What jellyfish doesn't die from old age?

All animals die..

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No, there is two species of jellyfish, Turritopsis Dohrnii, and Turritopsis Nutricula that reaches a point where it reverts to an early stage of development and then continues as if it is young. It is not clear if this process can continue without end for an individual jellyfish, but the possibility cannot be categorically rejected. To be fair it has to be pointed out that most of these jellyfish live and die the old fashioned way. But if they are under stress for some reason, the reverting process can kick in.