Jürgen Karvak, wading knee-deep in a small river, pulls 20×20 cm transparent plastic squares from metal rods in the stream and flushes them in the water. These are the so-called dormitories of tiny freshwater pearl mussel babies that are taking part in a conservation project of this endangered species. Karvak, a PhD student at Tartu University, is one of the scientists taking care of the barely visible mussels from spring to autumn: “We have six spots in the river with the mussel dormitories that need to be cleaned weekly from all sorts of sediments and biofilm that flow and grow in the river.”
As a biologist, Karvak’s main passion is all sorts of freshwater creatures who are not getting so much attention as the oceans or forests, even though they are putting up a fight for survival worldwide due to climate change and human interference in their habitats. “Freshwater biodiversity is disappearing twice as fast as that of forests and oceans. The freshwater pearl mussels are a perfect indicator species for the overall situation of a habitat,” says Karvak. “When the mussel populations are doing well, it’s a sign that the natural processes are functioning and the biodiversity in the area is healthy.”
In Estonia, the freshwater pearl mussels are listed as critically endangered and truly rare. They can only be found in one river that flows through Lahemaa National Park into the Baltic Sea on the North coast of Estonia. In fact, they are so protected that the conservationists don’t even want to disclose the name of the “pearl” river.
Ageing population
The Estonian State Forest Management Centre (RMK) organised a ‘census’ of the pearl mussels in 2022 and found that there are 25,500 freshwater pearl mussels in the 21 km long habitat of their native river. This is more than the previous pessimistic estimates and less than the optimistic guesses. What makes the scientists worry is the age of the mussels. A bit like the ageing Western society, the mussels have a reversed population pyramid. There are many middle-aged and old mussels but very few are younger than 40.
As seen in many other European regions, in such cases, the whole population can be wiped out suddenly. In Europe about 90% of the freshwater pearl mussels were lost by the end of the 20th century. To avoid that fate, conservation and re-population projects such as LIFE Revives of RMK, Tartu University, in cooperation with Finland and Sweden, and the CoastNetLIFE by the Environmental Board of Estonia, started in 2021 with the aim of improving the quality of mussel life in the rivers by reversing landscape interferences from earlier times to make the rivers flow more naturally and to eliminate the number of sediments that gets to the river from agricultural landscaping.
Studies have found that sediments are likely to be blamed for the ‘infant death’ of the mussels on the riverbed – they just don’t get enough oxygen due to the sediments. The other part of the programme is the mussel nursery and repopulation.
A biography full of challenges
Jürgen Karvak explains the amazing path that the tiny mussels have gone through by the time they are being re-settled into the river ‘dormitories’: “The adult mussels eject millions of larvae that are about half a millimetre long. Only very few of them succeed in finding a host – a trout or salmon – and attach them to the gills of the fish. They stay with the fish until next spring and drop to the river bed.”
Thus, the health of the trout population in the river is also crucial for the mussels, and trout are also the ‘little helpers’ of conservationists. They have caught trout from the river, transferred them to a research centre and waited until the larvae or glochidia let go of the hosts.
These tiny mussels are then taken care of in special aquariums and dishes. It’s also one of Karvak’s tasks to deliver fresh water from the native river for the mussel babies every week so that they get the original nutrients from their natural habitat. After a year in the lab, the strongest mussels were resettled into the river to the mussel dormitories. So far, about 6000 mussels have been moved back to their birth river. Karvak examines the glochidia under the microscope regularly to discard the dead ones. They are barely visible to the eye but do put on a real show under the microscope!
Karvak knows the favourite locations of the mussels in the river and shows us an elderly specimen. The grown-ups are about 10 cm long and have a ‘leg’ that occasionally pops out from the shell to move. One adult mussel filters about 40 litres of water per day to get the nutrients needed. Karvak explains: “They also offer habitats for other species within their colonies – small fish can find shelter; vertebrae live amidst them. Even shells of dead mussels serve as living quarters for other species. In small rivers, everything is connected as in other ecosystems, and the mussels are one very important link in this chain.”
Estonia’s oldest known freshwater pearl mussel was caught in 1993 when it was 134 years old. Karvak admires these creatures: “The mussels lead a slow life and thus live long. Perhaps a great example for us humans as well.” Taking care of the mussel babies is a good example of this kind of slow life – it requires a lot of patience but also brings him to the most beautiful and peaceful natural surroundings.
Pearls for the queen
One of these beautiful spots that the mussels inhabit is where the river makes a sharp bend around an old farm. The family of the farm has known for generations that the mussels have to be safeguarded and their habitats protected. Occasionally, they spotted damaged mussels on the shore — it turned out that the wild boars liked to munch on them. Farm owner Imbi Jäetma recollects a summer day several decades ago: “We were harvesting hay and putting hay bales into the barn.
During our lunch break, we lay down on the fresh hay, and suddenly, my husband noticed a matchbox stuck to the roof beams. He took it down as a precaution, and when he opened it, there were several pearls inside!” The family cherishes the treasure even though it does not exactly look like shimmering diamonds. An old aunt immediately remembered that the farmhands occasionally tried to open the mussels to find pearls and sell them to goldsmiths in pre-WW2 Tallinn. According to Jäetma, there is even a legend that the lord of Kolga manor gave a pearl from this river as a gift for Catherine II’s crown in the 18th century.
The freshwater pearl mussels create pearls when a corn of sand or some other foreign body gets inside the mussel, and it can’t expel it. The mother-of-pearl layer inside the shell starts to grow around the intruder to insulate it from the mussel’s body.
The conservation project has run for only a few years, and the outcome is far from clear. Success depends on the survival of the tiny mussels in the dormitories and on reinstating their natural habitat to survive and grow on the riverbed once resettled. Karvak says that the local community members are potentially the best conservationists and hopes to include and involve the locals in looking after the mussels.
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