Freshwater pearl mussel ©Linda Pitkin/2020VISION
Freshwater pearl mussel
Freshwater pearl mussels spend their adult lives nestled in the river bed of fast flowing, rocky streams, filter feeding the water.
Scientific name
Margaritifera margaritiferaWhen to see
January to DecemberSpecies information
Category
Statistics
Life span of up to 150 years! One of the longest-lived invertebrates in existence.Maximum size up to 17cm long
Conservation status
Listed as endangered by IUCN. Critically endangered on the island of Ireland.
Protected in the UK under the Wildlife and Countryside Act, 1981.
Priority Species under the UK Post-2010 Biodiversity Framework.
Protected in the UK under the Wildlife and Countryside Act, 1981.
Priority Species under the UK Post-2010 Biodiversity Framework.
Habitats
About
The freshwater pearl mussel is an endangered species of ultra long-lived mollusc, found in the fast-flowing rocky upper sections of clean, low nutrient rivers. They have a fascinating life cycle: once released into the water, larval mussels (or glochidia to use their scientific name), have just 24 hours to latch onto the gills of a salmon or trout. At this stage they look just like a minuscule mussel and are able to open and close their tiny shells in a process known as snapping. The vast majority perish but those that successfully attach to the gills of salmonid fish essentially ‘hitch a ride’ for up to 10 months, before dropping off and burrowing down into the river bed.This complicated life stage protects the vulnerable young mussels, but also prevents the population from constantly shifting downstream into inappropriate habitats.
Like most bivalves, these mussels are filter feeders so over the next few years the young mussels develop gills. By 3-5 years of age, they will have made their way up to the top of the river bed, and will stay in roughly that spot for the rest of their lives, filtering out algae and organic sediment from river water. Young mussels don’t reach sexual maturity 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.
Small freshwater pearl mussel populations have clung on in the hillier, more remote parts of the UK. Older mussels are still relatively widespread but they are struggling to successfully reproduce because their young are much more vulnerable to too much silt and excess nutrients.
How to identify
Freshwater pearl mussels are incredibly rare and only exist in a few river systems in the UK, mostly in Scotland, Northern Ireland and the Lake District. Adult mussels have matte, brown-black shells, which are rounder and larger than a sea mussel. Usually these shells will be slightly open, with their siphons (the tubes through which they inhale, exhale and reproduce) sticking out. The inside of their shells is coated in a thick layer of nacre (or mother of pearl).Very occasionally, if an irritant enters their shell, a mussel will coat it in nacre, producing a pearl but this only happens within roughly one in 7000 individuals. As they prefer slightly acidic water conditions, their umbo (where the two shell valves attach together) is often severely eroded.
Distribution
Worldwide these mussels are classified as endangered but, in the UK and Ireland, they are on the IUCN red list of critically endangered species. In many of our more rugged west coast regions of Scotland, Wales and Ireland where agriculture and industry didn’t gain traction, small populations have clung on, so the distribution is still quite wide. But the adults generally aren’t managing to successfully reproduce because young mussels are highly susceptible to water quality issues, particularly excess sediment in the river from soil erosion, as it essentially suffocates them.The only viable population left in England is in the Lake District, but even here they have recently required a helping hand through a breeding programme managed by West Cumbria Rivers Trust.
Did you know?
Freshwater pearl mussels can filter up to 50 litres of water a day, roughly enough to fill a bath. Through their filtration, they can improve water quality for other species like fish, eels, otters and even us. Legend has it that Julius Caesar was persuaded to invade the British Isles by the promise of their pearls!How people can help
Under the Habitats Directive, it is illegal to survey for mussels or take them out of the water without a license. The Wildlife Trusts work with planners, water companies, landowners, statutory bodies and anglers to help make our waterways and waterbodies as good for wildlife as they are for people. By working together, we can create Living Landscapes: networks of habitats stretching across town and country that allow wildlife to move about freely and people to enjoy the benefits of nature. Support this greener vision for the future by joining your local Wildlife Trust.Northern Ireland's reintroduction programme
Northern Ireland has the longest running reintroduction programme anywhere in the world, operating in the Ballinderry River. At their breeding centre, set up with Peace & Reconciliation funds from the EU after the Troubles, the river is diverted through a series of channels.
Beginning 30 years ago, wild mussels were brought in to release their larvae and “infect” the gills of the local trout species. The baby mussels, which drop off after nine months are then raised entirely in captivity until they reach roughly 10 years of age, at which point they are tagged and put into the wild at release sites. This process has vastly increased the breeding success of freshwater pearl mussels in this part of Ireland.
To hear the story of Northern Ireland’s freshwater pearl mussels and the people that care for them, you can listen to Caitlin Kennedy’s (Comms & Communities Lead - Temperate Rainforest) radio documentary for Future Ecologies.
