Freshwater pearl mussels are molluscs that help to filter the water of our rivers and lakes. They have a fascinating life cycle that depends on using salmon and trout as nurseries! Sadly, the mussel is sensitive to environmental change, and is threatened by pollution of the water in which it lives. By managing the waste products of farming and forestry, we can help to not only save the mussel, but also the pristine waterways in which it lives.

Freshwater pearl mussels (Margaritifera margaritifera) are bivalve molluscs – this means they are animals with a body enclosed in a hinged shell with two sides, and belong to the same general group of animals as octopus, squid, slugs, and snails.

Where can pearl mussels be found?

They are found in pristine freshwater ecosystems (mostly in rivers, but also in lakes), those with low levels of suspended solids, and low in nutrients. Freshwater pearl mussels are sessile filter feeders – they remain in one place and suck in water from their environment, filtering out small pieces of organic material to eat. In this way, they act as cleaners, removing free-floating particles from the water and improving its clarity. In Ireland, they can be found mainly in the north and south of the country, with gaps in their distribution in the midlands and east coast.

The Ownagappul catchment in Cork is one of the top 8 catchments in the country for freshwater pearl mussels. 

Freshwater pearl mussels in the wild. Photo from NPWS website, taken by Áine O’Connor

The fascinating life cycle of the pearl mussel

Freshwater pearl mussels have a fascinating life cycle that depends heavily on the pristine nature of their environment, but also on their ability to hitch a ride on fish they share it with! 

Mussel larvae (called glochidia) must attach to the gills of a salmonid fish (in Ireland, an Atlantic salmon or brown trout) to survive the first nine months of their life, enjoying the oxygen-rich environment in this part of the fish’s body. Following this, they drop off and totally bury themselves in clean, well-oxygenated gravel/sand beds, where they remain for at least 5 years before emerging as adults, able to withstand flowing water.

Threats to the pearl mussel

If rivers and lakes become loaded with nutrients, algae and plants can grow quickly, trapping silt in the watercourse, clogging up the clean gravel beds that juvenile mussels need to settle on and bury themselves into. In addition, juvenile mussels can be choked by plants and algae preventing oxygen exchange between water and the riverbed, and by the depletion of oxygen within the sediment itself by respiring plant roots.

These problems prevent juvenile mussels from growing to adulthood, endangering the population as a whole. This sensitivity to water chemistry and sediment levels mean that freshwater pearl mussels in Ireland are highly threatened by pollution. Addition of nutrients and sediment to rivers often occurs due to farming and forestry activities – livestock waste and fertiliser from farms can enter water through runoff or dumping, while forestry systems that fell and replant trees regularly risk adding fertiliser and decaying tree material into waterways. 

Diagram showing the life cycle of the freshwater pearl mussel

The freshwater pearl mussel is endangered worldwide (the species has declined by 90% over the past century) and is protected under the Irish Wildlife Act and EU Habitats Directive.  There are 27 populations in protected areas in Ireland, 8 of which contain 80% of the Irish population. These populations are in slow decline, and in danger of extinction if nothing is done to help them. So, what is doing to help? 

How to protect the pearl mussel?

The Kerry LIFE project aims to help the Irish population of freshwater pearl mussels through encouraging establishment of more permanent forestry systems, which would cut down on the potential for fertiliser and decomposing tree material entering waterways. In addition, they advocate for improvements to farming such as nutrient and drainage management, installation of livestock drinking water facilities (to keep livestock and their waste further from rivers), and management of riverside habitats to keep riverbanks stable (preventing excess sediment entering the water). 


Written by Tom Murphy