There is a certain type of crayfish that some will keep as a pet that if just dumped in the wild will multiply rapidly and it doesn't need a mate to do it.
There's the danger of mercury poisoning and if you were to only feed canned tuna to your cat, they wouldn't get all of the nutrients they need. This is very unhappy news for Juniper the Cat, who only has cans of tuna on her Christmas wish list.