The presence of freshwater mussels, also known as river mussels, is sometimes only apparent upon finding the single valves of shells on the shores of a body of water. As living organisms, freshwater mussels lead quiet lives at the bottom of the river or pond bed. Yet mussels aren't as immobile as they seem: they do wander around the sediment, burrowing in sand or mud.
Using its strong foot, Unio sp. can propel itself forward like a snail at a surprisingly fast pace, or burrow down into the soil or sediment into a more convenient or safer place. Its light-coloured soft parts are protected by two hinged shell halves which may be shut tightly thanks to a muscle used for this purpose. The elliptical or egg-shaped shell can be up to 10cm long and is around twice as long as it is high. The thick shell valves of young mussels are greenish to yellow-brown, while older specimens have turned darker due to exposure to iron and manganese, and also exhibit a markedly corroded back. If Unio sp. is submerged under sediment, its respiratory aperture will still be exposed.
Freshwater mussels are native to all parts of Central Europe with the exception of the British Isles. They may even be found at elevations higher than 600m. In the Donau-Auen National Park, the Swollen river mussel (Unio tumidus) and the Painter's mussel (Unio pictorum) have been observed. The Swollen river mussel is similar to the thick shelled river mussel (Unio crassus), yet can tolerate inferior water quality and prefers very slow-moving waters. The Painter's mussel (Unio pictorum) has settled both in the Danube's main stem and its backwaters.
Endangerment and Conservation Status
Unio crassus is critically endangered (CR) and is classified within the Annex II of the Habitats Directive (Council Directive 92/43 EEC on the Conservation of natural habitats of wild fauna and flora) and is thus protected in all EU countries. To this end, conservation programmes must be developed and dedicated conservation areas must be made available. Unio crassus – formerly a common sight yet now near extinct – is an extremely sensitive organism requiring the highest water quality in order to survive. It is extremely sensitive to pollutants such as chemical fertilisers from run-off and drainage.
Freshwater mussels feed primarily on plankton and minute floating organic particles which they catch using their gills. In order to do so, up to 20 litres water per hour is pumped through their bodies. Thanks to their considerable biomass – in large rivers up to two-thirds – they help keep the waters they inhabit clear. The venerable Unio crassus can even live up to 20 years.
Reproduction takes place from spring to the beginning of summer for organisms which are at least four years old. After the males deposit their sperm in the water, females absorb it through their gills. A grown female can release up to 50,000 glochidia (mussel larvae) into the waters. With a bit of luck, and using the horn on each side as a hook, some of the glochidia manage to attach themselves to the gills of a host fish. After a six-week stint as a parasite to the host fish (often a bullhead, chub, or dart), they are transformed into juvenile mussels and subsequently burrow into the sediment. This phase can last several years, whereby it is important that the sand and gravel layers below ground are not too clogged (colmatation, e.g. soil or riverbed clogging), as this otherwise may lead to oxygen deficiency with subsequently fatal results for the juvenile mussels.