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Thick shelled river mussel

(Unio crassus)

Relationsship and features of typical shells (taxonomy)


Shell reaching a total length of 4 - 7 cm (rarely up to11 cm). The height of the shell is usually less then half the length. The shape of the shell is ovate-oblong and swollen. The lower margin is often forming a straight line in the centre and is equally curved up to the posterior- and anterior ends. The anterior end is slightly tipped down. Occasionally the lower margin can be curved inwards, which gives the shell the shape of a "kidney", to be compared with the shape of the shell from the Freshwater pearl mussel (Margaritifera margaritifera). The Thick shelled river mussel can though be distinguished from the Freshwater pearl mussel by its larger radius (thickness) at the centre. The valves are very thick and heavy. The colour is dark green-brown-black. Its surface is often covered with a layer of lime deposit or an iron- manganese coating. The umbonal rugae are strong and consist of close wavy rows with ridges. In larger specimens the rugae are mostly lost by corrosion. Powerful hinge-teeth are present. The anterior lateral and cardinal teeth are well developed and in the left valve they form a line. In the right valve the anterior lateral tooth is strongly pronounced and shaped like a very big triangular wedge. To observe ! There is a possible risk of confusion between the Thick shelled river mussel and the Freshwater pearl mussel. The different shape of the hinge-teeth is a distinct feature when separating the two species.


The species inhabits brooks, streams and rivers in Sweden but can also be found in the in- and outflow of lakes. It prefers a bottom with sand and gravel. The glochidia are released in "packages" taken in by the fish as a "nutrient". Some of the larvae will then attach to the gills of the host fish. The Thick shelled river mussel is the most endangered species of the large freshwater mussels in Sweden. It is Red listed as an "Endangered" (EN). It has probably disappeared from several places of occurrence during recent years, especially from habitats in the northern part of its distribution area. The species has recently been discovered in two additional stream waters in the eastern parts of the Country. Pollution and acidification of the habitats, destroying of suitable bottom substrate and loss of host fish are the main threats to the species.


Very rare in Sweden. A few scattered and small isolated populations. Appearance in some waters ranging from Scania in the very south up to the middle part of the Country. Not present north of "Limes Norrlandicus". Huge gaps between different (isolated) populations.

Photo: Håkan Holmberg
Scientific basis: Ted von Proschwitz

Left valve
Left valve
Right valve
Right valve
Dorsal view
Dorsal view
Ventral view
Ventral view