Snrub- I think that the snail thing is smart from the way I'm thinking now. (some will certainly disagree) The clams are definitely eaten by RES. As far as the mussels, I've learned that there a lot of different kinds of freshwater mussels that have various life cycles and can be good for your pond or horrible for it so if you are going to add them like me you better be sure which kind you are adding.

1 way they could be in your pond is they could have been attached to the gills of your fish, frogs, or salamanders that you added or came naturally to your pond. That is part of some of the life cycles of different mussel species.