Freshwater Pearl Mussel

Freshwater Pearl Mussels Life History

Protected under Schedule 5 of the 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 and Species 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.

The freshwater pearl mussel is a fascinating and important part of our biodiversity and cultural heritage. It is one of the world’s most critically endangered species and Scotland is one of their few remaining strongholds. In Scotland freshwater pearl mussel populations have declined due to illegal pearl fishing, poor water quality and habitat loss. Over the last 100 years more than one third of Scottish rivers that used to support freshwater pearl mussel no longer do so.

They are indicators of a healthy river. They live in the bed of clean, fast- flowing rivers, where they can be completely or partly buried. They feed by drawing in river water and filtering out fine particles. Each day an adult mussel can filter around 50 litres of water making them vitally important to other species that need clean water especially salmon and trout. They have a complex lifecycle and, in their first year, they harmlessly live on the gills of young salmon or trout before detaching and burying themselves in suitable substrates where they continue to grow. Looking after freshwater pearl mussel benefits the whole river.

It matures at 10–15 years, when the length generally exceeds 65 mm. In early summer (June to July), the males shed sperm into the water, and it is inhaled by the females. The fertilised eggs develop in a pouch on the gills for several weeks, and are released from July to September as tiny larvae, measuring (0.6–0.7 mm), known as glochidia. These resemble tiny mussels, but their shells are held apart until they encounter a suitable host, when they snap shut on to the host’s gill filaments.

They drop off in May and early June, and must land in clean sandy or gravelly substrates to settle and start to grow. This association does not appear to harm the fish, and enables young mussels to colonise new areas upstream. Young mussels are typically yellowish brown, becoming darker with maturity. Freshwater pearl mussels develop very slowly and can live for more than 100 years, reaching 12–15 cms in length. The life span and maximum size reached are highly variable between populations, depending on environmental conditions. The huge losses involved in this unusual life cycle make the freshwater pearl mussel particularly vulnerable to adverse conditions.