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The horse mussel

Modiolus modiolus facts

  • Modiolus modiolus is the latin or scientific name given to the horse mussel, a marine bivalve also known in Scotland as Clabach Dubh and in the Shetlands as the Yaug (Comely, 1981).
  • Largest European mussel, size ranges from 35 to 200mm.
  • Long-lived species, many of the larger mussels in Strangford Lough are well over 20 years of age. It is likely that horse mussels can live as long as 50 years of age.
  • Lives semi-infaunally or epifaunally on a variety of seabed subtrates including bedrock, gravel, sand and, most rarely mud.
  • Occurs in pools in the lower eulittoral zone to depths over 100m.
  • Forms infaunal or epifaunal biogenic reefs.
  • Widely distributed in the Northern Hemisphere (distribution map). In Europe is found from the Barents and White Seas to the Bay of Biscay. In the UK and Ireland is absent south of the Irish Sea and Humber Estuary. Outside Europe the horse mussel is present off the Atlantic coast of North America, from Labrador to South Carolina and in the Pacific (Bering Sea, Japan and west coast of North America).

The Modiolus reefs of Strangford Lough - a diverse marine ecosystem

The horse mussel Modiolus modiolus forms aggregations or 'beds' that under the EC Habitats Directive (92/43/EEC) are classified as sublittoral biogenic reefs. These Modiolus reefs appear as a result of the build up of a mound of faecal mud and shell debris over several years forming large and continous banks or isolated clumps. In the UK, Modiolus reefs usually occur on sandy or coarse substrates.

In Strangford Lough - a large (150km²) shallow sea lough situated on the east coast of County Down, Northern Ireland - M. modiolus forms clumped biogenic reefs which are unusual as they are located in areas of very soft substratum. Only in the Fith of Forth (Scotland) and the San Juan Channel (North-America) are Modiolus reefs found in soft substrates. Strangford Lough has a high detritus and plankton load which combined with a high water movement provides excellent conditions for filter feeders such as oysters and mussels.

Horse mussel reefs provide important 'ecosystem services' by their high filtration capacity (1 mussel filters about 1L water per hour while 1000 mussels can filter about 24 tonnes water per day) and by channelling primary phytoplankton productivity to the benthos, increase biodiversity. The Modiolus reefs of Strangford Lough enable a very rich faunal community that includes most of the major groups of organisms and this habitat is regarded as one the most luxuriant biotopes in the North-East Atlantic.


Horse mussel reefs are considered to be one of the most diverse marine ecosystems in Europe. Top photo: Modiolus clump with Antedon bifida; bottom images, from left to right: Dahlia Anemone Urticina felina, sea urchin Echinus esculentus and starfish Solaster paposus (Photo: Bernard Picton).

Contact Information

Modiolus Restoration Research Group
Queen's University Marine Laboratory (QUML)
12, The Strand
Portaferry
BT22 1PF
Northern Ireland
Phone: +44 (0) 28 90972249
E-mail: d.roberts@qub.ac.uk

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