Freshwater pearl mussel

The freshwater pearl mussel has a complex life cycle that includes a parasitic larvae stage. The larvae, called glochidia, attach to the gills on host fish where they develop into juvenile mussels. Only a small proportion of the larvae survive to adulthood, but this is compensated for by the species’ high reproductive capacity and long lifespan. The species requires clean, oxygen-rich and nutrient-poor water with stable pH levels to reproduce.

Has disappeared from many waters
In Sweden, the freshwater pearl mussel has disappeared from about a third of its historical range since the 20th century. Today, it is found in approximately 600 rivers in the country. Local populations can range from just a few individuals to several millions. The species relies on trout and salmon as host fish not only for reproduction but also for dispersal and genetic exchange between different mussel populations.

Threats to the species
The survival of the species is threatened by factors such as siltation, riverbed destruction, acidification, hydropower development and pollution. Modern forestry and agriculture often have a negative impact by increasing sedimentation and degrading water quality. Invasive species such as signal crayfish and muskrat can also negatively affect mussels through predation.

The freshwater pearl mussel needs salmon or trout as hostfish to have a successful reproduction.

Conservation measures
The freshwater pearl mussel is fully protected under Swedish law and also under the EU Habitats Directive. Strengthening existing populations requires extensive conservation measures. These includes preserving and restoring their natural habitats, but also the habitats and distribution of their host fish by removing migration barriers.

Helping with host fish
The freshwater pearl mussel also functions as an bioindicator storing information about water quality in its shells over time. The species is of great scientific and ecological value, but extensive efforts are needed to conserve and restore functioning populations. One way is to catch host fish and place them in tanks with gravid mussels allowing more larvae attach to their gills. These fish are then released back into the river, where the larvae can develop on the gills of their host fish and eventuelly drop off as juvenile mussels  and settle in the riverbed. After a few years, they have developed into adult mussels and can contribute to the reproduction.