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Anodonta cygnea
Balaton lake
This is a very cool find, it's a kind of mussel called a swan mussel. That white thing coming out on the right of the shell is its foot. I'm not sure what those other things on it are, hard to tell from this photo, but I'd guess it's another smaller species of bivalve that's latched on to it. Thanks for sharing, I'll get the ID suggested for you!