Invertebrate species: molluscs


1029 Freshwater pearl mussel    Margaritifera margaritifera   
   Follow link for UK biodiversity information on this species.

Background to selection

Distribution of SACs with species 1029 Margaritifera margaritifera.  Click image for enlarged map.
Distribution of SACs with species 1029 Margaritifera margaritifera. Click image for enlarged map.

Description and ecological characteristics

 

The freshwater pearl mussel Margaritifera margaritifera grows to 140 mm in length, and burrows into sandy substrates, often between boulders and pebbles, in fast-flowing rivers and streams. It requires cool, well-oxygenated soft water free of pollution or turbidity. The mussel spends its larval, or glochidial, stage attached to the gills of salmonid fishes. The larvae attach themselves during mid to late summer and drop off the following spring to settle in the riverbed gravel where they grow to adulthood.

 

Population declines have been caused by factors such as pearl-fishing, pollution, acidification, organic enrichment, siltation, river engineering, and declining salmonid stocks. An additional serious threat in parts of mainland Europe is predation by introduced muskrat Ondatra zibethicus (Zahner-Meike & Hanson 2001). M. margaritifera is now a rare species whose conservation is giving rise to concern, and its increasing rarity in mainland Europe gives extra significance to UK populations.

 

As this species does not reach reproductive maturity until at least 12 years old and may live for over 120 years (Bauer 1992; Beasley & Roberts 1999), population age-structure is vitally important when assessing viability. The presence of juveniles (a feature essential to the long-term sustainability of mussel populations) is a clear indicator of the structural and functional features of the habitat required for the survival and reproduction of the species.

European status and distribution

 

The freshwater pearl mussel Margaritifera margaritifera is widely distributed in Europe, Fennoscandia and north-eastern North America, but has suffered serious decline and is threatened with extinction or is highly vulnerable in every part of its former range. It is listed as ‘vulnerable’ by IUCN, but is believed to be even more threatened. Outside Britain and Ireland, recruiting populations of international importance survive in probably fewer than 50 rivers world-wide (Young et al. 2001). Only a few viable populations survive in mainland Europe, most remaining continental populations consisting of mussels at least 30 years old with few signs of recent recruitment – although an apparent absence of juveniles may in some cases be partly due to difficulties in surveying (Hastie & Cosgrove 2002). A distinct hard-water form, M. margaritifera durrovensis, is restricted to a few river systems in southern Ireland.

UK status and distribution Click to view UK distribution of this species

 

Margaritifera margaritifera was formerly widespread throughout western and northern parts of Great Britain. However, England and Wales are each now believed to support only a single recruiting population. In Northern Ireland the species formerly occurred widely in several catchments, but is now restricted to a few sites. Many UK rivers now contain only scattered individuals, with no juvenile mussels recorded; such populations may become extinct due to lack of recruitment. Despite serious declines in both range and total population, Scotland is the remaining European stronghold for M. margaritifera, supporting functional populations in over 50 rivers, mainly in the Highlands.

Site selection rationale

 

The selection of sites for freshwater pearl mussel has been influenced by the importance of the UK for conserving this species in Europe. Site selection has been based on an analysis of past records and more recent survey of potentially important rivers. Most of the selected sites hold large populations which contain juvenile mussels (indicating the presence of functional populations). Consideration has been given to the geographical distribution of the species; the predominance of Scottish rivers reflects the presence of more large viable populations here than in other parts of the UK.

 

While the SAC series makes a contribution to securing favourable conservation status for this Annex II species, wider measures, including implementation of the Species Action Plan, are also necessary to support its conservation in the UK.


Site accounts

Abhainn Clais an Eas and Allt a' Mhuilinn Highlands and Islands
This site in the north-west Highlands of Scotland includes two burns (streams) that each support functional freshwater pearl mussel Margaritifera margaritifera populations. Both burns are relatively small yet support high-quality populations. Abhainn Clais An Eas has pearl mussels present along a 1 km stretch from the mouth of the river to a large waterfall impassable to migratory fish. Allt a’Mhuilinn supports pearl mussels along a 2 km stretch from the sea to an upstream loch. Both burns contain good populations of juveniles indicating that present conditions support recruitment.
Afon Eden - Cors Goch Trawsfynydd West Wales and The Valleys
This tributary of the Afon Mawddach lies within a little-modified catchment and supports the only population of freshwater pearl mussel Margaritifera margaritifera in Wales that is regarded as viable. Recruitment is evident, with some juveniles recorded in recent years, although the population is comparatively small at an estimated 1500 individuals.
Ardnamurchan Burns Highlands and Islands
Ardnamurchan Burns includes four streams within one area of the extreme west of Scotland. Each stream supports a high-quality functional freshwater pearl mussel Margaritifera margaritifera population that includes juveniles.
Cladagh (Swanlinbar) River Northern Ireland
The Cladagh (Swanlinbar) River rises on Cuilcagh Mountain and flows through County Cavan in the Republic of Ireland before crossing the border into County Fermanagh in Northern Ireland, and eventually entering Upper Lough Erne. It is a moderately large river, being ultra-oligotrophic in its upland reaches within the Republic of Ireland, before gradually becoming oligotrophic and oligo-mesotrophic through its middle and lower reaches within Northern Ireland. The freshwater pearl mussel Margaritifera margaritifera population, which is estimated to have a minimum number of 10,000 individuals, is confined to 6 km of undisturbed river channel in the middle section of the river. It is one of the largest known populations surviving in Northern Ireland.
Mingarry Burn Highlands and Islands
Mingarry Burn on the Isle of Mull is the only site in the Inner Hebrides selected for freshwater pearl mussel Margaritifera margaritifera. The stream is about 3 km long and contains a functional pearl mussel population along its entire length. The site is a relatively recent discovery, with no published historical records prior to the 1997 Scottish national survey (Cosgrove & Young 1998). This is regarded as one of the most undisturbed Scottish pearl mussel sites.
North Harris Highlands and Islands
North Harris includes two rivers that support high quality, functional freshwater pearl mussel Margaritifera margaritifera populations. One of the rivers contains the most important Western Isles population and is therefore the most significant pearl mussel component of the site. The other river also has a significant functional population, but the relatively low number of juveniles indicates that recruitment may be limited. The two rivers represent the north-western extremity of the species’ range in the UK and Europe.
Owenkillew River Northern Ireland
The Owenkillew River rises in the eastern Sperrin Mountains in Northern Ireland and flows westwards, forming part of the Lough Foyle system. It is a large river, being ultra-oligotrophic in its upland reaches, and then becoming oligotrophic and oligo-mesotrophic through its middle and lower reaches. The freshwater pearl mussel Margaritifera margaritifera population, which is estimated to have a minimum number of 10,000 individuals, is confined to 4 km of undisturbed river channel in its upper reaches. It is the largest known population surviving in Northern Ireland.
River Borgie Highlands and Islands
Freshwater pearl mussel Margaritifera margaritifera are found throughout the main stem of the Borgie, from just above the estuary to the outflow of Loch Slaim, the lowest of a series of lochs on the river. Mussels are comparatively rare in the lower reaches and most abundant in the middle reaches, with mean densities in some places of 1–9 individuals m2, exceptionally attaining 35 m2 at one location. Although juveniles were absent at lower and upper sites, they were common in the middle reaches occurring at densities of up to 7 m2. It is likely that pearl mussels are restricted to the main stem of the river. At the time of survey in 1996, most tributaries were dry, and only one contained pearl mussels, in very low numbers.
River Dee North Eastern Scotland
The Dee is a major east coast Scottish river, which flows uninterrupted for some 130 km from its upland reaches in the high Cairngorms to the North Sea. It supports a functional population of freshwater pearl mussel Margaritifera margaritifera, which is common in the Dee, recorded from a location approximately 30 km from the river source to approximately 6-7 km upstream from its mouth. Juveniles make up approximately 30% of the recorded population, among the highest proportions recorded in Scotland. This indicates that the population is recruiting strongly and is one of the most important in the UK.
River Ehen Cumbria
The River Ehen supports the largest freshwater pearl mussel Margaritifera margaritifera population in England. Exceptionally high densities (greater than 100 m2) are found at some locations, with population estimates for the entire river exceeding 100,000. The conservation importance of the site is further enhanced by the presence of juvenile pearl mussels, indicating recruitment since 1990.
River Evelix Highlands and Islands
The Evelix is the only remaining small east coast river in Scotland that supports a surviving functional freshwater pearl mussel Margaritifera margaritifera population, particularly within the upper reaches of the river. Mussels have also occur in the lower stretches of the river, but in lower numbers than upstream and with fewer juveniles were present. Pearl-fishing is thought to be the principal reason for the decreased numbers in the lower stretches, but the presence of juveniles in the upper sections indicates that despite losses, the population is successfully recruiting.
River Kerry Highlands and Islands
Freshwater pearl mussels Margaritifera margaritifera occur within the Kerry downstream from a hydro-electric dam. They are at their most abundant in the middle reaches of the river where they attain mean densities of 13 m2. It is estimated that the total pearl mussel population of the Kerry is in excess of 400,000. Not only is the population important in terms of its size, but it also has a favourable age-structure with a relatively high proportion of juvenile mussels.
River Moidart Highlands and Islands
Freshwater pearl mussels Margaritifera margaritifera are plentiful in the Moidart, from Loch nan Lochan downstream. The highest mussel densities of 2030 m2 occur in the middle to upper sections of the river. The Moidart is one of the few rivers where, in addition to an abundant adult mussel population, there is evidence of juvenile recruitment.
River Moriston Highlands and Islands
The River Moriston flows into the northern side of Loch Ness, and supports a functional freshwater pearl mussel Margaritifera margaritifera population. Pearl mussels are present from downstream of a hydro-electric dam to the confluence with Loch Ness. Due to illegal pearl-fishing the population is not abundant but survey results show that 40% of the population is composed of juveniles. This is the highest percentage recorded in any Scottish pearl mussel population and indicates that recent successful recruitment has taken place.
River Naver Highlands and Islands
With the River Borgie, this site in Sutherland represents the northern extreme for freshwater pearl mussel Margaritifera margaritifera in the UK. The Mallart River is a tributary of the River Naver and they flow through a wide floodplain of moorland and conifer plantations. Both rivers support high quality pearl mussel populations that include many juveniles, indicating recent successful recruitment. Pearl mussels have been recorded throughout much of the length of both rivers, indicating that they can support good populations, despite a history of relatively intensive pearl-fishing.
River Oykel Highlands and Islands
The Oykel is a long, meandering river in the northern Highlands of Scotland that flows into the Kyle of Sutherland on the east coast. The river supports an excellent, high-quality freshwater pearl mussel Margaritifera margaritifera population with high densities recorded at some locations, including a bed numbering several thousand individuals. Surveys have also recorded high percentages of juveniles within the population, indicating that there has been recent successful recruitment. There is also evidence of unsurveyed pearl mussel populations in deep water that may increase the conservation importance of the river.
River South Esk Eastern Scotland
Freshwater pearl mussels Margaritifera margaritifera are abundant in the River South Esk, representing the south-eastern range of the species in Scotland. The pearl mussel population is most abundant in the middle reaches of the river where they attain densities > 20 m2. The conservation importance of the site is further increased by the abundance of juveniles which comprise approximately 20% of the population. The presence of juvenile pearl mussels less than 20 mm long indicates that there has been successful recruitment since 1996.
River Spey Eastern Scotland, Highlands and Islands
The River Spey is a large Scottish east coast river that drains an extensive upland catchment and supports an outstanding freshwater pearl mussel population in its middle to lower reaches. In parts of the River Spey, extremely dense mussel colonies have been recorded (225 m2) and the total population is estimated at several million. As the population also shows evidence of recent recruitment and a high proportion of juveniles, the River Spey is considered to support a pearl mussel population of great international significance.
Upper Ballinderry River Northern Ireland
The Upper Ballinderry River rises in the Sperrin Mountains in Northern Ireland and flows eastwards into Lough Neagh. It is a moderately large river with nutrient conditions that range from oligo-mesotrophic to mesotrophic. The freshwater pearl mussel Margaritifera margaritifera population, which is estimated to have a minimum number of 1,000 individuals, is confined to 7.5 km of undisturbed river channel in its middle reaches. It is one of the largest known populations surviving in Northern Ireland.

SACs where this Annex II species is a qualifying feature, but not a primary reason for site selection

Ardvar and Loch a' Mhuilinn Woodlands Highlands and Islands
Foinaven Highlands and Islands
Glen Beasdale Highlands and Islands
Inverpolly Highlands and Islands
Rannoch Moor Eastern Scotland, Highlands and Islands
River Clun Herefordshire, Worcestershire and Warwickshire, Shropshire and Staffordshire
River Kent Cumbria
 

Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.