system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do immortal jellyfish stay alive?
gpt
It's kind of played up in the media...they probably are functionally immortal, but it's not that special.  

Jellyfish (actually a hydrozoan in this case but that doesn't really matter much here) have two life stages, a polyp stuck on a rock like a little anemone (the juvenile), and a floating jellyfish stage (the adult)

Immortal jellyfish can revert from the adult phase back to the juvenile phase...but that doesn't necessarily make them immortal, it's just changing the shape of their body.  Actual immortality requires keeping your _cells_ healthy and renewed.

But immortal jellyfish can probably do that so they probably are immortal anyway.  But that's nothing special, even their relative the lowly hydra, which can't do any special "turn back into a baby" tricks, is probably immortal.  We've tracked them for years in lab experiments and they show no sign of aging at all....not surprising since they regularly replace and renew _all_ their cells....easy to do when you are more or less just a bag with tentacles.  Probably a lot of animals in this group can just go on living forever...or at least until something eats them or they starve or whatever.