Red List habitat classification > RL - Marine habitats > RLBAL - Baltic > BAL20 Unvegetated communities on Baltic infralittoral shell gravel

Unvegetated communities on Baltic infralittoral shell gravel

Quick facts

Red List habitat type code BAL20
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)


This Baltic Sea benthic habitat occurs on with at least 90% coverage of shell gravel according to the HELCOM HUB classification.  Four biotopes and two sub-biotopes have been associated with this habitat. The sub-biotope 'Baltic photic shell gravel dominated by Mytilidae’ (AA.E1E1),  sessile/semi-sessile epibenthic bivalves cover at least 10% of the seabed and more than other perennial attached erect groups. This biotope is identified by a large representation of Mytilidae, at least 50% of the biomass among the epibenthic bivalves and often encountered in high energy exposure areas. It has been reported from the Baltic proper and Belt Sea. 'Baltic photic shell gravel dominated by vase tunicate (Ciona intestinalis) (AA.E1F1) is ony reported from the Belt Sea and occurs in areas where the bottom consists largely of mollusc shells or small shell fragments, often constituting small patches inside other sediments. Due to the combination of the extended interstitial space and the presence of biotic hard substrates, it is inhabited by a unique combination of endobenthic and epibenthic species, such as the vase tunicate (Ciona intestinalis). The biotopes 'Baltic photic shell gravel characterized by sparse or  by mixed epibenthic macrocommunities (AA.E2T and AA.E1V)' are characterised by a low (0-10%) coverage of macroscopic vegetation or sessile macroscopic epifauna and most often encountered in high energy exposure areas.

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 integrated indices 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.

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

Threat status

Synthesis of Red List assessment

This habitat is only present in the EU 28. The overall assessment for this EUNIS level 4 habitat has been based on the HELCOM (2013) assessments for the associated HELCOM HUB biotopes. Draft assessments were derived using a weighted approach whereby the HELCOM assessment outcomes were assigned a score. This was averaged across the relevant biotopes. The outcomes were reviewed by Baltic experts to reach a final conclusion. HELCOM (2013) assessed the associated biotope AA.E1E1 as Least Concern (A1) and biotope AA.E1F1 as Vulnerable (B1A[ii]).
There is a lack of quantitative data on baseline conditions and trends in quantity and quality of this habitat but declines of between 30-50% have been reported for one of the associated biotopes. Because of an estimated overall decline of 25% and likely future declines (linked to ocean acidification, eutrophication and activities which directly disturb the seabed) this habitat has been assessed as Near Threatened for both the EU 28 and EU 28+.
Red List Category Red List Criteria
Near Threatened A1, A2a
Red List Category Red List Criteria
Near Threatened A1, A2a

Confidence in the assessment

Red List of habitat categories and criteria descriptions

Pressures and threats

  • Mining, extraction of materials and energy production
    • Exploration and extraction of oil or gas
  • 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)
    • Nutrient enrichment (N, P, organic matter)
    • Input of contaminants (synthetic substances, non-synthetic substances, radionuclides) - diffuse sources, point sources, acute events
  • Natural System modifications
    • Human induced changes in hydraulic conditions
    • Removal of sediments (mud...)
    • Dredging/ Removal of limnic sediments
    • Estuarine and coastal dredging
    • Siltation rate changes, dumping, depositing of dredged deposits
    • Dumping, depositing of dredged deposits
    • Other siltation rate changes
  • Climate change
    • Changes in abiotic conditions
    • pH-changes

Habitat restoration potential


Trends in extent

Average current trend in quantity

Decreasing Decreasing
EU28 EU28+

Trends in quality

Average current trend in quality

Unknown Unknown
EU28 EU28+

Conservation and management needs

To conserve this habitat, its distribution should be mapped in order to better understand its environmental requirements. Where it occurs, it should be protected and bottom trawling prohibited. Further eutrophication should be stopped or reversed to improve the oxygen conditions and reduce overgrowth for example of annual brown algae on vase tunicates.

List of conservation and management needs

  • Measures related to wetland, freshwater and coastal habitats
    • Restoring/Improving water quality
  • Measures related to spatial planning
    • Establish protected areas/sites
  • Measures related to hunting, taking and fishing and species management
    • Regulation/Management of fishery in marine and brackish systems
  • Measures related to special resouce use
    • Regulating/Managing exploitation of natural resources on sea


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)
Baltic Proper Present Unknown Unknown Decreasing
Belt 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 Unknown Unknown Unknown There is insufficient quantitative data to make an accurate estimate of EOO and AOO.
EU28+ Unknown Unknown There is insufficient quantitative data to make an accurate estimate of EOO and AOO.
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.
Invertebrates Ciona intestinalis
Invertebrates Modiolus modiolus
Species scientific name English common name Species group
Ciona intestinalis Invertebrates
Modiolus modiolus Invertebrates

Vegetation types

Relation to vegetation types (syntaxa)

Not available

Other classifications

Not available
European Environment Agency (EEA)
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