Northern Ireland's Priority Species

Margaritifera margaritifera – freshwater pearl mussel

Margaritifera margaritifera

Margaritifera margaritifera (L., 1758)
Family: Margaritiferidae

The freshwater pearl mussel is a large (up to 140mm) and solid bivalve shell which lives in fast-flowing, clean rivers. Like some marine bivalves including the oyster, it can produce pearls from the mother-of-pearl nacre secreted on the inside of its valves. There have been semi-commercial fisheries for pearls in areas where it is common until quite recently, but severe decline over most of its range has placed it on the conservation agenda and commercial exploitation has been halted.

In brief

  • Only three substantial populations remain in Northern Ireland, in the Cladagh (Swanlinbar) River, the Owenkillew River and the Upper Ballinderry River

  • One of these has recently suffered a 55% decline

  • Endangered and in severe decline throughout its range in Europe

  • Listed as a UK Priority species

  • Lives in fast-flowing, clean rivers

  • Requires large aggregations in a local area to breed successfully

  • Also requires good populations of salmonid fish to be present

  • Larvae (glochidia) attach to the gills of juvenile salmonid fish for a short period during the summer breeding period

  • Threatened by river drainage schemes, fertilizer run-off, conifer planting on upland peat, pearl fishing, climate change and salmonid over-fishing.

Species description
A large, solid shell which is usually embedded with large aggregations of other individuals in suitable fine sediment in the beds of clean, fast-flowing, highland rivers (Figure 1).

Life cycle
Successful breeding requires healthy and abundant salmon and trout populations to be present. Pearl mussels produce free-swimming larvae which must find the gills of a young salmonid fish within a short time during the summer breeding season. They encyst in the gills of the young fish and are carried around until the following spring when they hatch and fall to the bottom of the river to start life as a mussel. Apart from this bizarre start to life mussels sit unmoving in sand and gravel beds on the bottom of streams for up to 50 years filtering microscopic plants from the water as food.

Similar species
The swan and duck mussels (Anodonta species) are of a similar size (Figure 2) but inhabit slow-moving, lowland waters and lack the complex life history of the pearl mussel. They are both thinner-shelled and more angular with a greener or more yellow cast and shiny coating (periostracum) to the shell. The pearl mussel is heavier and has a much rougher and darker, duller periostracum and a more rounded, less angular, shape.

How to see this species
A visit to the trout hatchery at Coagh on the Ballinderry River is probably the only way to see this species nowadays. An experimental hatchery for pearl mussel has been set up with a view to supporting struggling wild populations or to reintroducing the species to rivers from which it has disappeared. Details are given below.

Current status
In Northern Ireland there are records of the species, since 1970, for 14 10km squares. Two rivers were found to support populations of around 10,000 individuals and one a population in excess of 800 individuals. The total for all remaining sites was only between 1000 and 2000 individuals. Very recently there has been a severe decline in one of the large populations with about 55 per cent fewer individuals than previously estimated. Essentially, the species is in severe decline and on the verge of extinction in Northern Ireland. This is especially true because the species has a complex life cycle and is able to breed successfully only when there are large concentrations of individuals present in a section of river (see below). Populations are becoming increasingly scattered and most remaining individuals are old with very few young specimens present.

In Ireland, the species is still widespread in parts of the west and south-west with at least three rivers containing populations in excess of 1,000,000 individuals. A large, thick-shelled form of the species called var. durrovensis, is unique to Ireland and is confined to the River Suir, where it is now thought to be close to extinction.

It is now reduced to single populations in England and Wales but still common in parts of Scotland where there are estimated to be 50 rivers supporting viable populations. Recent surveys have indicated that the species may still exist in 151 10km squares.

Historically, the species was extremely common in suitable rivers throughout most of Europe from the arctic and temperate regions of western Russia, to the Iberian peninsula and on the north-eastern seaboard of North America. But its range and abundance are now much reduced.

Why is this species a priority in Northern Ireland?

  • Listed as a UK Priority Species

  • Declining steadily in Northern Ireland with only three medium-sized populations remaining, plus scattered smaller populations, that is, rapid decline and scarce.

Threats/Causes of decline

  • Damage to suitable areas of streambed by river engineering and drainage operations

  • Modern intensive agriculture with high stocking rates and use of fertilizer or slurry exacerbated by field drainage operations, leading to enrichment and pollution

  • Planting of forests in deep peat in the uplands leading to increases in enrichment and siltation

  • Pearl fishing, despite being prohibited by law, may still be carried on in some catchments.

  • Climate change represents a threat, with summer water and oxygen levels declining in streams and temperatures rising.

Conservation of this species

Current action
There is a Northern Ireland Species Action Plan which was published in March 2005. There is also a UK Species Action Plan which was published in 1995.

  • The Cladagh (Swanlinbar) River, the Owenkillew River and the Upper Ballinderry River are designated as ASSIs and SACs, with M. margaritifera identified as a SAC selection feature.

  • Hatchery and reintroduction programme at Ballinderry Fish Hatchery

  • Monitoring of existing populations

  • Implementation of the Nitrates and Water Framework Directives to reduce enrichment and pollution.

Proposed objectives/actions

  • Maintain the size of the 3 existing significant populations of M. margaritifera

  • Maintain the range of existing populations of M. margaritifera at seven 10km² squares.

  • By 2010, increase the size of each of the 3 populations above by 50 per cent

  • By 2015, re-establish a population of M. margaritifera in one former known locality for the species

  • By 2020, re-establish a population of M. margaritifera in a further suitable site.

What you can do
This is a relatively easy species to identify. If you find something resembling this species please note the locality from an Ordnance Survey map and report the record of the species to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, Tel: 028 9039 5256, [at] or to roy.anderson [at]

Further information


ASSIs Code=S1029 oring.pdf


Text written by:
Dr Roy Anderson

iNaturalist: Species account : iNaturalist World Species Observations database