Margaritifera (Margaritifera) margaritifera | Freshwater Pearl Mussel



Conservation status

Future Prospects
Overall Assessment of Conservation Status
Overall Trend in Conservation Status
Source: NPWS 2013.

IUCN Conservation Status

Ireland (1)Critically Endangered [A3cde+4cde]
Europe (2)Critically Endangered [A2b]
Global (3)*Endangered [A1ce+2c]

*This assessment is acknowledged as needing to be updated.

Sources: (1) Byrne, A. et al 2009; (2) Moorkens, E., 2011 (3) Mollusc Specialist Group, 1996.

Legal status

Protected by the following legal instruments:

  • Habitats Directive [92/42/EEC] Annex II, Annex IV
  • Wildlife Act (1976)
  • European Communities Environmental Objectives (Freshwater Pearl Mussel) Regulations (2009)
  • Wildlife (N.I.) Order (1985)
  • Environment (N.I.) Order (2002).

Native status


Species Biology


One of three Irish freshwater bivalves over 50mm. The Freshwater Pearl Mussel (Margaritifera margaritifera) is the ony one with a heavy, black shell (as an adult) with the umbone (apex) often eroded.  Shell length can reach 9-14cms.

Source: Moorkens, E. A., 1999.


Margaritifera margaritifera populations are found in relatively fast flowing water with a high oxygen content, low nutrient and low calcium content, mainly over non-calcareous rock. Substrate is ideally gravel or gravel and coarse sand with no silt and free of extensive filamentous algae coverage.

Watercourses with high water quality (Q value) are strongly associated with Freshwater Pearl Mussel populations and especially those populations where there is juvenile recruitment. In Ireland rivers with high Q values are generally at higher elevations with lower stream order.

Populations comprised solely of adults, where juveniles cannot be recruited due to siltation of stream-bed interstitial spaces, still exist in lowland rivers due to the great age achievable by the species.These populations would have been extant before the threats currently faced by the species became prevalent.

Sources: Moorkens, E. A., 2000; Moorkens, E., 2011.

Habitats are;

  • Eroding / upland rivers (FW1)
  • Depositing / lowland rivers (FW2)

Source: Moorkens, E. A.,  2000; Fossitt, J.A. 2001.


Male releases sperm through exhalant siphon which are received  by the female through inhalant siphon.

After fertilization the eggs develop into a larval stage known as 'glochidia'. These are brooded by the female in her gills until being released at about .08mm in length.

In Ireland once released into the the water column the glochidia, if inhaled by Atlantic Salmon (Salmo salar) or native Brown Trout (Salmo trutta), will attempt to attach to the gills of the fish.

The release of glochidia by female Freshwater Pearl Mussel in Ireland is timed between August and September and c. 9 million can be released per female.

The glochidia will remain attached to the fish host until of a size to survive independently. Within a year from attatchment to the fish host the glochidia will drop off and bury themselves completely in the substrate where they will remain for about 5 years. Buried in interstitial spaces and filter feeding renders this young stage of the Freshwater Pearl Mussel highly vulnerable to stress or death in the event of silt build-up.

Sexually mature between 7 and 15 years and can live to over 100 years of age.

Sources: Moorkens, E. A. 1999; Freshwater Pearl Mussel FPM Project Website

Threats faced

Modification of hydrographic functioning, generalJ02.05High
Other human induced changes in hydrographic conditions
Restructuring agricultural landholding
Water abstractions from groundwaterJ02.07High
Diffuse pollution to surface waters due to agricultural and forestry activities
Pollution to surface waters by industrial plants
Diffuse pollution to surface waters due to other sources not listed
Pollution to surface water by storm overflows H01.02Medium
Other point source pollution to surface water
Diffuse pollution to surface waters due to household sewage and waste waters
Surface water abstractions for public water supply
Collapse of terrain, landslide
Dredging / removal of limnic sediments
Other outdoor sports and leisure activities
Changes in abiotic conditions

Source: NPWS 2013.

These threats accord well with the threats outlined in the 2011 European level IUCN Red List for the species which also included as threats;

  • Interspecific Faunal Relations [with regards changes in population structure of the host fish species for M. margaritifera glochidia - principally Salo salar.]
  • Genetic Pollution [with regards to competition between the host fish species for M. margaritifera glochidia and introduced fish species].
  • Climate Change
  • Antagonism arising from the introduction of species

Source:Moorkens, E. (2011).

The 2009 Irish Red list of non-marine molluscs identified the following as major threats to the species;

  • Reduction in water quality
  • Increases in siltation and
  • Physical interference with habitat 

The finding that 90% of populations in Ireland are unlikely to breed again as a result of the impact of these threats was re-itereated in the account for Margaritifera margaritifera.

Source: Byrne, A. et al 2009.

Conservation actions

In the Article 17 Habitats Directive reporting for the period 2007-2012 Conservation Measures implemented or being implemented during the period were;

  • legal measures to protect the species
  • legal measures to restore / improve water quality,
  • administrative measures to produce single species or species group management measures
  • administrative measures to manage urban and industrial waste.

The Conservation  Actions section in the 2011 Europe level IUCN Red List suggests;

  • management plans on a catchement by catchment basis with a compensation policy for farmers undertaking compatible practices included as part of the management plan.
  • a captive breeding program for areas where populations have become greatly depleted, in a addition to a management plan.

Sources: NPWS 2013.


World distribution(GBIF)

Found throughout the north temperate zone, ranging from the east and west coast of North America to the British Isles and Scandinavia, with populations scattered throughout mainland Europe.

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

Temporal change

Records submitted to Data Centre in 2021

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

How can you help

The National BiodiversityData Centre is trying to improve our knowledge on the distribution of the Freshwater Pearl Mussel in Ireland. Should you observe this species, please submit sightings to add to the database. Detailed observations will assist us gaining a better insight into where the Freshwater Pearl Mussel 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



Byrne, A., Moorkens, E.A., Anderson, R., Killeen, I.J. & Regan, E.C. (2009) Ireland Red List No. 2 – Non-Marine Molluscs. National Parks and Wildlife Service, Department of the Environment, Heritage and Local Government, Dublin, Ireland.

Fossitt, J.A. (2001) A Guide to Habitats in Ireland. The Heritage Council.

Mollusc Specialist Group 1996. Margaritifera margaritifera. The IUCN Red List of Threatened Species. Version 2014.3. <>. Downloaded on 06 January 2015.

Mollusc Specialist Group 1996a. Margaritifera margaritifera ssp. durrovensis. The IUCN Red List of Threatened Species. Version 2014.3. <>. Downloaded on 06 January 2015.

Moorkens, E. A. (1999) Conservation Management of the Freshwater Pearl Mussel Margaritifera margaritifera. Part 1: Biology of the species and its present situation in Ireland.Irish Wildlife Manuals, No. 8.

Moorkens E. A.(2000) Conservation Management of the Freshwater Pearl Mussel Margaritifera margaritifera. Part 2: Water Quality Requirements. Irish Wildlife Manuals, No. 9.

Moorkens, E. (2011). Margaritifera margaritifera. The IUCN Red List of Threatened Species. Version 2014.2. Downloaded on 19 September 2014.

NPWS (2013) The Status of EU Protected Habitats and Species in Ireland.  Species Assessments Volume 3. Version 1.0. Unpublished Report, National Parks & Wildlife Services. Department of Arts, Heritage and the Gaeltacht, Dublin, Ireland.

Additional comments

In most locations in Ireland Margaritifera margaritifera occurs in rivers that are more acidic in nature. There is however a population occuring in calcareous conditions along a short stretch of the river Nore. Apart from the difference in pH status of the environment, there are also morphological differences in this population, most notably the lack of erosion to the umbone in adult individuals. This population has variously been described as a species (Margaritifera durrovensis), a subspecies (Margaritifera margaritifera ssp. durrovensis) or as an ecophenotype of Margaritifera margaritifera. It has been referred to by the common name Nore Freshwater Pearl Mussel.

In 1996 the IUCN assessed this as Margaritifera margaritifera ssp. durrovensis and the 2013 Habitats Directive article 17 reporting for Ireland assessed it as Margaritifera durrovensis in line with recommendations on Article 17 reporting. At a national level a red list assessment in 2009 assessed it as Margaritifera durrovensis. There is still (2014) not full agreement on the taxonomic status of this population.

The 2009 national red listing assessed the Nore Freshwater Pearl Mussel as Critically Endangered. The 1996 IUCN global assessment was also critically endangered. The 2013 Habitats Directive article 17 reporting for Ireland Overall Assessment of Conservation Status was bad and Overall Trend in Conservation Status as declining.

Sources: Byrne, A. et al 2009; Mollusc Specialist Group 1996a; NPWS 2013; European Topic Centre on Biological Diversity.