The common (or blue) mussel is a medium-sized edible marine bivalve mollusc. It is characterised by a smooth wedge, or pear-shaped shell (of two halves), usually purple, blue or dark brown, which has concentric growth lines emanating from the hinge. The common mussel is usually found in sub-tidal and inter-tidal beds on rocky shores and at depths no greater than 10m.

Behaviour

The common mussel is a sessile species, permanently settling on substrates as adults; they are often found in dense colonies called mussel beds, where they are somewhat protected from predation by virtue of their numbers. Fibrous brown byssal threads extending from the closed shell are used both to attach it to surfaces and for defence, by tying down predators.

It is a filter feeder and uses its cilia to funnel sea water into its mouth opening, where it filters minute food particles for digestion. They can filter up to 65 litres of water a day and have an important role in nutrient cycling as the filtering process removes potentially toxic algae.

Mussels have separate sexes. Once the sperm and eggs are fully developed they are released into the water column for fertilisation. An individual female can produce 5 to 8 million eggs, large proportions of which are never fertilised and of those that are, as few as 1% of larvae actually reach adulthood.

Size

• Length: 5-12cm
• Weight: 1.5-6.5g
• Average lifespan: 2-3 years; sometimes more than 10 years

Status

Common; no specific conservation measures in place

Distribution

The common mussel, as its name suggests, is the most common mussel found throughout Scotland. Sub-tidal blue mussel beds are, however, restricted to a few scattered locations in lochs and firths including the Solway Firth, Firth of Clyde, Loch Creran, Loch Ailort, Dornoch Firth, Moray Firth, Firth of Tay and, Whiteness Voe in Shetland.

When to see

January to December; mussels are at their largest and fleshiest in October and smallest in March.

Facts

• Mussels have been cultivated for almost 800 years in Europe, and have been used as a food source for more than 20,000 years. In fact, prehistoric settlements in Scotland can often be identified by the large mounds of mussel shells found nearby.
• The byssal threads are produced as a liquid which sets in seawater. They resemble soft rubber at one end and rigid nylon at the other and are five times tougher than a human tendon. Scientists are developing a mussel-based adhesive for use in eye surgery.
• Dense mussel beds stabilise the underlying sediment creating a highly complex habitat for a diverse community of plants and animals. The hard substrate they provide, in otherwise sedimentary areas, increases the overall biodiversity of the area.