Mussel beds Modiolus modiolus on Atlantic sublittoral sediment
|Red List habitat type||code NEAA5.67|
|Source||European Red List habitat factsheet|
|European Red List of habitats reports|
|European Red List of habitats (Excel table)|
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.
Synthesis of Red List assessment
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.
|Red List Category||Red List Criteria|
|Near Threatened||A2b, C/D2.|
|Red List Category||Red List Criteria|
|Near Threatened||A2b, C/D2.|
Confidence in the assessment
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 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
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 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
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)|
|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.|
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|
Relation to other habitat types mentioned in legal instruments
|Legal text||Annex||Name in legal text||Code in legal text||
|Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora - consolidated version 01/01/2007||Annex I: natural habitat types of community interest whose conservation requires the designation of special areas of conservation||Reefs||1170||Narrower||http://ec.europa.eu/environm...rective/index_en.htm|