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Inhabits small cold rivers and clear streams > Length up to 150 mm width 70 mm width, Thick walled valves . The small glochidia live as parasites in fish gills . Pearls may be formed in the mantle . Grows very slowly only a few mm per year , but has a life span of up to 80 years .
Feeds on Microorganisms and debris on the beds of rivers and streams
The freshwater pearl mussel is distributed from the arctic and temperate regions of Wn Russia through Europe to the NE seaboard of North America. Recent studies have revealed that there have been dramatic declines throughout its Holarctic range. Substantial populations with active recruitment are now found on fewer than 50 rivers in Canada, northwest Russia and northeast Scandinavia, with a handful of sites in Bavaria, the Czech Republic and Austria.
Previously widespread and abundant in areas of England and Wales, recent surveys revealed most former populations to be virtually extinct, with little active recruitment. Major declines have occurred in Northern Ireland, with recent survey work again showing population declines and scant recruitment, including the rare hard-water form, Margaritifera margaritifera durrovensis (Phillips) which is unique to Ireland.
Originally distributed throughout Scotland, a comprehensive survey from 1996 to 1999 revealed that the freshwater pearl mussel is now extinct in most of the lowlands and scarce everywhere except for a handful of Highland rivers. If the present rates of extinction continue, it has been estimated that surviving Scottish populations may only persist for a further 25 years. Up to half the worlds known remaining populations with active recruitment now occur in Scotland.
The cause of early decline was likely pearl fishing and industrial pollution. The current lack of recruitment may be due to increasing siltation and eutrophication of rivers. The recent declines in stocks of migratory salmonid fish upon which the larvae are dependent is also concerning , River engineering for hydro-schemes, flood defence purposes and fishery improvements continues to pose a serious localised threat. Forestry operations, acidification, effluent from fish farms and chemical sheep dip are further threats to the declining populations.
This species is Protected under Schedule 5 of the UK Wildlife and Countryside Act (1981) and the Wildlife (Northern Ireland) Order 1985, the freshwater pearl mussel is also listed on annexes II and V of the EU Habitats Directive and Appendix III of the Bern Convention. It is included on the IUCN Invertebrate Red List, where its status is described as Vulnerable (IUCN 1990). Classified as a priority species by the UK Biodiversity Steering Group, a national Species Action Plan has been prepared to encourage measures for its survival.
By Tom Meijer (Own work) [GFDL, CC-BY-SA-3.0 or CC BY-SA 2.5-2.0-1.0], via Wikimedia Commons