Swollen river mussel
(Unio tumidus)Relationsship and features of typical shells (taxonomy)
Appearance
Shell reaching a total length of 5 - 8 cm (rarely up to 12 cm). The height of the shell is approximately half the total length. The lower margin of the shell is often markedly curved all over its length. More seldom is this curved shape less pronounced. The lower margin bends softly up to the posterior end, that often is wedge-shaped. Colour from yellow, yellow-green to olive-green. When comparing the colour with the Painters mussel (U. pictorum) the Swollen river mussel is often darker. Older specimens are often dark brown. The species can be very thick shelled in running water. The umbonal rugae consist of two irregular radiating rows of elongated wavy tubercles, some of which join up to form undulating ridges. The anterior adductor muscular impression is not situated so close to the anterior end as is seen in the Painters mussel. Fairly powerful hinge-teeth are present. In the right valve the anterior lateral and cardinal teeth are well developed, thin and wedge-shaped. In the left valve the cardinal tooth mostly is relatively elongated and lower than the anterior lateral tooth, which mostly is shorter. The posterior laterals are thin but proportionally much broader than in the Painter´s mussel.Biology
The species inhabits the same habitats as the Painter´s mussel (i.e. brooks, streams, rivers and lakes) in Sweden. It is not abundant in water of strong current. It prefers waters that are rich, to medium rich in nutrient but can somehow be found also in habitats of more poor nutrient. It is mostly found in shallow water but moves also to greater depth (9 - 10 m) than the Painters mussel.Distribution
Rather common in Sweden. The species occures from Scania in the very south to the middle part of the Country. Not present beyond "Limes Norrlandicus
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