Is it summer already? Juvenile freshwater pearl mussels are born in the middle of the winter

The Ģֱ–led LIFE Revives project has succeeded in accelerating the freshwater pearl mussel (FPM) larvae development’s parasitic phase on salmonid gills, therefore moving up the time when juveniles can be set free into independent life.
Simojoen aikuisia raakkuja allaskasvatuksessa Konneveden tutkimusasemalla. (Kuva: Emmi Ventelä.)
Published
21.2.2023

In nature, FPM larvae, or glochidia, mature into juvenile FPM as parasites on salmonid gills and release from their host fish in early summer when the river water reaches a high enough temperature. Juvenile development in nature requires up to 11 months. The Ģֱ’s Konnevesi Research Station tested whether it is possible to accelerate the development of glochidia-larvae into juveniles and predict the time for harvesting juveniles based on past years’ temperatures when salmonids are exposed to glochidia. The temperature of the host fish pool was systematically raised after a short “winter period”, 2 to 3 months after the beginning of the parasitic phase.

“Our trial yielded results and the first Simojoki small FPMs to be born at a research station saw the light of day immediately after Christmas 2022,” says technical assistant Saana Tepsa. By the end of January 2023, 33,000 individual juveniles representing the Simojoki FPM population had been collected at the research station. The juveniles are cared for and raised at the station until next summer, when they will be moved in gravel boxes or buddensiek plates in a controlled manner to grow in their home river. “We follow the juveniles’ growth for a few years, after which this new generation is released into the river bottom to strengthen Simojoki’s FPM population,” says Professor Jouni Taskinen, who is leading the project.

Accelerated maturing among juveniles makes aged FPM populations younger

The Simojoki FPM population consists mainly of older individuals whose natural reproduction is often limited by, among other things, the low number of host fish in the river needed during the species’ the larvae phase. The amount of young individuals is particularly small, which indicates that the birth rate of new FPM generations has been in decline for decades. Adult FPMs were brought to the Konnevesi Research station in 2022, ensuring their reproduction took place in controlled circumstances. The glochidia-larvae these individuals produce were infected the very same autumn on salmonid gills to develop further.

One method that has been developed and adopted in the LIFE Revives project is the continuous year-round FPM cultivation, which is based on raising the temperature in stages. This method offers a new approach to maintaining and growing FPM population sizes in our river waters.

“In many FPM rivers, natural reproduction is weak or completely absent, so bringing juveniles to reproduce in laboratory circumstances in an accelerated and flexible manner regardless of the season creates entirely new possibilities for saving freshwater pearl mussel populations which are in a weak state,” says Professor Taskinen.

The LIFE Revives project, coordinated by the Ģֱ, has received funding from the European Commission’s LIFE programme.

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