Red List habitat classification > RL - Marine habitats > RLNEA - Atlantic > NEAA5.67 Mussel beds Modiolus modiolus on Atlantic sublittoral sediment

Mussel beds Modiolus modiolus on Atlantic sublittoral sediment

Quick facts

Red List habitat type code NEAA5.67
Threat status
Europe Near Threatened
EU Near Threatened
Relation to
Source European Red List habitat factsheet
European Red List of habitats reports
European Red List of habitats (Excel table)

Summary

Modiolus modiolus is an Arctic-Boreal species, with a distribution ranging from the seas around Scandinavia (including Skagerrak & Kattegat) and Iceland south to the Bay of Biscay. Beds of M. modiolus occur with a patchy distribution in the colder waters of the North-East Atlantic, with a range that extends from Russia in the southern parts of the Barents and White Seas, off Norway and down to Sweden (the Sound and Kattegat) and off the UK, in the southern North Sea and Irish Sea. Beds are also known to occur around Iceland and the Faeroes.

Modiolus beds form when horse mussels (Modiolus modiolus) aggregate together by attaching to each other and the substratum with byssal threads. The degree of aggregation at which individuals are considered to be sufficiently dense to constitute a “bed” has been determined to be >30% cover. The depth range over which M.modiolus forms beds is unknown, but they typically occur from the lower shore to about 70 m, with clumps found below 100 m in the Irish Sea. Off the Faeroes, they occur to about 200m depth, being most dense between 65-95 m. The majority of beds are located in current- swept fully-saline locations, although some can be found in sheltered bays, fjords or lochs, with some beds restricted to depths below haloclines.

Modiolus beds occur on a range of substrata, most often on cobbles through to muddy gravels but have also been found on bedrock. Beds are often persistent features which build up through accumulating faecal pellets, shell and trapped sand, so that they may become de-coupled from the substratum on which they were originally founded. They can be self-sustaining to the extent that spat survival is greatest in the crevices amongst the byssal threads of the mature clumps. A diverse range of epibiota and infauna often exists in these communities including hydroids, red seaweeds, solitary ascidians and bivalves such as Aequipecten opercularis and Chlamys varia. The clumping of the byssus threads of the M. modiolus creates a stable habitat that also attracts a very rich infaunal community including areas with a high density of polychaete species. 

 

Indicators of quality:

The condition of Modiolus beds may be judged in several different ways:

- Spatial integrity, such as whether fishing gear tracks cut across a bed.

- Topographic integrity, such as the continued presence of ridges, mounds and other biogenic relief.

- The size distributions of the mussels and whether the populations are being adequately renewed by successful spat settlement and juvenile survival through the first years when they are most vulnerable to predation.

- The abundance, composition, condition and diversity of the associated biota.

For physical disturbance impacts, changes in soft epifauna are more likely. Some of the vagile epifauna, such as brittlestars (e.g. Ophiothrix fragilis) are known to fluctuate markedly in abundance, so caution is needed when interpreting change. Damage also leads to increased abundance of scavengers, which are attracted to disturbed areas.

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

Threat status

Synthesis of Red List assessment

Modiolus modiolus is an Arctic-Boreal species, with a distribution ranging from the seas around Scandinavia (including Skagerrak & Kattegat) and Iceland south to the Bay of Biscay. Beds of M. modiolus occur with a patchy distribution in the colder waters of the North East Atlantic, with a range that extends from Russia in the southern parts of the Barents and White Seas, off Norway and down to Sweden (the Sound and Kattegat) and off the UK, in the southern North Sea and Irish Sea. Beds are also known to occur around Iceland and the Faeroes.
While some beds are well known previously unknown small beds of M. modiolus continue to be found even in areas where the distribution of coarse-scale benthic habitats is considered to be well documented. Maps have been produced for several countries and regions, but there remains uncertainty over the extent and occurrence of this habitat. Available evidence indicates that fully developed “beds”, where the mussels cover most of the sea floor creating biogenic reef structures, are scarce.
Very few individual beds have been studied over decadal time periods, so there is little firm evidence for decline across the entire habitat range. There are, however, clear instances where severe local declines have been caused by physical damage from trawling. There is also some evidence to suggest that the quality of Modiolus beds have declined over both historical and shorter timescales and that they continue to be vulnerable to human activities, particularly demersal fishing activity, but there are insufficient reliable data to quantify trends. OSPAR have identified this habitat as being under threat and/or declining in the all parts of the OSPAR area where it occurs and Modiolus modiolus is assessed as Vulnerable under the HELCOM species red list.
This habitat has been assessed as Near Threatened in both the EU 28 and EU 28+ because of recent and future threats.
EU
Red List Category Red List Criteria
Near Threatened A2b, C/D2.
Europe
Red List Category Red List Criteria
Near Threatened A2b, C/D2.

Confidence in the assessment

low
Red List of habitat categories and criteria descriptions

Pressures and threats

  • Biological resource use other than agriculture & forestry
    • Fishing and harvesting aquatic resources
    • Professional active fishing
    • Benthic or demersal trawling
    • Benthic dredging
  • Pollution
    • Pollution to surface waters (limnic, terrestrial, marine & brackish)
    • Diffuse pollution to surface waters due to agricultural and forestry activities
    • Diffuse pollution to surface waters due to household sewage and waste waters
    • Nutrient enrichment (N, P, organic matter)
  • Climate change
    • Habitat shifting and alteration

Habitat restoration potential

M.modiolus is a long-lived species and individuals within beds studied around the UK are frequently 25 or more years old. The species is considered to be highly intolerant to substratum loss, abrasion and physical damage. As recruitment is sporadic, varying with season, annually, with location, and hydrographic regime and is generally of low intensity, it may take many years for a population to recover from damage, if at all. Recovery has not been recorded from any damaged beds that have been studied. Efforts to restore beds through relocation have not been successful to date.

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 most effective conservation and management measure for this habitat is the restriction and control of activities that physically damage the bed structure. This would include regulation of demersal fisheries and shell fisheries, including exclusion of these activities from areas where this habitat occurs, both within and outside marine protected areas. This will also be important to enabling the recovery of this habitat in degraded areas. Mapping as well as long-term monitoring is needed to assess the condition of this habitat.

List of conservation and management needs

  • Measures related to marine habitats
    • Restoring marine habitats
  • Measures related to spatial planning
    • Establish protected areas/sites
    • 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

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)
Celtic Seas Present Unknown Decreasing Decreasing
Kattegat
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 365,273 69 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+ >69 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 Phycodrys rubens
Invertebrates Abietinaria abietina
Invertebrates Aequipecten opercularis
Invertebrates Alcyonium digitatum
Invertebrates Amphipholis squamata
Invertebrates Antedon bifida
Invertebrates Aonides paucibranchiata
Invertebrates Ascidiella aspersa
Invertebrates Buccinum undatum
Invertebrates Chlamys varia
Invertebrates Ciona intestinalis
Invertebrates Corella parallelogramma
Invertebrates Dendrodoa grossularia
Invertebrates Glycera lapidum
Invertebrates Hyas araneus
Invertebrates Kirchenpaueria pinnata
Invertebrates Laonice bahusiensis
Invertebrates Mediomastus fragilis
Invertebrates Modiolus modiolus
Invertebrates Ophiocomina nigra
Invertebrates Ophiopholis aculeata
Invertebrates Ophiothrix fragilis
Invertebrates Pagurus bernhardus
Invertebrates Paradoneis lyra
Invertebrates Sertularia argentea
Invertebrates Spisula elliptica
Invertebrates Timoclea ovata
Invertebrates Urticina felina
Species scientific name English common name Species group
Phycodrys rubens Algae
Abietinaria abietina Invertebrates
Aequipecten opercularis Invertebrates
Alcyonium digitatum Invertebrates
Amphipholis squamata Invertebrates
Antedon bifida Invertebrates
Aonides paucibranchiata Invertebrates
Ascidiella aspersa Invertebrates
Buccinum undatum Invertebrates
Chlamys varia Invertebrates
Ciona intestinalis Invertebrates
Corella parallelogramma Invertebrates
Dendrodoa grossularia Invertebrates
Glycera lapidum Invertebrates
Hyas araneus Invertebrates
Kirchenpaueria pinnata Invertebrates
Laonice bahusiensis Invertebrates
Mediomastus fragilis Invertebrates
Modiolus modiolus Invertebrates
Ophiocomina nigra Invertebrates
Ophiopholis aculeata Invertebrates
Ophiothrix fragilis Invertebrates
Pagurus bernhardus Invertebrates
Paradoneis lyra Invertebrates
Sertularia argentea Invertebrates
Spisula elliptica Invertebrates
Timoclea ovata Invertebrates
Urticina felina 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