Zebra mussells are one non-indigenous life form in the Great Lakes. And, yes, they actually clean the water better than anything else that has been tried so instead of eliminating them, they want to control their numbers since they have a tendency to breed out of control. Then there's the Asian carp. One was recently found in a river near one of the Lakes but, as far as I know, none have been found in the Lakes themselves and may it ever be so.

Ships from all sorts of distant ports bring in critters not indigenous to the Lakes.