@pawian, that's a nice blue colour :)
These are native to (or common in) Poland with colonies in the Wisła and Odra. The szczeżuja (Eng: duck mussel).
We found it once in a river near my grandma's village when I was a kid or a teen. We simply called it "małż". Btw, some women call their husbands in such a humorous way, because in Polish one of the words for a husband is "małż
The don't have brains either. maybe they should start posting here ;-)