WWF

The freshwater pearl mussel’s scientific name is Margaritifera margaritifera, which means ’bearer of pearls’. The mussel can indeed produce pearls if a grain of sand or a parasite manages to get inside the shell. In some cases, but very rarely, they develop into nice, round pearls. The ability to develop pearls has made the freshwater pearl mussel into one of the few invertebrates that has interested people. Swedish cultural history has been affected by the pearl fishing and all the jewellery and artwork that have been made using the pearls. The legislation regarding pearl fishing has at times been very detailed. During the 18th century, there was a specific royal pearl inspector that overlooked handling of pearls. The penalty for illegal pearl fishing could be very severe. If you owned a river or stream where the pearl mussel was found, you could be taxed for it annually, paid in mussels. Today, pearl fishing is completely forbidden and the freshwater pearl mussel has been protected throughout Sweden since 1994.

The freshwater pearl mussel lives primarily a very still life, two-thirds of it buried in the river bottom substrate of sand, gravel or stone. The mussel feeds itself by filtering water. Through the filtering, the gills absorb oxygen for their blood at the same time as food in the form of organic particles is collected. The mussel needs to have a permanent flow of water and therefore lives only in running water. It has high standards for its living environment and can therefore be used as a good environmental indicator. The freshwater pearl mussel requires clear, oxygen-rich, nutrient-poor and well-buffered water with a stabile pH. In order for it to be able to reproduce, it needs a host fish such as brown trout or salmon. If it likes its place, it will stay there its entire life, but if necessary, it can move itself by walking with its "digging muscle" or so-called foot. It can even pull in its foot and let itself be carried by the current to a new place.

The freshwater pearl mussel can be found in most of Europe, the western part of Russia, and in eastern North America. The largest known populations of freshwater pearl mussel in the EU are found in Sweden and Scotland, but their numbers are declining throughout their entire range. In Sweden, pearl mussels are found from Scania to Lappland. They are only found in running water, from small, narrow forest streams to larger, wider rivers.

In Sweden, there are eight species of large mussels. The freshwater pearl mussel could be mistaken for the Thick-shelled river mussel that is only found in southern Sweden.

The freshwater pearl mussel has a sturdy, dark, kidney-shaped shell. The colour can range from brown to black. In older individuals, the oldest portion of the shell is corroded. They grow slowly, but can grow up to 15 cm long. The mussels stand on their ends in the river bottom in sand, gravel or stone. Since the mussel more or less only grows during the summer, you can see annual growth rings in their shells, making it possible to see the age of mussels. The oldest known specimen was found outside Jokkmokk. Its aged was determined to be more then 256 years. The freshwater pearl mussel is one of the longest living species in the Swedish fauna.

Here you can see some mussels standing in between sand and small stones on the bottom of a river. Photo: Lennart Henrikson

The freshwater pearl mussel lives primarily a very still life, two-thirds of it buried in the river bottom substrate of sand, gravel or stone. The mussel feeds itself by filtering water. Through the filtering, the gills absorb oxygen for their blood at the same time as food in the form of organic particles is collected. The mussel needs to have a permanent flow of water and therefore lives only in running water. It has high standards for its living environment and can therefore be used as a good environmental indicator. The freshwater pearl mussel requires clear, oxygen-rich, nutrient-poor and well-buffered water with a stabile pH. In order for it to be able to reproduce, it needs a host fish such as brown trout or salmon. If it likes its place, it will stay there its entire life, but if necessary, it can move itself by walking with its ”digging muscle” or so-called foot. It can even pull in its foot and let itself be carried by the current to a new place.

 

The mussel grows slowly. These small ones might be around 10 years old. Photo: Jakob Bergengren

The freshwater pearl mussel’s reproduction is fascinating. In Sweden, reproduction takes places in July when the male mussels release their sperms. The female gets the sperm through its breathing water, at which point the egg is fertilised and develops into tiny larvae, so-called glochidia. The larvae are ejected from the female’s shell a month later. After that, the small larvae must make their way to a brown trout’s gills. They live as parasites for 9 to 10 months on the brown trout’s gills and when it has grown to a half millimetre in size, the mussel drops off of the fish. With some luck, it will eventually sink to the river bottom and land at a suitable place where it can bury itself. If the conditions are right, the mussel will have grown to a centimetre in size after 8 years and will then stick up a little from the river bottom material. It is not sexually mature until it is 15-20 years old, but can then reproduce for the rest of its life, as long as it is in the right environmental conditions.

This picutre shows the reproduction of the freshwater pearl mussle. Illustration: Åsa-Maria Karlsson.

The freshwater pearl mussel cannot reproduce if it cannot find brown trout or salmon. It can seem risky or impractical to have such a complicated reproductive strategy. However, there is an important point to such a strategy and that is that the mussel can then spread upstream in rivers and streams. It can also follow the host fish out to sea and then into a new river or stream. This might explain the wide geographical range of the freshwater pearl mussel.  
 

 

Human activities are the main reason behind the marked decrease in number of freshwater pearl mussel. In particular, activities that cause changes in the mussel’s living environment, worsen the water quality, and change relations to other species. The biggest threats to the mussel’s habitat come from effects from forestry, agriculture, acidification, and dams. Nutrient leaching from agriculture and the influx of particles as a result of forestry drainage, increased run-off from felling trees, and erosion from driving too close to the water’s edge all increase the risk for siltation. Siltation results in decrease in the levels of oxygen and the small spaces between pebbles become clogged, which makes things very difficult for the young, small mussels. Siltation of river bottoms even affects brown trout negatively, particularly during the egg and juvenile stages, which in its turn affects the pearl mussel’s chances for reproduction.
Regulation of the water flow, from such things as hydropower dams, also leads to increased siltation and poorly placed road culverts act as migration obstacles for host fish.
Young mussels are very sensitive to acidification, although older individuals can handle it better. On the other hand, acidification can also mean that bound aluminium precipitates, killing both young and old mussels (as well as many other living organisms in rivers and streams).
The introduction of alien species in watercourse can affect the mussel negatively, both directly and indirectly. The introduction of the signal crayfish has proven to affect the mussel population because the crayfish eat small mussels. The introduction of non-nativie trout populations can also negatively affect the mussel as it could outcompete the native brown trout or salmon or spread disease. The most common species to be introduced are the American Brook trout and the rainbow trout, neither of which can be used as a glochidial host for the mussel larvae.

Due to these set-backs, the freshwater pearl mussel is threatened and classified as vulnerable (VU) in the Swedish Red List of endangered species. During the 20th century, the freshwater pearl mussel disappeared from half of the watercourses where it was previously found in Sweden. In those rivers and streams where the mussel is still found, its survival is not secured over the long-term. In fact, in many places where the mussel is found today, they are simply “living dead populations” – populations where no reproduction is occurring.

 

The freshwater pearl mussel is a very good indicator of clean, running water where the ecological interplay between species exists, undisturbed. If the freshwater pearl mussel is found to be disappearing, it is a warning sign that an entire type of ecosystem might be disappearing. It is also not only the freshwater pearl mussel that is dependent on this type of ecosystem, but many other species as well. Some researchers believe that that the freshwater pearl mussel itself has an important water-cleansing effect because of its filtering.

The freshwater pearl mussel is also an environmental historical archive. It stores substances from the river or stream in its shell. Through the annual rings in its shell, it is therefore possible to see when the storage of different substances took place. In this way, it is possible to measure water chemistry and the effects of land use from long ago.

Every species and ecosystem has a right to exist for purely ethical reasons. Species can obviously disappear because of natural reasons, but the freshwater pearl mussel has existed for 80 million years, and it is only during the last hundred years that it has become threatened by human activities.

 

The Swedish Environmental Protection Agency has released a new action plan in 2005 for the conservation of the freshwater pearl mussel. The goal is for the freshwater pearl mussel to remain in viable populations throughout Sweden where the natural conditions make it possible.
In order to make this possible, the following measures will be undertaken:

The most effective and realistic way of conserving the freshwater pearl mussel and its ecosystem in a national perspective is by getting increased awareness and consideration from forestry and agriculture.
It is important for consideration to be taken in smaller rivers and streams, high up in the water system, because all effects there are carried downstream.