This paper contains a couple of mentions about Marmorkrebs (though they are so brief that doesn’t quite rate a separate entry in the abstracts). The most important one is this:
At least three new populations of marbled crayfish Procambarus sp. have been found in the wild in Germany in the past two years, all aquarium discards (Chucholl, pers. comm.).
“Chucholl” is Chris Chucholl, who is studying ecology and invasive species issues.