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Freshwater pearl mussel

Freshwater pearl mussel ©Linda Pitkin/2020VISION

Freshwater pearl mussel

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Scientific name: Margaritifera margaritifera
Freshwater pearl mussels spend their adult lives anchored to the river bed, filtering water through their gills and improving the quality of the water for other species.

Top facts

Stats

Life span of over 100 years!
Maximum size up to 17cm long

Conservation status

Listed as endangered by IUCN.
Protected in the UK under the Wildlife and Countryside Act, 1981.
Priority Species under the UK Post-2010 Biodiversity Framework.

When to see

January to December

About

The freshwater pearl mussel is an endangered species of mollusc, found in clean, nutrient poor low-calcium rivers. They have a fascinating life cycle; their larvae attach to the gills of salmonid fish and ‘hitch a ride’ for up to 10 months of the year. When they are ready, they need to drop off in to pristine river habitat where they will use their muscular foot to rasp algae and bacteria from the gravel. At around 3-5 years they will have developed gills and will be able to filter free-flowing river water. The mussels do not mature sexually until the age of 12-15 years, or about 65mm long. Each female can produce up to 4 million larvae which are released into the water column every May/June.

What to look for

Freshwater pearl mussels are incredibly rare and only exist in a few river systems in the UK. Adult mussels have a robust, brown-black shell of elongate elliptical shape with a concave ventral margin. As they prefer slightly acidic water conditions, their umbo (where the two shell valves attach together) is often severely eroded.

Where to find

The only viable population of freshwater pearl mussels in England is in Cumbria, though they are also found on a few other rivers throughout England, Wales and Northern Island. There are more viable populations in Scotland but these are also under threat of decline.

Habitats

Did you know?

Freshwater pearl mussels can filter up to 50 litres of water a day. Through their filtration, they can improve water quality for other species such as fish, eels, otters and more.