Phymatolithon calcareum | Common Maërl



Conservation status

Future Prospects
Overal Assessment of Conservation Status
Overal trend in Conservation Status

Source: NPWS 2013.

IUCN Conservation Status*
Not evaluated
Europe (1)
Not evaluated
Global (1)Not evaluated

Source: (1) The IUCN Red List of Threatened Species. Version 2014.2 2014.

* There is currently no IUCN Irish, European or global Red List assessment for Phymatolithon calcareum. The IUCN has identified Marine macro-algae as a priority taxon for assessment. A Global Marine Species Assessment program was initiated by the IUCN in 2005, however as of 2014 global Marine macro-algae assessments have not been completed, although some national assessments for macro-algae, e.g. by Australia, have been done.

Sources: The IUCN Red List of Threatened Species. Version 2014.2; Global Marine Species Assessment website.

Legal status

Protected by the following legal instruments:

  • Habitats Directive [92/42/EEC] Annex V
  • The Convention for the Protection of the marine Environment of the North-East Atlantic (OSPAR)

Native status


Species Biology


The identification of individual maërl bed forming species in the field may be fraught with difficulty as, depending on a range of environmental factors as well seasonality and growth stage, closely related species may look very dissimilar while more distantly related species may look very similar. With experience and knowledge of local factors identification to species level in the field may be possible. However de-calcifying and microscopical examination may be necessary, and in some cases examination under Scanning Electron Microscope may be recommended, in order to resolve samples to species level.

  • Phymatolithon calcareum is a red algae of the order Corallinales an order characterised, in part, by the laying down of calcite in the cell walls of vegetative cells.
  • P. calcareum is included in the sub-family Melobesioideae, which is characterised, in part, by species with non-geniculate thalli.
  • Microscopically, some cells within contiguous filaments in the thallus are joined by cell fusions rather than by secondary pit connections (i.e. pit connections formed between unrelated adult cells in contiguous filaments, not to be confused with primary pit connections formed between daughter cells of a dividing filament cell).
  • Tetra-sporangial conceptacles are multi-porate. (currently bi-sporangial and gametangial plants are unknown for P. calcareum in Irish waters).
  • Crustose thalli for P. calcareum have not been recorded in Irish or British waters as of 2014.
  • Epithallial cells appear domed or block shaped in vertical section, not winged.
  • The thallus in  Phymatolithon species are monomerous with multi-layered 'medullary' filaments, the thallus in general over 10 cells in thickness.

  • Thallus cells are not arranged concentrically.
  • Species in the genus Leptophytum share the characteristics outlined above. Phymatolithon species may be differentiated from Leptophytum based on shape of the outermost epithallial cells. In Phymatolithon these are disctinctly rectangular or block shaped, in Leptophytum these are rather compressed dorso-ventrally. There are also differences in the shape of cells surrounding the plugs blocking the pores to the tetrasporangial conceptacles, but a Scanning Electron Microscope may be necessary to examine this character.
  • In the field Phymatolithon calcareum can closely resemble Lithothamnion corallioides and Lithothamnion glaciale which are in the same sub-family but microscopically epithallial cells in those appear winged in vertical section.
  • P. calcareum is often a purple-brown when fresh, with a very matt surface, and robust branches that are generally over 1mm in diameter.  Thallus surface may be slightly bumpy.
  • Lithothamnion corallioides, when fresh, is a dull brown-pink, with an almost glossy, slightly bumpy surface and has rather brittle branches generally under 1mm in diameter.
  • Lithothamnion glaciale however is often a richer pink, occasionally bluish-pink with a smoother, matt surface. Branches are more robust and somewhat club shaped and of varying thickness.

Source: Irvine, Linda M. and Chamberlain, Yvonne M. 2011.


Habitats may include but are not necessarily limited to;

  • Sea inlets and bays (MW2)

Source: NPWS 2013a; Fossitt, J.A. 2001.


In British and Irish waters only tetrasporangial conceptacles are known. Tetraspores, as the name implies are spores produced in groups of four. Spore production occurs in enclosed 'conceptacles' or chambers which are located near the surface of the thallus with openings to allow the spores to be dispersed. Spores are presumed to be haploid, produced by meiosis.

It may be the case that at least in British and Irish waters that most reproduction for unattached plants is by vegetative propogation from fragments of branched thalli.

Source: Irvine, Linda M. and Chamberlain, Yvonne M. 2011.

Threats faced

Fishing and harvesting aquatic resources
Suspension culture
Intensive fish farming, intensification
Bottom culture
Nautical sports

Source: NPWS 2013.

There are as yet no IUCN Red List assessments available at a national, European or global level as yet (see Conservation Status above).

'Maërl beds' appear on the OSPAR List of Threatened and / or Declining Species and Habitats. In Irish waters Phymatolithon calcareum is a major component of such maërl beds. The following threats identified in a Background Document prepared following the inclusion of 'Maërl beds' on the OSPAR list;

  • Damage to the surface of beds by;
  1. heavy demersal fishing gear.
  2. pollution by inshore finfish and shellfish aquaculture operations.
  3. suction dredging for bivalves.

  • Increase in sediment loads and / or excessive growth of ephemeral species of macroalgae resulting from coastal construction and increases in agricultural and sewage discharges.

Source:Hall-Spencer et al 2010.

The E.U. funded BIOMAERL Project (1996-1999) listed a number of threats to Maërl beds . These included the following;


Very important
Fishing trawl gears
Very importantDirect
Very importantDirect
Direct effluent discharges
Very importantDirect
Land reclamation
Very importantDirect
Coastal structures
Very importantDirect
Alien species
Off-shore dumping

Source: BIOMAERL team, 2003.

Conservation actions

In the Article 17 Habitats Directive reporting for the period 2007-2012 no Conservation Measures in place or in the process of being implemented during the period were listed for this species.

There are as yet no IUCN Red List assessments available at an Irish, European or global level as yet (see Conservation Status above).

The E.U. funded BIOMAERL Project (1996-1999) suggested a number of conservation measure that may go some way to reversing declines in Maërl bed conservation status. These included the following;

  • Apply higher conservation status to maerl habitats and maerl forming species in European legislation.
  • Carry out of further research on maerl ecosystems.
  • Recognise maerl beds as non-renewable resources that cannot sustain direct exploitation.
  • Place prohibitions on the use of towed gear on maerl grounds.
  • Place moratoria on the issue of further permits for the siting of aquaculture units above maerl grounds.
  • Monitor existing exploited or impacted maerl beds.
  • Designate ‘no-take’ reserves.
  • Put in place measures to limit the impacts that might affect water quality above maerl beds.
  • Discourage the mooring of fixed-cage aquaculture facilities for fin-fish or bivalve molluscs above maerl beds.
  • Develop a programme of monitoring of the ‘health’ of European maerl beds.
  • Develop an awareness campaign on the biological importance of maerl beds.
  • Discourage the mooring of fixed-cage aquaculture facilities for fin-fish or bivalve molluscs above maerl beds.
  • Ensure that statutory environmental quality standards require existing mariculture facilities over maerl grounds to monitor impacts on the maerl ecosystem .
  • Discourage permanent anchorages over maerl beds.
Source: BIOMAERL team, 2003.


World distribution(GBIF)

Most records are north west Eurpoean, centered mainly on Britain and Ireland , Norway and southern Sweden. Also records from Atlantic coasts of France and Portugal as well as Mediterranean coasts of France and Spain. There are scattered reports from the eastern and western seaboards of North America as well as western Australia.

Accuracy of world distribution shown in the Global Biodiversity Information Facility (GBIF) map below will be constrained by, amongst other factors, data held but not shared by countries and organizations not participating in the GBIF.

Irish distribution

Most records are from counties with Atlantic coastlines, principally Cork, Kerry, Galway and Donegal, but also on north east coast of Northern Ireland.

Temporal change

Records submitted to Data Centre in 2023

The following map is interactive. If you would prefer to view it full screen then click here.

How can you help

The National Biodiversity Data Centre is trying to improve our knowledge on the distribution of Common Maërl (Phymatolithon calcareum) in Ireland. Should you observe this species, please submit sightings to add to the database. Detailed observations will assist us in gaining a better insight into where the species is most abundant in Ireland and we might also be able to detect regional variations. Please submit any sightings and photographs at:

All records submitted online can be viewed on Google Maps – once checked and validated these will be added to the database and made available for conservation and research.

Further information

For further information contact Dr. Liam Lysaght



BIOMAERL team, 2003. Conservation and management of N.E Atlantic and Mediterranean maerl beds. Aquatic Conservation: Marine and Freshwater Ecosystems, 13 (S1): 65-76

De Grave S., Fazakerley H., Kelly L., Guiry M. D., Ryan M., & Walshe J. 2000. A Study of Selected Maërl Beds in Irish Waters and their Potential for Sustainable Extraction. Marine Institute.

Hall-Spencer, Jason M., Kelly John, Mags, Christin A. 2010. Background Document for Maërl Beds. Department of Environment, Heritage & Local Government.

Irvine, Linda M. and Chamberlain, Yvonne M. 2011. Seaweeds of the British Isles, Volume I Rhodophyta Part 2B Corallinales, Hildenbrandiales. Pelagic Press.

The IUCN Red List of Threatened Species. Version 2014.2. <>. Downloaded on 04 November 2014.

Additional comments

Phymatolithon calcareum is one of a number of species of calcified red seaweeds that contribute to the formation of extensive beds made up of dead and living thalli of these seaweeds. The species are collectively termed maërl. The sometimes extensive beds, which are intermingled with non-living substrate along with other biota, may be considered a habitat of a type. Although maërl beds are not specifically listed in Annex I of the Habitats Directive, which deals with habitats, inclusion of Phymatolithon calcareum and Lithothamnion corallioides in Annex V offers some degree of de facto protection to the habitat type. 

Additionally the broader habitat type in which both maërl species and maërl beds occur coincide with habitat types listed in Annex I of the Habitats Directive.