BIOTIC Species Information for Modiolus modiolus
Researched byLizzie Tyler Data supplied byUniversity of Sheffield
Refereed byThis information is not refereed.
Distribution and Habitat
Distribution in Britain & IrelandFound off all British coasts. Extensive beds are most common on northern or western coasts but absent south of the Irish Sea and Humber estuary.
Global distributionRecorded from the White sea and Norway, off the Faroes and Iceland, south to the Bay of Biscay and occasionally North Africa. Also from Labrador to North Carolina in the Atlantic and from the Bering Sea south to Japan and California in the Pacific.
Biogeographic rangeNot researched Depth range0 - 150 m
MigratoryNon-migratory / Resident   
Distribution Additional InformationModiolus modiolus was also recorded as a dominant member of the species rich benthic assemblage found throughout the gravely sediments of St. George's Channel, and the outer regions of Cardigan Bay and Caernarfon Bay (Mackie et al., 1995). Holt et al., (1998; Table 1) lists the predominant forms of horse mussel beds in British waters.

Modiolus modiolus is adapted to live semi-infaunally with an endobyssate attachment to the substratum but may also be found attached to hard substratum, epifaunally in a manner similar to the common mussel, Mytilus edulis. Modiolus modiolus beds vary in size, density, thickness and form.
  • Epifaunal horse mussels carpet steep surfaces and replace Mytilus spp. on offshore structures in cold waters, e.g., in Loch Duich, Loch Long and Loch Alsh (Holt et al., 1998).
  • Semi-infaunal reefs of large individuals about two-thirds embedded into sediment, forming an irregularly clumped layer and younger individuals between the byssal threads of larger individuals.
  • Reefs form due to the build up of a mound of faecal mud and shell debris over several years. Reefs vary in size and height, depending on the build up of faecal mud and water flow rates. In strong tidal streams the beds may not retain the faecal mud and do not form raised beds (Holt et al., 1998).
  • On coarser grounds and in strong currents the horse mussels bind together the coarse sediment forming banks and live nested infaunally within the deposit.
  • Coarse sediment banks formed by horse mussel beds form wave-like mounds or bioherms in the Bay of Fundy up to 3m high and 20m wide and 10s -100s of metres in length, which are visible on sidescan sonar (Wildish et al., 1998; Wildish & Fader, 1998). Similar banks, up to 1m high occur of the north east Isle of Man and Codling Bank, Ireland (Holt et al., 1998).
  • Horse mussel beds may occur as large and continuous banks (biogenic reefs) or as scattered clumps.
  • Holt et al., (1998) noted that extensive horse mussel beds or reefs were restricted to between 5 -50m in UK, although the large reefs recorded in the Bay of Fundy, Nova Scotia occurred at ca 80m (Wildish & Fader, 1998; Wildish et al., 1998).

Substratum preferencesBedrock
Large to very large boulders
Small boulders
Gravel / shingle
Muddy gravel
Coarse clean sand
Fine clean sand
Sandy mud
Muddy sand
Biogenic reef
Artificial (e.g. metal/wood/concrete)
Under boulders
Physiographic preferencesOpen coast
Offshore seabed
Strait / sound
Ria / Voe
Enclosed coast / Embayment
Biological zoneLower Eulittoral
Sublittoral Fringe
Upper Infralittoral
Lower Infralittoral
Upper Circalittoral
Lower Circalittoral
Wave exposureModerately Exposed
Very Sheltered
Tidal stream strength/Water flowStrong (3-6 kn)
Moderately Strong (1-3 kn)
Weak (<1 kn)
Very Weak (negligible)
SalinityFull (30-40 psu)
Habitat Preferences Additional Information
Distribution References Fish & Fish, 1996, Tebble, 1976, Hayward et al., 1996, Holt et al., 1998, Anonymous, 1999(u), Brown & Seed, 1977, Davenport & Kjørsvik, 1982, Comely, 1978, Brown, 1984, Wildish et al., 1998, Seed, 1976, Mackie et al., 1995, Jones et al., 2000, Wildish & Fader, 1998, Bayne et al., 1976b, Hayward & Ryland, 1990, Julie Bremner, unpub data,
About MarLIN | Contact, Enquiries & Feedback | Terms & Conditions | Funding | Glossary | Accessibility | Privacy | Sponsorship

Creative Commons License BIOTIC (Biological Traits Information Catalogue) by MarLIN (Marine Life Information Network) is licensed under a Creative Commons Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales License. Permissions beyond the scope of this license are available at Note that images and other media featured on this page are each governed by their own terms and conditions and they may or may not be available for reuse. Based on a work at