Horse mussel (Modiolus modiolus) beds

Horse mussel beds © Paul Kay

Horse mussels are common on UK coasts, and are important to conservation where they form extensive beds in otherwise featureless sandy and muddy seascapes.

The beds are formed of live mussels and dead shells, mixed with sand and mussel waste, and all held together by byssus threads (the hair-like ‘beards’ by which the mussels attach to the seabed). Some mussel beds can extend over hundreds of hectares, and can grow in height to several metres above the surrounding seabed.  Mussel beds stabilise the soft seabed and provide food and shelter for other marine life.

Exactly which other animals and plants are found in the beds depends on bed size and factors such as water depth and current strength.  Creatures attaching to the surface of shells include sponges, soft corals, anemones, barnacles, and, in shallower water, coralline algae and other red seaweeds.  Heart urchins, spider crabs, other crabs, starfish, whelks, fish and worms are also found within horse mussel beds.

Horse mussels themselves are not widely collected for food, but the beds have been extensively damaged by scallop dredging and trawling.  Horse mussel beds are also threatened by aggregate extraction, dredge spoil dumping, cable laying and other activities that cause serious seabed disturbance.  Pollution and the increasing water temperature resulting from climate change may also be a threat.

Horse mussels are long-lived (up to 25 years) and slow growing, and so are vulnerable to disturbance.  Studies have shown that when extensive areas are cleared, recovery of the beds is unlikely.

For the official habitat definition please see the documents listed below.

 

European distribution

Extensive horse mussel beds are found only in Shetland, Orkney, the Hebrides and other parts of western Scotland, the Ards Peninsula, Strangford Lough, the Isle of Man, north-west Anglesey and north of the Lleyn Peninsula.  Dense beds of young horse mussels have been found in the Bristol Channel, but beds of adults are not known to occur there.  The full European range of the horse mussel extends from the Arctic to the Bay of Biscay.

 

Conservation status/need

Horse mussel beds fact

  • This is a UK BAP Priority Habitat (BAP habitats are now Habitats of Principal Importance/Priority Habitats).
  • OSPAR List of Threatened and/or Declining Species and Habitats (entire OSPAR area)
  • Horse mussel beds can also be key features of habitats listed in Annex I of the Habitats Directive

 

Official definition

UK Biodiversity Action Plan; Priority Habitat Descriptions. BRIG (ed. Ant Maddock) 2008 (updated December 2011)

Descriptions of habitats on the OSPAR list of threatened and/or declining species and habitats (OSPAR agreement 2008/07).

 

Further information

JNCC - UK BAP Priority Species and Habitats

OSPAR Commission - Background Document for Modiolus modiolus beds

World Register of Marine Species - Modiolus modiolus

Marine Life Information Network - Horse mussel Modiolus modiolus

Marine Species Identification Portal - Modiolus modiolus

Marine Life Information Network - Modiolus modiolus beds with hydroids and red seaweeds on tide-swept circalittoral mixed substrata

JNCC EUNIS habitat correlations table