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Marine Habitat Classification

Description of biotope or habitat type

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   Foliose red seaweeds with dense Dictyota dichotoma and/or Dictyopteris polypodioides on exposed lower infralittoral rock

Physical habitat description

Salinity Full (30-35 ppt)
Wave exposure Exposed, Moderately exposed
Tidal streams Moderately strong (1-3 kn), Weak (>1 kn)
Substratum Bedrock; large boulders
Zone Infralittoral - lower
Depth Band 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 IR.HIR.KFaR.FoR.Dic Foliose red seaweeds with dense <I>Dictyota dichotoma</I> and/or <I>Dictyopteris polypodioides</I> on exposed lower infralittoral 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.


A dense turf of foliose red seaweeds mixed with a dense turf of the foliose brown seaweeds Dictyota dichotoma and/or Dictyopteris polypodioides on exposed and moderately exposed lower infralittoral rock, generally at or below the lower limit of the kelp zone. In some areas the lower infralittoral is subject to a moderate amount of scour from nearby sand. D. dichotoma is relatively tolerant of such scour and in such areas a zone forms with other sand-tolerant seaweeds. D. membranacea is confined to south-western coasts. Typically brown seaweeds dominate the seabed or are at least in equal abundance to the red seaweeds, some of which may also form dense stands such as Plocamium cartilagineum, Calliblepharis ciliata, Cryptopleura ramosa, Bonnemaisonia asparagoides, Heterosiphonia plumosa, Delesseria sanguinea and Vertebrata byssoides. The urchin Echinus esculentus can be found grazing the rock surface which can be covered in coralline algae. The anthozoans Caryophyllia smithii and Alcyonium digitatum are usually present in this biotope along with the tube-building worm Spirobranchus sp. which is more common in sand-scoured areas. The starfish Asterias rubens and Henricia sp. and sponge crusts including Cliona celata can also be found here. D. dichotoma also occurs in the kelp park, and records should only be assigned to this biotope where kelp such as Laminaria hyperborea is sparse or absent and a relatively high density of D. dichotoma and/or D. membranacea is present.


This biotope usually occurs at or below the lower limit of kelp L. hyperborea (LhypR.Pk or Lhyp). In south-west England a zone of mixed kelp forest L. hyperborea and Laminaria ochroleuca may occur above the dense foliose algae (LhypR.Loch). FoR.Dic marks the lower limit of the lower infralittoral zone.

Temporal variation

Like many of the red seaweeds found in this biotope the dominant brown seaweeds D. membranacea and D. dichotoma have annual fronds which tend to die back in the autumn and regenerate again in the spring. This produces a seasonal change in the density of the seaweed cover, which is substantially reduced over winter months and reaches its most dense between April and September.

Characterising species

Taxon Typical abundance - SACFOR scale % of core records where taxon was recorded
Laminaria hyperborea Occasional 41-60%
Dictyota dichotoma Common 81-100%
Dictyopteris polypodioides Frequent 81-100%
Delesseria sanguinea Frequent 81-100%
Vertebrata byssoides Frequent 61-80%
Heterosiphonia plumosa Frequent 61-80%
Plocamium cartilagineum Frequent 61-80%
Asterias rubens Occasional 61-80%
Alcyonium digitatum Occasional 21-40%
Corallinaceae Frequent 41-60%
Calliblepharis ciliata Occasional 21-40%
Caryophyllia smithii Occasional 61-80%
Cliona celata Occasional 41-60%
Cryptopleura ramosa Frequent 41-60%
Henricia oculata Occasional 41-60%
Echinus esculentus Occasional 41-60%
Bonnemaisonia asparagoides Occasional 21-40%
Spirobranchus triqueter Occasional 21-40%

Similar biotopes or habitat types

This biotope occurs at similar depth and conditions as IR.HIR.KFaR.FoR.Dic. The abundance of the brown seaweeds Dictyopteris polypodioides (Occasional) and D. dichotoma is higher (Frequent) without the presence of the kelps Laminaria hyperborea and Saccharina latissima.

This biotope occurs on shallow, silted infralittoral bedrock and boulders in areas of turbid water dominated by dense red seaweeds, with the notable absence of kelp. Individual species of red seaweeds such as Plocamium cartilagineum or Calliblepharis ciliata often dominate. The fauna is both less diverse and at lower abundance.

This biotope occurs on mixed, mobile substrata and is characterised by red seaweeds such as Halarachnion ligulatum, Lomentaria orcadensis, Naccaria wiggii and Compsothamnion thuioides which thrive in these conditions.

Classification history of this biotope or habitat type

Classification version Code
2015 (15.03) IR.HIR.KFaR.FoR.Dic
1996 (6.95) EIR.Dic

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