The parasite on fish and worm muscles has neither a mitochondrial genome, we are told, nor a way to breathe:
A microscopic and genomic analysis of the creature revealed that, unlike all other known animals, H. salminicola has no mitochondrial genome — the small but crucial portion of DNA stored in an animal’s mitochondria that includes genes responsible for respiration.
While that absence is a biological first, it’s weirdly in character for the quirky parasite. Like many parasites from the myxozoa class — a group of simple, microscopic swimmers distantly related to jellyfish — H. salminicola may have once looked a lot more like its jelly ancestors but has gradually evolved to have just about none of its multicellular traits.
Brandon Specktor, “Scientists discover first known animal that doesn’t breathe” at LiveScience
Paper. (paywall)
That’s called devolution, when life forms simply junk complex equipment they never use. One wonders if there is any characteristic of live that some life form or other has not devolved to get rid of. But they will, of course, likely be parasites like salminicola.
See also: Devolution: Getting back to the simple life