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Marine Habitat Classification

Description of biotope or habitat type

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   Modiolus modiolus beds with fine hydroids and large solitary ascidians on very sheltered circalittoral mixed substrata

Physical habitat description

Salinity Full (30-35 ppt)
Wave exposure Moderately exposed, Sheltered, Very sheltered, Extremely sheltered
Tidal streams Weak (>1 kn), Very weak (negligible)
Substratum Boulders, cobbles and shells on muddy sediment
Zone Infralittoral - lower, Circalittoral
Depth Band 5-10 m, 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 SS.SBR.SMus.ModHAs <I>Modiolus modiolus</I> beds with fine hydroids and large solitary ascidians on very sheltered 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.


Beds or scattered clumps of Modiolus modiolus in generally sheltered conditions with only slight tidal movement. Typically occurs in sealochs and the Shetland voes. Brittlestars Ophiothrix fragilis and Ophiocomina nigra, as well as Ophiopholis aculeata are often frequent, sometimes forming a dense bed as described in OphMx. The queen scallop Aequipecten opercularis is often present in moderate abundances. Large solitary ascidians (Ascidiella aspersa, Corella parallelogramma, Dendrodoa grossularia) and fine hydroids (Kirchenpaueria pinnata) are present attached to the mussel shells. Decapods such as hermit crabs (Pagurus bernhardus) and spider crabs (Hyas araneus) are typically present. Coralline algal crusts may be found on the mussel shells, with some red seaweeds in shallower water such as Phycodrys rubens. 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.


No situation data available.

Temporal variation

No temporal data available.

Characterising species

Taxon Relative importance of taxon for defining this community (%) Typical abundance - SACFOR scale % of core records where taxon was recorded
Kirchenpaueria pinnata 1 Occasional 41-60 %
Terebellidae 1 Occasional 21-40 %
Pomatoceros triqueter 4 Frequent 21-40 %
Balanus balanus 2 Occasional 41-60 %
Balanus crenatus 2 Occasional 41-60 %
Pagurus bernhardus 6 Occasional 61-80 %
Hyas araneus 3 Occasional 61-80 %
Liocarcinus depurator 2 Occasional 41-60 %
Gibbula cineraria 1 Occasional 21-40 %
Buccinum undatum 3 Occasional 61-80 %
Modiolus modiolus 19 Common 81-100 %
Aequipecten opercularis 6 Frequent 61-80 %
Antedon bifida 1 Occasional 21-40 %
Crossaster papposus 2 Rare 41-60 %
Asterias rubens 5 Occasional 61-80 %
Ophiothrix fragilis 8 Frequent 61-80 %
Ophiocomina nigra 2 Frequent 21-40 %
Ophiopholis aculeata 1 Occasional 21-40 %
Echinus esculentus 8 Occasional 81-100 %
Corella parallelogramma 2 Occasional 41-60 %
Ascidiella aspersa 1 Frequent 21-40 %
Dendrodoa grossularia 3 Occasional 41-60 %
Corallinaceae 2 Frequent 41-60 %
Phycodrys rubens 1 Occasional 41-60 %

Similar biotopes or habitat types

ClloModHo occurs in similar physiographic features, although seems to be in softer sediment in some cases and with a much lower abundance of Modiolus and a lower diversity in general.

ModCvar is a more species rich biotope with far more sponges and hydroids growing on and amongst the Modiolus and large numbers of Chlamys varia

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