Horse mussel beds

In Gaelic, horse mussels (Modiolus modiolus) are called 'clabaidh-dubha' ('clabby doos'), meaning 'big black mouths'! This probably refers to how they look on a plate, but is also an apt description of them in the seabed, with the edge of the living mussel forming pale 'lips' inside the dark shell. Horse mussels are bivalve molluscs, similar to the familiar blue mussels of the seashore, but much larger at 10-20cm long, and they usually live below low water. The shell is solid and dark blue, with a glossy brown covering on the shells of live animals.

Why are they important?

Horse mussels form beds and reefs which stabilise mobile seabeds, creating a home for many other creatures, and good feeding grounds for young fish. Bottom trawls and dredges, particularly those used for scallops, are known to have caused widespread and long-lasting damage to some horse mussel beds, the best documented being in Strangford Lough, Northern Ireland. Horse mussel beds are a priority habitat for UK Biodiversity Action Plans  , now taken forward by the Scottish Government as part of the Scottish Biodiversity Strategy  .

Where do they live?

Horse mussels occur all around the British Isles, but they are more common in the north. The most extensive beds are off northern and western coasts, in depths of 5-70m, so in Scotland we have a particular responsibility for their care. Beds of mature horse mussels are not known south of the Severn and Humber Estuaries, and records from the far south are likely to be of single individuals or juveniles.

The mussels usually live part-buried in soft to coarse sediments, but can also attach to rock, from the lower shore to around 280m depth. They can live singly, or in small clumps, or can form extensive beds and reefs, especially in sheltered sites with tidal streams, as in the narrows of sealochs and the sounds between islands

Nature of the reef

The strong byssus threads of a horse mussel are like many tiny guy ropes, attaching it firmly to stones, shells and other mussels nearby, and anchoring it in the seabed. Many mussels together can form extensive beds. Sediments, shell fragments and mussel droppings accumulate between the live mussels, and the beds can build up over time into mounds or reefs which can be considerably higher than the original seabed. Young mussels settle between, and on top of, the older ones. When the older mussels die, their shells remain, helping to build up the height of the reef, and modifying the seabed considerably. Horse mussels are long-lived - many survive for more than 25 years old, and some may live to more than 50 years. Young mussels are a favourite meal for crabs and starfish, but once the mussels grow to more than 6cm long, they are relatively safe from these predators.

The mussels' large shells provide a solid foundation for many sessile animals, including soft corals, tubeworms, barnacles, sea firs, sea mats and seaweeds. Young scallops attach to the sea firs. Between the shells, and inside older ones, brittlestars, crustaceans, worms, molluscs and many other small animals find shelter, attracting young fish (including commercial species like cod) to feed.