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Marine Habitat Classification

Description of biotope or habitat type

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   Modiolus modiolus beds with hydroids and red seaweeds on tide-swept circalittoral mixed substrata

Physical habitat description

Salinity Full (30-35 ppt)
Wave exposure Moderately exposed, Sheltered, Very sheltered
Tidal streams Strong (3-6 kn), Moderately strong (1-3 kn)
Substratum Cobbles, pebbles and Modiolus shells
Zone Infralittoral - lower, Circalittoral
Depth Band 5-10 m, 10-20 m, 20-30 m, 30-50 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 SS.SBR.SMus.ModT <I>Modiolus modiolus</I> beds with hydroids and red seaweeds on tide-swept circalittoral mixed substrata

  • 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.


Modiolus beds on mixed substrata (cobbles, pebbles and coarse muddy sediments) in moderately strong currents or wave exposed areas, typically on the open coast but also in tide-swept channels of marine inlets. Ophiothrix fragilis are often common in this biotope along with the calcareous tubes of Pomatoceros triqueter, anemones such as Alcyonium digitatum and Urticina felina and hydroids such as Abietinaria abietina and Sertularia argentea. Buccinum undatum may also be important and in some areas the clam Chlamys varia may be frequent but not in the same abundances as in ModCvar. Little information on the infaunal component is given here although it is likely that it is very rich and may highlight more subtle differences in the Modiolus biotopes. This biotope is typified by examples off the north-west Lleyn Peninsula in N Wales and off Co. Down, Northern Ireland.


No situation data available.

Temporal variation

No temporal data avaiable.

Characterising species

Taxon Relative importance of taxon for defining this community (%) Typical abundance - SACFOR scale % of core records where taxon was recorded
Abietinaria abietina 3 Frequent 41-60 %
Sertularia argentea 1 Occasional 21-40 %
Alcyonium digitatum 10 Common 61-80 %
Urticina felina 2 Occasional 41-60 %
Pomatoceros triqueter 4 Frequent 41-60 %
Balanus crenatus 1 Frequent 21-40 %
Pagurus bernhardus 3 Occasional 61-80 %
Buccinum undatum 4 Occasional 61-80 %
Modiolus modiolus 27 Abundant 81-100 %
Crossaster papposus 1 Rare 21-40 %
Asterias rubens 8 Occasional 81-100 %
Ophiothrix fragilis 13 Common 61-80 %
Ophiocomina nigra 2 Frequent 41-60 %
Psammechinus miliaris 2 Common 21-40 %
Echinus esculentus 2 Occasional 41-60 %
Ciona intestinalis 1 Rare 21-40 %
Phycodrys rubens 2 Common 21-40 %

Similar biotopes or habitat types

Tends to occur in deeper water than ModT and has a more divesre infauna as a result of the lower tidal current strength.

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