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Marine Habitat Classification

Description of biotope or habitat type

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   Mixed kelp and red seaweeds on infralittoral boulders, cobbles and gravel in tidal rapids

Physical habitat description

Salinity Full (30-35 ppt), Variable (18-35 ppt)
Wave exposure Sheltered, Very sheltered, Extremely sheltered
Tidal streams Very strong (>6 kn), Strong (3-6 kn), Moderately strong (1-3 kn)
Substratum Boulders, cobbles and gravel
Zone Infralittoral
Depth Band 0-5 m, 5-10 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 IR.MIR.KT.XKTX Mixed kelp and red seaweeds on infralittoral boulders, cobbles and gravel in tidal rapids

  • 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.


Mixed substrata of boulders, cobbles, pebbles and gravel, typically found in tidal rapids with kelp Saccharina latissima and Laminaria hyperborea and red seaweeds. L. saccharina usually dominates this habitat although L. hyperborea may occur in equal abundance at some sites. The kelp in these tidal rapids does not form the same dense canopies associated with stable tide-swept bedrock, but generally occurs at lower abundance (Frequent). Other brown seaweeds occur in significant amounts in these tidal rapids including Dictyota dichotoma, Halidrys siliquosa and Chorda filum. These mixed substrata support a greater diversity of species than scoured bedrock narrows (XKT). In particular, there is an increase in red algal species such as Corallina officinalis, Bonnemaisonia hamifera and Ceramium nodulosum, although none occur in any great abundance. Red seaweeds common to both XKT and this biotope include Chondrus crispus, Delesseria sanguinea, Plocamium cartilagineum and Phycodrys rubens. Good examples of this biotope often have maerl gravel (Lithothamnion sp.) or rhodoliths between cobbles and boulders. Where maerl dominates, the biotope should be recorded as a maerl bed (SS.SMP.Mrl). The sponges associated with more stable, tide-swept conditions are generally absent, but the anthozoan Anemonia viridis might be present. Cobbles and pebbles are encrusted by the ubiquitous polychaete Spirobranchus triqueter and provide shelter for scavenging crabs such as Carcinus maenas and the hermit crab Pagurus bernhardus, gastropods such as Steromphala cineraria and echinoderms such as Echinus esculentus, Asterias rubens, Ophiocomina nigra and Ophiothrix fragilis which favour these sites of increased water movement. Additional infaunal species, inhabiting the sediment pockets, include Lanice conchilega and Sabella pavonina, which can be locally abundant.


Where stable rock fringes the shallows the tide-swept Laminaria digitata biotope often occurs (LdigT). Adjacent areas of stable bedrock or boulders in these sheltered, tide-swept narrows can support a similar kelp community, often with a greater percentage of L. hyperborea (XKT). Maerl fragments are often found amongst the mixed substrata of XKTX and this biotope may abut more extensive areas of maerl bed (Phy & Lgla).

Temporal variation


Characterising species

Taxon Relative importance of taxon for defining this community (%) Typical abundance - SACFOR scale % of core records where taxon was recorded
Saccharina latissima 11 Frequent 81-100%
Spirobranchus triqueter 10 Frequent 61-80%
Lithothamnion 9 Common 81-100%
Asterias rubens 6 Occasional 61-80%
Carcinus maenas 5 Frequent 41-60%
Ophiocomina nigra 4 Common 41-60%
Ulva lactuca 4 Occasional 41-60%
Corallinaceae 3 Frequent 41-60%
Chondrus crispus 3 Rare 61-80%
Dictyota dichotoma 3 Frequent 41-60%
Pagurus bernhardus 3 Occasional 41-60%
Phycodrys rubens 3 Occasional 41-60%
Anemonia viridis 2 Occasional 41-60%
Bonnemaisonia hamifera 2 Frequent 21-40%
Ceramium nodulosum 2 Rare 41-60%
Corallina officinalis 2 Occasional 41-60%
Echinus esculentus 2 Occasional 41-60%
Delesseria sanguinea 1 Occasional 41-60%
Steromphala cineraria 1 Frequent 41-60%
Halidrys siliquosa 1 Occasional 21-40%
Laminaria hyperborea 1 Frequent 21-40%
Ophiothrix fragilis 1 Occasional 21-40%
Rhodophyta indet.(non-calc.crusts) 1 Frequent 21-40%

Similar biotopes or habitat types

This biotope occurs on the open coast in deeper water.

Stable, tide-swept rock in the sheltered narrows and sills of Scottish sealochs, characterised by a dense kelp forest of L. hyperborea (Common) and/or Saccharina latissima (Frequent) on scoured, coralline-encrusted rock. The brown seaweed Chorda filum is not present. Filter-feeders like the sponge Halichondria panicea, hydroids (Obelia geniculata) and anthozoans like Urticina felina and Sargartia elegans are present (Frequent-Occasional) due to the more stable substrata of this biotope compared to IR.MIR.KT.XKTX.

Classification history of this biotope or habitat type

Classification version Code
1997 (97.06) SIR.K.Lsac.T (part)

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