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Common or Blue Mussel

Mytilus edulis Linnaeus, 1758

 
Blue mussel bank in Zeeland, Netherlands.
Photo: Maaike Verschueren (iNaturalist).
Contents

Systematics

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Class: Bivalvia
 Subclass: Autobranchia
Infraclass: Pteriomorphia
Order: Mytilida
Superfamily: Mytiloidea
Family: Mytilidae
 Mytilus edulis Linnaeus, 1758

Source: MolluscaBase eds. (2025): Mytilus edulis Linnaeus, 1758.

Introduction


Blue mussels overgrown with barnacles.
Photo: Keith Hiscock, Marine Life Network.		  
Mussels (Mytilidae) are a remarkable form of life: in the Wadden Sea they can form kilometre-wide aggregations of millions of individual organisms covering the seabed. However, mussels are by no means restricted to the Wadden Sea: several species occur along almost all coasts of the North Atlantic, wherever a suitable substrate is available. Mussels even overgrow artificial structures such as piles and harbour walls (see picture above), as well as chains.

Description

 
Inhalant (left) and exhalant (centre) siphons of a living mus-
sel. In the right part of the picture the mussel's foot can be
seen. Source: Tiere der Ostsee: Die Miesmuschel.
When looking more closely at individual mussels, one finds that they are bluish-black shells up to 10 cm long, composed of two almost identical halves - it is therefore difficult to tell the upper and lower sides apart. The shells of mussels are often themselves covered by other marine organisms, such as the ubiquitous barnacles (Balanus), algae and seaweed. In fact, mussel beds in the Wadden Sea form an important habitat for more than a hundred species of animals living on and between the individual mussels.

From the soft body of the mussel, one usually sees only the strong foot protruding from between the shells. At its end is the byssus gland, which produces the byssus thread - a tough, protein-based fibre that is one of the secrets of the mussel’s success. Using these byssus threads, mussels can attach themselves not only to the substrate and to each other, but also move about by laying out a thread and then contracting it, thereby pulling the shell forward.

Besides the foot, the two siphons can be seen when the mussel opens its shells wide enough. Mussels do this only under water, when they breathe and feed.

Ecology

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Experiment with Blue Mussels


Harbour water from Juist is filled in two glasses. In the
right one, a handful of mussels are placed. The left glass
is used as comparison.		  
  

After an hour, the mussels have filtered the water, so it
becomes transparent.

Source: Aldebaran (1997).		  
Like other bivalves, mussels are filter feeders that extract both oxygen for respiration and nutrients from the seawater. In doing so, they make an important contribution to maintaining the Wadden Sea ecosystem. Each individual mussel filters about 2 to 3 litres of seawater per hour. Taking into account that mussels can be exposed to air during low tide, this amounts to a filtration rate of roughly 10 to 20 litres of seawater per mussel per day. It is estimated that all the mussels living in the Wadden Sea can filter the entire volume of water within just a few days. Mussels are therefore rightly referred to as the "purification plants" of the tidal flats.

But what happens to the suspended particles that the mussel filters from the seawater? From the respiratory current, the mussel separates digestible particles and directs them to its digestive tract, while indigestible material is expelled through the exhalant siphon. As a result, a layer of mud accumulates around the mussel - composed of fine particles that once floated in the seawater.

Distribution

The common mussel (Mytilus edulis) was originally restricted to the coastal waters of the eastern North Atlantic, from about Aquitaine northwards to Norway, the White Sea and Spitsbergen. In the western North Atlantic it occurs from Washington, D.C. to about Maine. From Nova Scotia northwards it is replaced by the Pacific mussel (Mytilus trossulus). In contrast, the common mussel has also been recorded in southern Greenland and Iceland. Through accidental transport and deliberate introduction in aquaculture, it is now also found in parts of the North Pacific. In the Mediterranean, the Mediterranean mussel (Mytilus galloprovincialis) occurs instead. As the individual Mytilus species are very closely related and can hybridise with one another, they are often referred to collectively as the Mytilus edulis complex.

Typical habitats extend from the high-tide line downwards along rocky coasts to the vicinity of river mouths, where the mussels often form dense beds together with barnacles. Mussels are also found in the Baltic Sea, but due to the lower salinity they reach only a comparatively small size of up to five centimetres, while their relatives in the North Sea can grow to about twice that size and develop stronger shells.

Their preferred habitat is in the intertidal and shallow subtidal zones, down to depths of around 20 metres, where they attach themselves to solid substrates by means of byssus threads. These threads are produced by a gland at the base of the foot. By anchoring themselves, mussels protect against being washed away and can even pull themselves free from the mud that accumulates around them as a result of their own filtering activity. They are able to detach their byssus threads and move short distances with the help of the foot before reattaching. Mussels require summer water temperatures of at least 4 °C. In mussel beds, they may form very dense populations of up to 2,000 individuals per square metre.

Reproduction and Development

Like their feeding, the reproduction of mussels also depends on the surrounding water. Mussels are dioecious, meaning there are separate males and females. In spring and several times during summer, each female mussel releases about 5 to 12 million eggs into the water, where they are fertilised by sperm cells from nearby males. This can result in a concentration of roughly one mussel larva per millilitre of seawater!

 
Veliger larva of Mytilus galloprovincialis.
Source: Ferrara University, Italy.
From the fertilised egg develops the trochophore larva, which transforms through metamorphosis into the veliger larva. These planktonic larval stages may be carried by sea currents over several hundred kilometres. However, about 99.9% of the planktonic mussel larvae are eaten. Even after this "murderous" selection, around 10,000 larvae remain, which develop into young mussels about 3 mm in size that settle on algae filaments or polyp colonies.

Until they reach about 5 cm in length, young mussels may still change location several times. They then attach permanently to a suitable substrate, preferably close to other mussels. Millions of individuals connected by their byssus threads form the well-known mussel beds. The advantage is obvious: by living in dense aggregations, the chances that sperm released by a male will encounter eggs from another mussel and fertilise them are greatly increased.

Predators

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Even after settling, mussels are exposed to numerous predators from both the water and the air. Above the surface, gulls and oystercatchers are among their enemies; underwater, eider ducks, starfish and crabs prey on mussels.

A major mussel predator is the common whelk (Buccinum undatum). This largest predatory snail of the North Sea lies in wait in front of a mussel, waiting until it must open its shells to breathe. Then it inserts its siphon and proboscis between the shell halves and begins to devour the now defenceless mussel.


Netted dog whelk (Hinia reticulata). Picture: Eric Walravens. 		 
Against smaller predatory snails, such as the dog whelk (Nucella lapillus) or the netted dog whelk (Hinia reticulata), mussels are, however, quite capable of defending themselves: they can immobilise the attacker with byssus threads until it eventually starves. About 30% of dog whelks found within mussel beds are caught in this way. Nucella lapillus therefore tend to avoid mussel beds whenever possible and are about twenty times more common outside them.

In addition to these biotic factors, abiotic factors also play an important role in the life and survival of mussels. Mussel beds that fall dry at low tide can be planed off and destroyed by drifting ice in winter. On the other hand, those exposed mussel beds - slowly raised above sea level by the accumulation of sediment resulting from the mussels’ own filtration activity - are safe from underwater predators such as whelks and starfish. The most favourable position for mussel beds is slightly above the low-water line. Such beds remain exposed for about four hours per tide - long enough to force starfish and snails to retreat.

Threat Situation

 
Respiration and nutrition of a common mussel (Mytilus edulis).
Source: Aquascope.
Like all bivalves, mussels are greatly affected by water pollution. Due to their filter-feeding lifestyle, they also tend to accumulate pollutants from seawater. Especially in the estuaries of heavily polluted rivers such as the Elbe River, as well as in harbour areas, the concentration of heavy metals in mussels can exceed the normal level by more than tenfold.

Economic Importance

The scientific name Mytilus goes back to the Greek philosopher Aristotle. "Μυτίλος" (mytilos) already referred to an edible mussel in ancient Greece. The Latin epithet edulis further emphasises the fact that mussels are edible - and very tasty indeed. Mussel fishing has been practised along the German coast in Lower Saxony and Schleswig-Holstein for more than a hundred years. In addition to the traditional harvesting of natural mussel beds (which locally have become quite rare), artificial mussel beds have been established in sheltered areas since the 1930s. These cultivated beds are less exposed to predators and thus produce larger mussels, which are ready for harvest after at least one year.

In Europe, the leading countries in mussel farming are the Netherlands and Denmark, with around 100,000 tonnes of mussels per year, compared to about 20,000 tonnes from the German federal states of Schleswig-Holstein and 8,000 tonnes from Lower Saxony.

Links

Literature

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Latest Change: 05.11.2025 (Robert Nordsieck).