Let’s save the endangered Freshwater Pearl Mussel

Freshwater pearl mussels are fascinating creatures and play an important role for the ecosystem. But the freshwater pearl mussel is also endangered and without our help will struggle to survive.


Real freshwater pearls with their different shapes and colours make a beautiful piece of jewellery. For centuries humans have been intrigued by these unique beautiful pieces of natural art. Some historians even say that Caesar’s admiration for pearls was one of the reasons for the first Roman Invasion into Britain in 55BC. Later, in the 16th century people used some of the most beautiful pearls to adorn the Scottish crown jewels.


Sure, pearls are beautiful and fascinating. But the mussels producing these pearls are actually even more fascinating. Freshwater pearl mussels (Margaritifera margaritifera) can get more than 250 years old and are vital for a healthy ecosystem. Similar to trees, their shells grow an extra layer every year. In this way they store environmental information that otherwise maybe would have been long forgotten.


During an internship I got the great opportunity to participate in a project monitoring a population of the endangered freshwater pearl mussel in a river near Trondheim. I am not an expert on freshwater ecology and have to admit that it was more or less a coincident that brought me in contact with this project. When I started, I didn’t know anything about freshwater pearl mussels (except maybe that they produce pearls). And I never would have thought that mussels, these dull creatures, are actually quite amazing.


So read on if you are curious and want to know more. You will learn why they are amazing, why they are in  danger, why this matters to us and what we can do.

The disappearance of a fascinating species

Once present in many rivers throughout Europe, freshwater pearl mussels have become rare. Over the past century about 95% of the European population has disappeared. Unnoticed by most of us! Nowadays, the freshwater pearl mussel is classified as endangered on the IUCN red list of threatened species. 


Most of the remaining european populations are small and in poor condition. You can still find some of the bigger and healthier populations in Russia, Norway, Sweden, Finland, Ireland and Great Britain. But even in these countries population numbers and sizes are declining. Without conservation efforts, the freshwater pearl mussel is likely to get extinct in the nearer future.


In the past, people were obsessed with the freshwater pearl mussels for their beautiful pearls. But pearls are actually quite rare resulting in whole rivers being fished empty just for the small chance of finding one or two pearls. During this process the pearl fishers killed thousands of mussels and pearl fishing became one of the main reasons for the dramatic decline of the European population. After most countries prohibited pearl fishing in the 1990s, it only plays a minor role now.

The ongoing decline of the endangered freshwater pearl mussel

So why are numbers still declining then? There are two main reasons for the ongoing decline: The mussels’ inability to cope with environmental changes and their complex life cycle. 


Freshwater pearl mussels make high demands on their habitat. They thrive in pristine rivers with cool, clean running waters and require high levels of oxygen. High levels of nutrients, fine sediments and other pollutants reduce the water quality. Increasing human impact in combination with climate change represent two main drivers for the ongoing habitat degradation. 


As a result, the water quality of many European rivers has deteriorated over the past centuries. Ideal circumstances have become rare. Due to worsening conditions in most rivers, remaining mussel populations are often over-aged. This means that there might be a lot of old individuals but no younger ones. 


While older mussels can still tolerate the degradation of their habitat to some extent, the young mussels simply can’t. Without being able to reproduce, many mussel populations slowly decline over the years until they will have disappeared one day.

Little stream with vegetated banks and surrounded by forest
Typical habitat of the freshwater pearl mussel
shallow stream with shade-giving trees
The shade-giving trees prevent the shallow water from heating up too much during summer. Ideal conditions for the freshwater pearl mussel.

When it comes to their life span, freshwater pearl mussels can easily compete with other long living species. In the right conditions the slow growing mussels can easily become more than 100 years old. Some of the oldest individuals ever found were older than 250 years. But their life cycle is complex and especially in the beginning of their life the mussels are very vulnerable. As a result only very few individuals reach adulthood. You could say that their long lifespan is needed to compensate for the high mortality rate during the first few years of existence.

The freshwater pearl mussel: endangered due to a complex life cycle

Mussels become fertile at the age of about 10 to 20 years. The male mussels release their sperm into the water and fertilise the egg cells of the female mussels. After about 4 weeks the female mussels release the mussel larvae (glochidia) into the water. 


The first critical phase begins. To survive, the larvae have to find a suitable host fish. Salmonids such as Atlantic salmon or brown trout are suitable hosts. Finding a host isn’t easy though. Only about four out of a million mussels will manage. The rest are carried away by the river and die. Obviously, declining host fish populations don’t make the mussel’s life easier.

Lifecycle of the endangered freshwater pearl mussel
Life cycle of the freshwater pearl mussel

Once the larva finds a host, it attaches itself to the gills of the fish. Over the winter they develop into a young mussel. In spring it is time for them to let go of their host. 


Now, the most critical phase begins. The small mussel has to be lucky and land on clean sand or gravel where it can burrow itself in the river substrate. If the riverbed is muddy with low oxygen levels, the mussel will suffocate. If it lands on a suitable spot it will spend the first few years burrowed in the river substrate before reappearing at the surface of the riverbed.


 Adult mussels can actively move up to a meter a day. But once they found their spot they usually will not move anymore. Over the years they grow up to a size of 15 cm growing a new layer on their shell every year. Similar to tree rings, the shell rings store some interesting information. Based on their composition we can, for instance, learn about the occurrence of acid rain in the past.

Three older individuals of freshwater pearl mussel on gravel
Three older individuals on gravel

Conservation of a threatened species

Worsening environmental conditions and a complex life cycle with several critical phases pose a big threat to the specie’s survival. To safeguard its survival conservation measures are inevitable. 

But protecting a species never is an easy task. It costs time, money and effort and success isn’t always guaranteed. Many people would probably perceive the disappearance of iconic species such as tiger, panda or polar bear as a great loss. Significantly less people would care about a mussel getting extinct. 

So why should we even bother to protect the freshwater pearl mussel?

The answer is manifold. First of all, you could argue that all life on Earth is worth protecting. After all, it is the diversity that makes our planet so beautiful. Additionally, all species fulfil some kind of function within an ecosystem. All parts of an ecosystem are to some extent linked to each other. The disappearance of one species may affect the fragile balance of an ecosystem. And while the disappearance of some species might remain unnoticed by most people, the disappearance of others, or simply too many species, can be really bad and lead to the collapse of the entire ecosystems.

Well-functioning ecosystems, however, provide important services. These services are not only crucial for the health of our planet but also our mere existence and a comfortable living environment. Some of these so-called ecosystem services are very obvious, while others may be easily forgotten. Think for instance of clean drinking water, food, shelter, but also pollinating insects or recreational areas. All of these services and many more are provided by nature more or less for free. The only thing we have to do is taking good care of our ecosystems. By destroying our planet’s ecosystems, we deprive ourselves of our basis of life.

An ecosystem hero

As member of the river ecosystem, freshwater pearl mussels make high demands on their habitat. Therefore, a healthy freshwater pearl mussel population indicates a healthy ecosystem. 


But the freshwater pearl mussel does not only make high demands on the habitat quality, it also partially creates it. Freshwater pearl mussels are filter feeders, which means that they retain food from inhaling and expelling water. In this way an adult mussel can filter and purify about 50 litres of water a day. This means that a population of several thousand individuals can improve the overall water quality significantly. 


The high water quality also benefits other species. Many fish species for instance appreciate living in rivers with clean water. In addition to that the freshwater pearl mussel is part of the food web. Its faeces are eaten by insects and serve as nutrients for plants.


As the presence of freshwater pearl mussels affects the whole ecosystem, their absence does too. In turn, conservation measures targeting the freshwater pearl mussel do not only have a positive impact on the mussels. They benefit the entire ecosystem. This makes the freshwater pearl mussel the perfect target for conservation measures.

Saving the endangered freshwater pearl mussel

To have a long-lasting effect, conservation measures should be implemented on a catchment scale. Limiting conservation measures to the areas with mussel populations doesn’t make much sense as long as pollutants are still entering the river system further upstream.


Conservation measures should focus on existing freshwater pearl mussel populations to protect them and help them spread to other areas of the river. To maintain or improve the habitat quality, river engineering and other activities that can be linked to pollution, high amounts of fine sediments and increasing levels of nutrients should be strictly avoided.


A vegetative buffer zone along the river is an effective way to reduce leakage of unwanted substances into the river. At the same time trees and bushes provide extra shade. In summer they can prevent the water from warming up too much.

empty shells of endangered freshwater pearl mussel next to a stream
These freshwater pearl mussels probably died as a result of chemicals leaking into the water

In the past, many rivers have been permanently changed. Dams were built and rivers were straightened. Tree trunks, boulders or accumulations of gravel were removed; habitat characteristics that are important for thriving mussel populations. In these areas nature restoration might be the only way to restore the habitat quality and bring nature back to its pre-engineering state. 


Restoration measures include restoring the original meandering course of the river, adding tree trunks, boulders and gravel and the removal of dams. Such severe interventions don’t go without risk for the mussels. The mussels might suffer from disruptions and increased turbidity in the water. Therefore, careful planning is indispensable to prevent permanent damage to the mussel population instead of protection.

Breeding freshwater pearl mussels

Experiments with mussels from hatcheries and stocking of fish infested with young mussels are going on. But so far the success rate of these experiments is not very satisfying. The freshwater pearl mussel is very particular and well adjusted to the conditions of its “home river”. This makes it difficult to relocate mussels from one river to another. The same applies to transferring small mussels from a hatchery into the wild.

Increasing awareness

In areas with freshwater pearl mussel populations it is important to raise awareness of their existence and their endangered status. That’s where environmental communication plays a crucial role. By increasing awareness people are more conscious and consequently will not harm the mussels accidentally.


So, if you live or travel in an area with freshwater pearl mussel populations it might be worth keeping an eye out for them. If you spot one, take some time to observe it. Enjoy the sight of a fascinating creature that has become very rare. Usually, you will find a whole population. Think about what these mussels might have witnessed and watch how they breathe in the rhythm of the river. Be careful though not to touch or disturb them in any way.


At the end of the day the freshwater pearl mussel is only one amazing example to show the beauty and complexity of our planet’s ecosystems and the impact of our actions. Healthy ecosystems are the basis of a healthy planet and the basis of our existence.