Gårdlösaleden a footpath in Gårdlösa beautiful natural and cultural landscapes

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2. Thick-shelled river mussel

The thick-shelled river mussel (Unio crassus) is a rare and seriously endangered species that is found in part of the Tommarp stream, from just south of Gärsnäs up to the Smedstorp pond. The species is protected, which means that you may not even lift it up out of the water.
The thick-shelled river mussel can live for roughly 70 years, and is usually 4-7 cm long. In the Tommarp stream, most of the mussels are about 5 to 8 cm long. This indicates that the mussels in the stream are relatively old. Only a few young individuals have been found, which means that several ge- nerations are missing and that reproduction is not working in the stream.
Reproduction takes place during the period May – July/August. The male mussel releases its sperm into the water, and the sperm are taken in by the female. The eggs are fertilised inside the female mus- sel and develop into a larval stage called glochidia. When the glochidia have matured, they are released into the water. They attach themselves to the gills of a host fish, which they parasitize for roughly one month. When they have developed into small mussels approximately 1 mm long, they release from the fish and fall to the bottom of the stream, where they bury themselves.
In order to thrive, the mussels need clean streambeds of sand and gravel, with a good flow of oxygenated water. It takes several years for a mussel to grow, and when it has become sufficiently large, roughly 10-12 mm, it comes up and settles on the streambed in order to filtrate.
At present it is not clear what species of fish act as host fish for the thick-shelled mussel. Fish found in the Tommarp stream include the bullhead, the minnow and the ninespine stickleback. The survival of the thick-shelled mussel is mainly threatened by changes in the environment, such as deterioration in the quality of water, removal of shading trees, the clearing of watercourses, and low water flows during summer.

Tjockskalig målarmussla (thick-shelled river mussel)              Photo Marie Eriksson

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