Life Without Oxygen

There's a little jellyfish that lives on salmon. It's a benign parasite.

And it doesn't have mitochondria.

Which means it doesn't need oxygen. Now, it's possible that it's letting the salmon metabolize oxygen for it, but...this might have implications for extraterrestrial life.

Maybe.

I'd want to know more, a lot more.

(And find a free-living critter that can pull this stunt. I suspect there isn't one).