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Marine Habitat Classification

Description of biotope or habitat type

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   Red seaweeds and kelps on tide-swept mobile infralittoral cobbles and pebbles

Physical habitat description

Salinity Full (30-35 ppt)
Wave exposure Extremely exposed, Exposed, Moderately exposed, Sheltered
Tidal streams Moderately strong (1-3 kn), Weak (>1 kn)
Substratum Small boulders, cobbles and pebbles with gravel
Zone Infralittoral
Depth Band 0-5 m, 5-10 m, 10-20 m, 20-30 m
Other Features Seasonally-disturbed substrata

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.KSwSS.LsacR.CbPb Red seaweeds and kelps on tide-swept mobile infralittoral cobbles and pebbles

  • 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.


Shallow mixed substrata of cobbles and pebbles swept by moderately strong tidal streams in exposed areas, and characterised by dense stands of red seaweeds. Tide-swept infralittoral cobbles and pebbles which may be highly mobile, create an environment that is difficult for many algae to survive in. Foliose and filamentous seaweeds with an encrusting phase in their life history, or those that are able to withstand rolling of the substratum and scouring, can form dense turfs of seaweed in the more settled summer months. Characteristic red seaweeds include Halarachnion ligulatum which is able to survive attached to the pebbles and cobbles. Ephemeral algae grow rapidly in periods of relative stability. Other characteristic red seaweeds include Plocamium cartilagineum, Hypoglossum hypoglossoides, Bonnemaisonia asparagoides and Brongniartella byssoides. Coralline encrusting algae cover many of the cobbles and pebbles; some areas of cobbles may be quite barren, dominated only by encrusting coralline algae and brittlestars. Of the brown seaweeds scattered Laminaria spp. and Desmarestia spp. may be present on more stable large boulders or bedrock outcrops. Chorda filum and Halidrys siliquosa may be present in low abundance but where these seaweeds occur in greater abundance (typically >Frequent) refer to MIR.LsacChoR and MIR.HalXK respectively. Although the faunal component of this biotope is usually relatively sparse it can include a wide variety of species. Turfs of hydroids (Nemertesia spp., Aglaophenia tubulifera) and bryozoans (Crisia spp. and Bugula spp.) are the major components but sponges and anemones may also occur. Brittlestars, sea-urchins, hydroids and solitary ascidians are more prominent in the Scottish examples of this biotope, which tend to occur in deeper water, due in part to clearer waters.


Although not common, this biotope is widely distributed from Sussex to the shallow areas of the Sarns in Cardigan Bay, the west coast of Scotland and the north-east coast of Ireland. Despite the wide distribution, the red seaweed composition remains remarkably constant. In areas such as the Sarns, in Wales, where mixed substrata continue into the shallows, dense swathes of HIR.LsacChoR can be found. More stable but highly scoured areas adjacent to LsacR.CbPb can support the Halidrys biotope HIR.XKHal. Where bedrock or large boulders occur above the mixed substrata of LsacR.CbPb it may support a kelp forest or park (HIR.LhypR or MIR.Lhyp). At many sites the mixed substrata supporting the dense seaweed turf gives way to sediment of varying composition.

Temporal variation

This biotope will take on a much more depauperate appearance during the winter months, once the ephemeral seaweeds have died back in late summer/autumn. Storms can mobilise the loose pebbles and cobbles, removing all but the most resilient of seaweeds and animals. By summer, under more stable conditions, new growth will flourish and dense stands of seaweeds dominate the seabed.

Characterising species

Taxon Relative importance of taxon for defining this community (%) Typical abundance - SACFOR scale % of core records where taxon was recorded
Urticina felina 1 Occasional 21-40 %
Pomatoceros triqueter 2 Occasional 41-60 %
Balanus crenatus 1 Frequent 21-40 %
Pagurus bernhardus 1 Occasional 21-40 %
Gibbula cineraria 4 Occasional 41-60 %
Asterias rubens 4 Occasional 41-60 %
Ophiothrix fragilis 1 Occasional 21-40 %
Clavelina lepadiformis 2 Occasional 41-60 %
Botryllus schlosseri 2 Occasional 41-60 %
Bonnemaisonia asparagoides 3 Occasional 41-60 %
Corallinaceae 4 Frequent 41-60 %
Calliblepharis ciliata 1 Occasional 21-40 %
Rhodophyllis divaricata 2 Occasional 41-60 %
Halarachnion ligulatum 7 Occasional 61-80 %
Callophyllis laciniata 2 Occasional 21-40 %
Plocamium cartilagineum 4 Occasional 41-60 %
Lomentaria orcadensis 1 Occasional 21-40 %
Pterothamnion plumula 1 Occasional 21-40 %
Heterosiphonia plumosa 1 Occasional 21-40 %
Cryptopleura ramosa 2 Occasional 21-40 %
Delesseria sanguinea 1 Occasional 21-40 %
Hypoglossum hypoglossoides 2 Occasional 41-60 %
Brongniartella byssoides 5 Occasional 61-80 %
Rhodomela confervoides 2 Occasional 21-40 %
Dictyota dichotoma 3 Occasional 41-60 %
Desmarestia aculeata 1 Occasional 21-40 %
Laminaria saccharina 1 Occasional 21-40 %

Similar biotopes or habitat types

LsacR.Gv has a more dense covering of L. saccharina with less robust and some foliose red algal species present.

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