Red List habitat classification > RL - Marine habitats > RLMED - Mediterranean > MEDA5.51 Rhodolith beds in the Mediterranean

Rhodolith beds in the Mediterranean

Quick facts

Red List habitat type code MEDA5.51
Threat status
Europe Data Deficient
EU Data Deficient
Relation to
Source European Red List habitat factsheet
European Red List of habitats reports
European Red List of habitats (Excel table)


Mediterranean rhodolith beds could be found between 20-150 m depth in normal marine conditions. The most common species is Lithothamnion corallioides and Phymatolithon calcareum is recorded less frequently. Different dominant species characterize the Mediterranean rhodolith beds, probably on the basis of biogeography and local environmental conditions.Characteristic associations include: Association with rhodoliths in coarse sands and fine gravels under the influence of bottom currents; and Association with rhodoliths on coastal detritic bottoms.

Rhodolith beds are defined by those sedimentary bottoms characterised by any morphology and species of unattached nongeniculate calcareous red algae (incompletely-coated grains excluded) with >10% of live cover. They occur in coarse clean sediments of gravels, clean sands and coastal detritic areas under the influence of bottom currents, which occur either on the open coast or in tide-swept channels of marine inlets (the latter often stony).

In general, fluent unidirectional hydrodynamism and laminar currents affect the seafloor. In the Mediterranean, the most favourable environment for this habitat to occur in biotopes with laminar bottom currents with a regular course. Changes in hydrodynamic conditions and intensity of light differentiate the presence of different associations, particularly in the community of the coastal detritic bottoms (circalittoral). The most frequent species of epiflora are Arthrocladia villosa and Sporochnus pedunculatus. Differences in intensity and typology of hydrodynamism and in species composition are able to determine differences in the growth form, structure and shape of the rhodoliths that can be summarized into three main morphologies: small and compact pralines, unattached branches, and large, irregular, boxwork rhodoliths. In the lower infralittoral zone this assemblage forms patches of organogenous pebbles of branched rhodoliths in various development stages. In the circalittoral zone the free calcareous algae formation can cover up to several square kilometres. Rhodolith beds can be considered as authentic carbonate factories, since they are among the highest producers of biogenic particles in European seas. The living part of the association is limited only to the surface of some centimetres of thickness. Subhabitat scomposed of non-nucleated, unattached growths of branching, twig-like coralline can be distinguished as maerl beds.

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

Threat status

Synthesis of Red List assessment

There are no published data on trends of this habitat and very little available information about its extent of occurrence or its area of occupancy. There have been no quantitative analyses examining the probability of extinction of Mediterranean rhodolith beds. As a result, there is insufficient data to properly assess the habitat against any of the red list criteria, and it is listed as Data Deficient for both EU 28 and EU +28.
Red List Category Red List Criteria
Data Deficient -
Red List Category Red List Criteria
Data Deficient -

Confidence in the assessment

Red List of habitat categories and criteria descriptions

Pressures and threats

  • Biological resource use other than agriculture & forestry
    • Suspension culture
    • Benthic or demersal trawling
    • Benthic dredging
  • Human intrusions and disturbances
    • Shallow surface abrasion/ Mechanical damage to seabed surface
    • Penetration/ Disturbance below surface of the seabed
  • Pollution
    • Pollution to surface waters by industrial plants
    • Pollution to surface waters by storm overflows
    • Other point source pollution to surface water
    • Toxic chemical discharge from material dumped at sea
  • Natural System modifications
    • Landfill, land reclamation and drying out, general
    • Removal of sediments (mud...)
  • Climate change
    • Temperature changes (e.g. rise of temperature & extremes)

Habitat restoration potential

Being among the slowest-growing organisms (up to a few mm per year), rhodolith algae are exceptionally vulnerable to any mechanical disturbance or those impacts that increase sedimentation and turbidity making the habitat less resilient to disturbance with a slow recovery capacity. Substantial deposits take centuries, or millennia, to accumulate. Moreover, the anthropogenic physical damage can change the species diversity and functional relationships.

Trends in extent

Average current trend in quantity

Unknown Unknown
EU28 EU28+

Trends in quality

Average current trend in quality

Unknown Unknown
EU28 EU28+

Conservation and management needs

A special Action Plan for the protection of Mediterranean coralligenous and maerl assemblages has been recently adopted within the framework of the United Nations Environment Programme’s Mediterranean Action Plan (UNEP-MAP). Besides, the European Commission's ‘Habitats Directive’ mandates the conservation management of two of the main European maerl-forming species, Phymatolithon calcareum and Lithothamnion corallioides and under European law (No 1967/2006) and destructive fishing is prohibited over Mediterranean maerl beds.
Marine reserves and MPAs have worked well in protecting local rhodolith beds from the effects of towed fishing gear as in the case of Tabarca Island. Rhodolith beds are considered as being vulnerable habitats by France and have been included within a proposed network of conservation areas termed ‘Natura 2000’ sites.
However, the lack of relevant geospatial data of the distribution of this habitat in the Mediterranean countries, significantly hinders the effective implementation of these policies. Among key conservation and management measures proposed are:
- Change the term "maerl" in the legal EU and Barcelona policy documents to "rhodoliths" as to conform the origin of the terms and their international scientific use, and to cover also the calcareous Peyssonnelia beds;
- Development of a moratoria on the issue of further permits for the siting of aquaculture units above rhodolith grounds;
- Setting up monitoring programmes of the health of rhodolith beds; including the collection of quantitative data about community composition;
- Designation of ‘no-take’ reserves and MPAs where these habitats occur;
- Establishing measures to limit the impacts that might affect water quality above rhodolith beds;
- Other Mediterranean endemics should be considered for protection (i.e.: Lithophyllum racemus, Lithothamnion minervae, Lithothamnion valens)
- Given its importance and current threats, should be evaluated as priority habitats for the Annex I of Habitat Directive

List of conservation and management needs

  • Measures related to wetland, freshwater and coastal habitats
    • Restoring/Improving water quality
  • Measures related to marine habitats
    • Other marine-related measures
  • 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)
Adriatic Sea Present Unknown Unknown Unknown
Aegian-Levantine Sea
Ionian Sea and the Central Mediterranean Sea
Western Mediterranean 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 2,182,108 136 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+ 146 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

Not available

Vegetation types

Relation to vegetation types (syntaxa)

Not available

Other classifications

Not available
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