional 61-80%
Nemertesia ramosa 2 Frequent 61-80%
Pododesmus patelliformis 2 Frequent 41-60%
Sycon ciliatum 2 Occasional 61-80%
Seraphsidae 2 Occasional 41-60%
Bugulina turbinata 1 Occasional 41-60%
Cancer pagurus 1 Occasional 61-80%
Cerianthus lloydii 1 Occasional 41-60%
Crisiidae 1 Frequent 41-60%
Amphilectus fucorum 1 Occasional 41-60%
Hydrallmania falcata 1 Occasional 41-60%
Ophiothrix fragilis 1 Occasional 41-60%
Stelligera montagui 1 Occasional 41-60%
Urticina felina 1 Occasional 61-80%

Similar biotopes or habitat types

This variant occurs on slightly less wave-exposed sites but subject to similar tidal streams. While CR.HCR.XFa.FluCoAs.Paur is found on circalittoral bedrock and boulders, CR.HCR.XFa.FluCoAs.X occurs on mixed substrata (boulders, cobble, pebble and gravel). Although both biotopes feature F. foliacea as a dominant feature of their fauna, CR.HCR.XFa.FluCoAs.X has a much more diverse associated fauna and lacks the abundant sheets of the colonial ascidian Polyclinum aurantium characteristic of CR.HCR.XFa.FluCoAs.Paur.

This variant occurs under slightly less wave-exposed conditions but subject to similar tidal streams. They both occur around similar depths but CR.HCR.XFa.FluCoAs.X is found on a mixed substratum (boulders, cobbles, pebbles and gravel) whereas CR.HCR.XFa.FluCoAs.SmAs tends to be found on bedrock or boulders. CR.HCR.XFa.FluCoAs.SmAs usually has a more diverse range of sponges, although it tends not to have the range of species found on more mixed sediment as in CR.HCR.XFa.FluCoAs.X (e.g. Cerianthus lloydii, Chaetopterus variopedatus and Mimachlamys varia).

This biotope occurs on moderately wave-exposed coasts subject to similar tidal streams. This biotope is found at much deeper depths than CR.HCR.XFa.FluCoAs.X, with a mean depth range of 20 m to 21 m. Unlike CR.MCR.EcCr.FaAlCr, LR.LLR.Flu CoAs.X is found on more mixed substrata, consisting of boulders, cobbles and pebbles. CR.HCR.XFa.FluCoAs.X also has a diverse sponge and bryozoan turf community.

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