One of the longest living invertebrates, the Freshwater Pearl Mussel (Margaritifera margaritifera) is a large, native, freshwater mussel which is Critically Endangered in the UK and is classified as Endangered globally under the IUCN red list criteria.
Pearl Mussels can live up to 100 years and thrive in fast-flowing streams and rivers characterised by low-nutrient levels and high water clarity. Water quality parameters including dissolved oxygen, pH levels and temperature are critical for the survival and growth of these mussels. The adult pearl mussels are filter feeders, and their infiltration activity clarifies water by removing suspended solids, algae and bacteria, enhancing habitat quality.
The Freshwater Pearl Mussel has a fascinating and complex four stage lifecycle. The larvae, called glochidia, form a parasitic symbiotic relationship with host fish like the Atlantic Salmon and Brown Trout. They are brooded by the females in their gills and released into the water between July and September. Upon contact, the larvae attach themselves onto the fins or gills of the host fish where they remain parasitic for about 10 months. During this time, they develop into young mussels, before detaching early the next summer, to live independently. These young mussels burrow into the riverbed to survive as adults.
Photo: Tom Clenche (CC BY – NC 4.0 via iNaturalist)
Action Needed
- Address the existing loopholes in planning and legislation. For example, planning applications rely on evidence and mitigation strategies from the applicants and developers and not regulators, which can result in inconsistencies in surveying and reporting. Pushing for mandatory independent ecological surveys is critical to addressing this loophole.
- Support funding for conservation breeding initiatives of this critically endangered species as well as restoration of host fish populations.
- Support projects that restore freshwater habitats, including native riparian woodland creation for climate adaptation, and to remove unnecessary or defunct structures from rivers.
- Improve regulation, monitoring and understanding of chemical pollution to maintain river health and water quality. Support Buglife’s veterinary medicines campaign.
Threats
Pollution, abstraction and siltation – Anthropogenic factors such as agricultural runoff, forestry operations and mining activities infiltrate streams with fine sediments reducing oxygen levels and impairing the filter-feeding efficiency of the juvenile mussels, subsequently inhibiting their growth and survival rates. Chemical pollution further degrades the water quality making the Pearl Mussel habitats unsuitable for survival.
Hydropower development contributes to fluctuations in river flow and substrate stability, degrading the habitat these critically endangered species depend on. Habitat fragmentation as a result of man-made barriers inhibits the upstream migration of host fish, while sediment accumulation reduces downstream flow which is critical for larval dispersal. Changes in water chemistry, flow regulation and sediment composition associated with dams and hydropower plants pose significant threat to freshwater Pearl Mussel populations, that persist beyond the construction phase of these facilities.
Commercial demands and overfishing – Historically, the Freshwater Pearl Mussel faced significant pressure from pearl-fishing in Scottish rivers where the species was abundant, until the commercial demand of freshwater pearls started contributing to its localised decline. The species is now strictly protected, and it is illegal to kill, take or disturb Freshwater Pearl Mussels from a river. Intensive exploitation of the host, Atlantic Salmon has significantly constrained the mussel population due to insufficient host availability, directly impacting the natural reproductive cycle and recovery of the endangered FPM populations.
Climate change – Extreme weather events and increase in water temperature in summer months constrain the survival and burrowing of the Freshwater Pearl Mussel. A catastrophic flood named ‘Storm Frank’ in December 2015 triggered river channel reconfiguration and erosion, resulting in 90% reduction in Freshwater Pearl Mussel abundance in Scotland’s River Dee. Restoration of riparian zones through tree planting and bank stabilisation is a solution that helps lower river water temperature in summer months and halts excessive siltation in vulnerable pearl mussel habitats.
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