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Marine Habitat Classification


Description of biotope or habitat type

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   Alaria esculenta, Mytilus edulis and coralline crusts on very exposed sublittoral fringe bedrock


Habitat (physical) description

Salinity Full (30-35 ppt)
Wave exposure Very exposed
Tidal streams Moderately strong (1-3 kn), Weak (>1 kn)
Substratum Bedrock
Zone Sublittoral fringe
Depth Band 0-5 m, Lower shore
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.Ala.Myt <I>Alaria esculenta</I>, <I>Mytilus edulis</I> and coralline crusts on very exposed sublittoral fringe bedrock

  • 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.

Description

Very exposed sublittoral fringe bedrock characterised by the kelp Alaria esculenta and dense patches of small individuals of the mussel Mytilus edulis, both of which grow over a dense cover of encrusting coralline algae. Foliose red seaweeds may also be present, but the species composition and their abundance vary between sites. Species such as Corallina officinalis occur widely. The kelp Laminaria digitata is usually absent, although stunted plants may be present at a few sites. The limpet Patella vulgata and the barnacle Semibalanus balanoides are often common. Patches of anthozoans and the hydroid Tubularia spp. occur in more wave-surged areas. In extremely exposed areas the A. esculenta zone can extend as deep as 15 m, where it has less S. balanoides, M. edulis and greater densities of Tubularia spp. (e.g. Barra and shallow areas of Rockall).

Situation

This biotope is most commonly found beneath the mussel-barnacle zone (MytB) of very exposed shores and above the upper infralittoral Laminaria hyperborea forest (LhypR or LhypFa). It is at the extremely wave-surged sites, such as St Kilda, that LhypFa occurs below Ala.Myt. Occasionally, the A. esculenta zone occurs below a narrow but dense band of red seaweeds: typically Mastocarpus stellatus and/or Palmaria palmata and Corallina officinalis (Mas) or very occasionally Himanthalia elongata (Him). A dense turf of C. officinalis (Coff) occurs above the A. esculenta zone at a few extremely exposed sites, particularly on steep or vertical rock. On less exposed shores, however, an A. esculenta dominated zone may lie immediately above a narrow L. digitata zone (Ldig). Ala.Myt can also occur on less exposed steep or vertical shores, where wave-surge restricts the growth of L. digitata which generally dominates the sublittoral fringe rock on moderately exposed shores. On seasonally unstable boulders or sites subject to disturbance by strong wave-action, a mixed kelp canopy that characterises LsacSac may occur beneath the Ala.Myt zone instead of the ubiquitous L. hyperborea forest; this is most common on the Shetland isles.

Temporal variation

At very exposed sites, A. esculenta may have been so wave-battered during the season as to be reduced to a tattered midrib with no blades, altering the general appearance of the biotope. Where Ala.Myt occurs on boulders and/or sites subject to disturbance during severe weather conditions, rock that is scoured clean may then be rapidly colonised by fast-growing green algae such as Enteromorpha spp. An assemblage of rapidly colonising species that characterise the disturbed Sac biotope may also develop in the sublittoral fringe. A species that can fluctuate in huge numbers at these sites is the starfish Asterias rubens, sometimes forming dense aggregations across the narrow A. esculenta band whilst feeding on the mussels.

Characterising Species

Taxon Relative importance of taxon for defining this community (%) Typical abundance - SACFOR scale % of core records where taxon was recorded
Semibalanus balanoides 5 Frequent 41-60 %
Patella vulgata 10 Common 61-80 %
Mytilus edulis 19 Abundant 61-80 %
Corallinaceae 3 Abundant 21-40 %
Corallina officinalis 10 Frequent 61-80 %
Lithothamnion 9 Abundant 61-80 %
Alaria esculenta 34 Abundant 81-100 %

Similar biotopes

IR.HIR.KFaR.Ala.Ldig
Occurs on less exposed shores. A. esculenta and M. edulis occurs at a lower abundance and Lithothamnion spp. is not usually present. L. digitata is present in high abundance and the diversity of red seaweeds are high.

IR.HIR.KFaR.AlaAnCrSp
Occur on very exposed bedrock shores. Dense anthozoans including Corynactis viridis and Phellia gausapata and sponges such as Haliclona urceolus and Myxilla fimbriata can be present. Ala.Myt is distinguished from the deep A. esculenta forest found on Rockall by its lack of short turf forming hydroids.

Biotope history

Classification Habitat code
6.95 LRK.AL

Photos

IR.HIR.KFAR.Ala.Myt Alaria esculenta, Mytilus edulis and coralline crusts on very exposed sublittoral fringe bedrock, North Gaulton Castle, SW Mainland, Orkney. Keith Hiscock © JNCC
IR.HIR.KFAR.Ala.Myt Alaria esculenta, Mytilus edulis and coralline crusts on very exposed sublittoral fringe bedrock, North Gaulton Castle, SW Mainland, Orkney. Keith Hiscock © JNCC
IR.HIR.KFAR.Ala.Myt Alaria esculenta, Mytilus edulis and coralline crusts on very exposed sublittoral fringe bedrock, Plumb Island, St Martins, Scilly. Roger Mitchell © JNCC
IR.HIR.KFAR.Ala.Myt Alaria esculenta, Mytilus edulis and coralline crusts on very exposed sublittoral fringe bedrock, Plumb Island, St Martins, Scilly. Roger Mitchell © JNCC

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