Red List habitat classification > RL - Marine habitats > RLNEA - Atlantic > NEAA2.72 Mussel beds in the Atlantic littoral zone

Mussel beds in the Atlantic littoral zone

Quick facts

Red List habitat type code NEAA2.72
Threat status
Europe Endangered
EU Endangered
Relation to
Source European Red List habitat factsheet
European Red List of habitats reports
European Red List of habitats (Excel table)

Summary

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. 

Characteristic species
For full habitat description, please download the habitat factsheet.

Threat status

Synthesis of Red List assessment

When determining trends in this habitat it is important to recognise that 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. Many mussel beds are subject to total destruction by storms, ice drifts and tidal surges and on occasion, this may involve hundreds of hectares. Trend analysis also needs to take account of the fact that many intertidal Mytilus beds are subject to relaying and commercial exploitation.
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+.
EU
Red List Category Red List Criteria
Endangered A1, C/D1
Europe
Red List Category Red List Criteria
Endangered A1, C/D1

Confidence in the assessment

medium
Red List of habitat categories and criteria descriptions

Pressures and threats

  • Urbanisation, residential and commercial development
    • Discharges
  • 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
    • 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

The mussel beds which characterise this habitat may be transient and dynamic or permanent and persistent. Their capacity to recover is generally strong where there are good spatfalls although development into established beds will be influenced by many factors, such as the presence of predators, local hydrographic conditions, and exposure of the location.
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

Decreasing Decreasing
EU28 EU28+

Trends in quality

Average current trend in quality

Decreasing Decreasing
EU28 EU28+

Conservation and management needs

The main management measures, which would assist the conservation of this habitat, are the regulation of the fisheries which target the mussel beds and protection from physical damage. Specific measures include control of fisheries through quotas, closed areas, specified fishing methods, regulations on the movement of spat including collection of spat for aquaculture, and prohibiting spat collection from intertidal areas.

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

Distribution

For each habitat a distribution map was produced from a wide variety of sources indicating known and potential occurrences of the habitat in 10x10 km grids within Europe. Occurrences in grid cells were given in two classes: actual distribution from relatively reliable sources (surveys, expert knowledge), and potential distribution based on models or less reliable indicators. Please download the fact sheet to see the map.

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
Celtic Seas
Greater North Sea
Kattegat

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.
AOO = the area occupied by a habitat measured in number of 10x10 km grid cells.
EOO = the area (km2) of the envelope around all occurrences of a habitat (calculated by a minimum convex polygon).

Characteristic species

The full list of characteristic species and genus are available above from the Summary. The species available in the EUNIS database are shown here.
Algae Fucus vesiculosus
Invertebrates Arenicola marina
Invertebrates Balanus crenatus
Invertebrates Carcinus maenas
Invertebrates Cerastoderma edule
Invertebrates Elminius modestus
Invertebrates Lanice conchilega
Invertebrates Littorina littorea
Invertebrates Mytilus edulis
Invertebrates Patella vulgata
Invertebrates Semibalanus balanoides
Species scientific name English common name Species group
Fucus vesiculosus Algae
Arenicola marina Invertebrates
Balanus crenatus Invertebrates
Carcinus maenas Invertebrates
Cerastoderma edule Invertebrates
Elminius modestus Invertebrates
Lanice conchilega Invertebrates
Littorina littorea Invertebrates
Mytilus edulis Invertebrates
Patella vulgata Invertebrates
Semibalanus balanoides Invertebrates

Vegetation types

Relation to vegetation types (syntaxa)

Not available

Other classifications

Not available
European Environment Agency (EEA)
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1050 Copenhagen K
Denmark
Phone: +45 3336 7100