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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 Hydrallmania falcata on tide-swept circalittoral mixed sediment

Physical habitat description

Salinity Full (30-35 ppt)
Wave exposure Exposed, Moderately exposed
Tidal streams Strong (3-6 kn), Moderately strong (1-3 kn)
Substratum Boulders, cobbles or pebbles with gravel and sand
Zone Circalittoral
Depth Band 5-10 m, 10-20 m, 20-30 m, 30-50 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.SMx.CMx.FluHyd <I>Flustra foliacea</I> and <I>Hydrallmania falcata</I> on tide-swept circalittoral mixed sediment

  • 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 represents part of a transition between sand-scoured circalittoral rock where the epifauna is conspicuous enough to be considered as a biotope and a sediment biotope where an infaunal sample is required to characterise it and is possibly best considered an epibiotic overlay. Flustra foliacea and the hydroid Hydrallmania falcata characterise this biotope; lesser amounts of other hydroids such as Sertularia argentea, Nemertesia antennina and occasionally Nemertesia ramosa, occur where suitably stable hard substrata is found. The anemone Urticina felina and the soft coral Alcyonium digitatum may also characterise this biotope. Barnacles Balanus crenatus and tube worms Spirobranchus triqueter may be present and the robust bryozoans Alcyonidium diaphanum and Vesicularia spinosa appear amongst the hydroids at a few sites. Sabella pavonina and Lanice conchilega may be occasionally found in the coarse sediment around the stones. In shallower (i.e. upper circalittoral) examples of this biotope scour-tolerant robust red algae such as Polysiphonia nigrescens, Calliblepharis spp. and Gracilaria gracilis are found. In offshore areas, such as in the Greater Gabbard North Sea Area, where there is circalottoral mixed sediment, with pebbles and gravels, the biotope may further support rich encrusting fauna, including bryozoans, Spirobranchus lamarcki, and the barnacle Verruca stroemia, and occasionally Sabellaria spinulosa. Alongside these encrusting fauna, infauna such as Lumbrinerids (Hilbigneris gracilis), Glycera lapidum, Echinocyamus pusillus, Amphipholis squamata, Caulleriella alata may be present, and may represent a transitionary form between SS.SMx.CMx.FluHyd and SS.SCS.CCS.MedLumVen.


This biotope is found around most coasts, although regional differences are seen where one or two similarly scour-tolerant species such as Styela clava and Crepidula fornicata (Solent) occupy the hard substrata. Further offshore in the North Sea Flustra foliacea is less dominant and a hydroid turf and encrusting species more prevalent.

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
Flustra foliacea 18 Frequent 81-100%
Hydrallmania falcata 11 Occasional 61-80%
Asterias rubens 10 Occasional 61-80%
Urticina felina 8 Occasional 61-80%
Nemertesia antennina 7 Occasional 41-60%
Pagurus bernhardus 6 Occasional 41-60%
Alcyonidium diaphanum 5 Occasional 41-60%
Alcyonium digitatum 5 Occasional 41-60%
Spirobranchus triqueter 3 Occasional 41-60%
Sertularia argentea 3 Occasional 41-60%
Crossaster papposus 2 Occasional 21-40%
Vesicularia spinosa 2 Frequent 21-40%
Balanus crenatus 1 Frequent 21-40%
Halecium halecinum 1 Occasional 21-40%
Nemertesia ramosa 1 Occasional 21-40%

Similar biotopes or habitat types

With increased scouring and more sand S. cupressina becomes more common and SS.SMx.CMx.FluHyd may develop into SS.SSa.IFiSa.ScupHyd.

On pebble plains, as tidal stream strength increases to a point at which the stones are regularly mobilised, all hydroids are scoured off leaving just Spirobranchus, bryozoan crusts, Balanus crenatus and coralline algae.

SS.SMx.IMx.CreAsAn is found in shallower water in slightly less exposed areas with a lower proportion of cobbles and pebbles and in slightly weaker currents.

Classification history of this biotope or habitat type

Classification version Code Habitat name
2015 (15.03) SS.SMx.CMx.FluHyd Flustra foliacea and Hydrallmania falcata on tide-swept circalittoral mixed sediment
2004 (04.05) SS.SMx.CMx.FluHyd Flustra foliacea and Hydrallmania falcata on tide-swept circalittoral mixed sediment
1997 (97.06) SS.MCR.ByH.Flu.SerHyd and part of SS.MCR.ByH.SNemAdia Flustra foliacea and other hydroid/bryozoan turf species on slightly scoured circalittoral rock or mixed substrata and part of Sparse sponges, Nemertesia spp., Alcyonidium diaphanum and Bowerbankia spp. on circalittoral mixed substrata

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