I haven't fed snails or shrimp to my ducks, but they eat many many slugs. And I have read that they do fine with snails. I would also suspect shrimp would not be a problem but you may want to look around a little more about that. But slugs and snails I would feel comfortable having them eat. That said, sometimes the worms and slugs they forage outdoors can carry parasites - again this is something I have read and we have not had a problem.