Margaritifera margaritifera L. 1758
River pearl mussels in Västernorrland, Sweden.
Picture: Joel Berglund.
Like the river mussel (Unio crassus), the river pearl mussel is also among the most threatened large fresh water bivalves in Europe. In addition to the threat to their environment, river pearl mussels in history have also been harmed by extensive exploitation for the pearls they produce. The river pearl mussel is the only European fresh water pearl mussel.
River pearl mussels are among the largest European fresh water mussels: Their very thick walled shells may grow to a size of 140 mm length. Also their age is astonishing: In Central Europe river pearl mussels may become as old as 80 to 100 years, in the north of their distribution area, for example in Sweden, they may even reach an age of more than 200 years! Also in Spain river pearl mussels may grow to see 100 years. This high life expectancy is because of their low rate of metabolism. On the other hand this requires water very rich in oxygen for the survival of the mussels. This generally may only be found in cold, fast flowing creeks and rivers.
River pearl mussels are rather inconspicuous: Younger specimens are dark brown, older ones generally are blackish in colour. The umbones near the connection between both shell valves in adult specimens are often corroded and worn.
Noble crayfish (Astacus astacus) in a meadow of algae.
Picture: W. Köstenberger.
River pearl mussels exclusively occur in low mountain ranges, in creeks cold in summer low in nutrients and limestone, such as on crystalline stone (e.g. granite). This part of European rivers is called the trout region. Especially the juvenile mussels are very little tolerant of changes in their environment, which makes the river pearl mussel a remarkable bio-indicator.
Under natural circumstances river pearl mussels in their common environment are accompanied by the noble crayfish (Astacus astacus) and the otter (Lutra lutra).
River pearl mussels are found between the 40. and 70. degree of northern latitude, the north polar circle is the climatic northern border of their area of distribution.
Waters for river pearl mussels must be fast flowing, cold in
summer and contain only little limestone. 
Worldwide, the distribution of the river pearl mussel is basically connected to that of the brown trout (Salmo trutta fario), the presence of which the mussel's development is based on.
In Europe, the river pearl mussel occurs in the lower mountain ranges from Spain as far east as Russia, from Scandinavia as far south as the Alps. In Austria and Germany the river pearl mussel is the only member of the Margaritiferidae family, while all other large fresh water mussels are part of the Unionidae family. In Spain there is another relative of the river pearl mussel, the large river pearl mussel (Pseudunio auricularis). It is estimated that world wide there are about 7 to 12 species of the Margaritiferidae family.
Like river mussels, river pearl mussels live as filtrators: From their respiratory water they filter plankton and detritus using their mucus covered gills. Like the blue mussels in the Wadden Sea, river pearl mussels so fulfil a very important ecological task. As are the blue mussels, river pearl mussels similarly are very susceptive to changes in water quality.
River pearl mussels only become sexually mature with the advanced age of about 15 years. Similar to other large fresh water mussels, fertilisation takes place in the female mussel's gills, after it had taken in male sperm cells with its respiratory water. The river pearl mussels develops passing a parasitic larval stage, the so-called glochidia, released by the female into the surrounding water. Each female river pearl mussel annually produces about 4 million glochidia approximately 0.07 mm in size, which already possess a bivalve shell equipped with shell hooks optimally serving to cling to a passing fish's gill filaments.
The brown trout (Salmo trutta fario) is indispensable for the development of river
pearl mussels. 
A river pearl mussel's glochidia are highly host specific: They are exclusively capable to infect a brown trout's (Salmo trutta fario) gills. Only in Northern Europe it is also possible for river pearl mussel glochidia to also infect a salmon (Salmo salar) and to continue their development in this fish's gills. Though the fish is unwillingly infected by the glochidia, this can still be regarded as a kind of symbiosis, as the fish profits greatly from the mussels living in the same water flow: The water quality is greatly improved by the mussels' filtration.
After the mussel glochidia have hibernated in the fish's gills, they fall off and during a metamorphosis change into juvenile mussels. Those continue their life in the creek floor and will not reappear until they are about 1 cm in size. During this time, about 95% of the juvenile mussels will have died.
River pearl mussel. Picture: Christoph Riegler, (Link).
The river pearl mussel's scientific name is Margaritifera, the pearl bearer. The production of pearls from fresh water pearl mussels has been known from Roman times. Suetonius already writes about it in his biography of Emperor Hadrian.
River pearl mussels' pearls develop like they do in sea-living pearl mussels: They are produced by the pallial epithelium. Foreign particles are coated with layers of aragonite crystals, so after a long time, a pearls comes into existence. A river pearl mussel needs about 20 to 25 years (!) to produce a pearl 4 mm in diameter! Because river pearl mussels were so frequent in historical times, exploiting them for their pearls was nevertheless economically feasible. But a very large number of mussels had to be "harvested" to obtain a sufficient number of pearls.
So in the Middle Ages pearl fishery soon became the monopoly of nobility and clergy, in short the proprietors of water flows with pearl mussels in them. Illegal pearl poaching was punished draconically, illustrated with ostentatious signs next to the riverside for the illiterate rural populace.
Not only the pearls were used, but also the mother-of-pearl (nacre), scientifically the hypostracum of the mussel shell, which until the discovery of plastic was used for the fabrication of many things, from buttons to spectacles.
River pearl mussel. Picture: Christoph Riegler, (Link).
Already harmed by pearl fishery, the European populations of river pearl mussels did not have much to oppose the growing water pollution and the overbuilding of water flows by agriculture and other human industry.
Similar to the river mussel, the river pearl mussel also was harmed by poisonous substances carried into the water because of the extensive use of fertilisers. The oxygen content of the water decreased, and also the juvenile mussels were suffocated under loads of sediment. Also the acidification of earth and water by acid rain and the planting of red pine monocultures had negative effects on the European populations of river pearl mussels.
Rainbow trout (Oncorhynchus mykiss). Picture: LfL, Starnberg.
Additional harm was done by introducing the American rainbow trout (Oncorhynchus mykiss) for fishery reasons. The rainbow trout on one hand decimates the juvenile brown trout. In contrary to the brown trout, the river pearl mussel glochidia are unable to use the rainbow trout as a host fish. Here the European otter (Lutra lutra) helps, as, where it is living, it decimates the rainbow trout and so helps the river pearl mussels, which are not part of its diet, other than, for example, the river mussel.
Similar to river mussels, also river pearl mussels have been severely harmed by muskrats (Ondathra zibeticus) introduced from America.
Threatened fresh water mussels.