Shrimp

The Shrimp make their appearance in Ocean of Fear. They live in the ocean. Ironically, even though the Land of Ooo is post-apocalyptic world and there is a remains of an underground city in the ocean, they seemed to actually survive the apocalypse of Ooo. It could be possible that they hid somewhere to avoid the nuclear war or they have a life like a cat has nine lives. It remains unknown about how they survive but it is most likely incognito.