Horse mussel (Modiolus modiolus) beds
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.
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.
- This is a UK BAP Priority Habitat (BAP
habitats are now Habitats of Principal Importance/Priority
- OSPAR List of Threatened
and/or Declining Species and Habitats (entire OSPAR
- Horse mussel beds can
also be key features of habitats listed in Annex I of the Habitats
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).
JNCC - UK BAP Priority
Species and Habitats
OSPAR Commission -
Background Document for Modiolus
World Register of Marine Species - Modiolus
Life Information Network - Horse mussel Modiolus
Marine Species Identification Portal - Modiolus
Marine Life Information Network - Modiolus modiolus beds
with hydroids and red seaweeds on tide-swept circalittoral mixed
JNCC EUNIS habitat correlations table