there's a species of jelly which can revert to its larval stage when it is under stress, or if it's old or sick. it can then carry on life as usual from then on, thus being practically immortal.
also they're jellies not jellyfish why the fuck would we call them fish they are just not fish like come on do they even look a bit like fish
or do we just have a compulsion to call everything that lives in the ocean a fucking fish - starfish, jellyfish, cuttlefish (whalefish?)