Mussel beds in the Atlantic littoral zone
|Red List habitat type||code NEAA2.72|
|Source||European Red List habitat factsheet|
|European Red List of habitats reports|
|European Red List of habitats (Excel table)|
Sediment shores characterised by beds of the mussel Mytilus edulis occur principally on mid and lower shore mixed substrata (mainly cobbles and pebbles on muddy sediments) but also on sands and muds. In high densities (at least 30% cover) the mussels bind the substratum and provide a habitat for many infaunal and epibiotic species. This habitat is also found in lower shore tide-swept areas, such as in the tidal narrows of sealochs. A fauna of dense juvenile mussels may be found in sheltered firths, attached to algae on shores of pebbles, gravel, sand, mud and shell debris with a strandline of fucoids. Two associated biotopes are M.edulis beds on littoral mixed substrata and M.edulis beds on littoral sand.
The temporal stability of mussel beds can vary a lot. Some beds are permanent, maintained by recruitment of spat in amongst adults. Other beds are ephemeral, for example in locations where large amounts of spat settle intermittently on a cobble basement. In such situations mussels rapidly build up mud, and are unable to remain attached to the stable cobbles and are then liable to be washed away during gales. A second example of ephemeral mussel dominated biotopes occurs when mussel spat ("mussel crumble") settles on the superficial shell of cockle beds.
‘Mussel mud’, composed of faeces, pseudofaeces and sediment, accumulates underneath mussel beds. In sheltered habitats, pseudofaeces (undigested, filtered particles) can build up forming a thick layer of anoxic mud. The layer of mud may prevent the attachment of mussels to the underlying substratum, but the silt layer often consolidates and forms a firm clay bank which is very erosion resistant including the mussels embedded into it. ‘Mussel mud’ (that is not anoxic) supports a diverse range of infauna.
Indicators of quality:
Both biotic and abiotic indicators have been used to describe marine habitat quality. These include: the presence of characteristic species as well as those which are sensitive to the pressures the habitat may face; water quality parameters; levels of exposure to particular pressure, and more integratedindices which describe habitat structure and function, such as trophic index, or successional stages of development in habitats that have a natural cycle of change over time.
There are no commonly agreed indicators of quality for this habitat, although particular parameters may have been set in certain situations e.g. protected features within Natura 2000 sites, where reference values have been determined and applied on a location-specific basis.
The overall quality and continued occurrence of this habitat is largely dependent on the presence of Mytilus edulis which creates the biogenic structural complexity on which the characteristic associated communities depend. The density and the maintenance of a viable population of this species is a key indicator of habitat quality, together with the visual evidence of presence or absence of physical damage. Monitoring programmes may include measures of biomass, coverage, length frequency distribution, a condition index for the mussels (a ratio between biomass versus shell lenght) and descriptions of the structure of a bed including vertical height profile, thickness and type of accumulated sediment, coverage and biomass of macroalgae.
Synthesis of Red List assessment
There has been a significant decline in the extent and biomass of this habitat, in the Netherlands, Denmark and Germany both historically and in recent decades. The quality of this habitat has also been reduced by fishing as this regularly depletes the mussel beds. Invasive species are also an issue in some locations. In the German Waddensea, for example all eulitoral mussel beds are now inhabited by the invasive species Crassostrea gigas and many are dominated by this species to the extent that the biomass of C.gigas is sometimes 4 to 5 times higher than biomass of blue mussels.
ICES found sufficient evidence for the decline and threat of this habitat over the whole OSPAR area, and this habitat is on the list of threatened and/or declining species in the OSPAR area.
Where there is good evidence, the decline in extent in recent years has been greater than 50% (and in some cases >80%) however, as this is not the case for all examples of this habitat, the overall decline over the last 50 years is estimated to be >50%. There has been a very substantial reduction in biotic quality of this habitat over the last 50 years in many locations. Expert opinion is that this is at least a severe decline affecting more than 50% of the extent of this habitat. On this basis the habitat is assessed as being Endangered for both the EU 28 and EU 28+.
|Red List Category||Red List Criteria|
|Red List Category||Red List Criteria|
Confidence in the assessment
Pressures and threats
- Urbanisation, residential and commercial development
- Biological resource use other than agriculture & forestry
- Marine and Freshwater Aquaculture
- Bottom culture
- Fishing and harvesting aquatic resources
- Professional active fishing
- Benthic dredging
- Leisure fishing
- Bait digging / Collection
- Pollution to surface waters (limnic, terrestrial, marine & brackish)
- Nutrient enrichment (N, P, organic matter)
- Natural System modifications
- Human induced changes in hydraulic conditions
- Modification of hydrographic functioning, general
- Climate change
- Changes in abiotic conditions
- Temperature changes (e.g. rise of temperature & extremes)
- Changes in biotic conditions
- Migration of species (natural newcomers)
Habitat restoration potential
Blue mussels are sessile, attached organisms that are unable to repair significant damage to individuals. They do not reproduce asexually and therefore the only mechanism for recovery from significant impacts is larval recruitment to the bed or the area where previously a bed existed. Recruitment is often sporadic, occurring in unpredictable pulses, although persistent mussel beds can be maintained by relatively low levels or sporadic recruitment. Recovery from human activity impacts may take at least 5 years, although in certain circumstances and under some environmental conditions (e.g. recurring physical disturbance or sporadic recruitment) recovery may take significantly longer. Nearly complete recovery from disturbance is a characteristic common to Mytilus beds throughout the world.
Trends in extent
Average current trend in quantity
Trends in quality
Average current trend in quality
Conservation and management needs
List of conservation and management needs
- Measures related to wetland, freshwater and coastal habitats
- Restoring/Improving water quality
- Restoring/Improving the hydrological regime
- Measures related to marine habitats
- Restoring marine habitats
- Measures related to spatial planning
- Legal protection of habitats and species
- Measures related to hunting, taking and fishing and species management
- Regulation/Management of fishery in marine and brackish systems
- Specific single species or species group management measures
- Measures related to special resouce use
- Regulating/Managing exploitation of natural resources on sea
Geographic occurrence and trends
|Seas||Present or presence uncertain||Current area of habitat (Km2)||Recent trend in quantity (last 50 years)||Recent trend in quality (last 50 years)|
|Bay of Biscay and the Iberian Coast||Present||Unknown||Decreasing||Decreasing|
|Greater North Sea|
Extent of Occurrence, Area of Occupancy and habitat area
|Extent of Occurrence (EOO) (Km2)||Area of Occupancy (AOO)||Current estimated Total Area||Comment|
|EU28||584,502||184||Unknown||EOO and AOO have been calculated on the available data. Although this data set is known to be incomplete the figures exceed the thresholds for threatened status.|
|EU28+||184||Unknown||EOO and AOO have been calculated on the available data. Although this data set is known to be incomplete the figures exceed the thresholds for threatened status.|
EOO = the area (km2) of the envelope around all occurrences of a habitat (calculated by a minimum convex polygon).
|Species scientific name||English common name||Species group|