Scientists have discovered an animal that doesn't breathe oxygen and appears to be evolving backwards.

Henneguya salminicola is a parasite related to jellyfish and corals which lives inside salmon muscle. Its genome was recently sequenced by scientists in Israel, who were surprised it has no mitochondria - which is essential for catching oxygen and turning it into energy.

Every single other animal known to science needs oxygen to live.

"Aerobic respiration was thought to be ubiquitous in animals, but now we confirmed that this is not the case," said Dorothee Huchon of Tel Aviv University.

"Our discovery shows that evolution can go in strange directions. Aerobic respiration is a major source of energy, and yet we found an animal that gave up this critical pathway."

Henneguya salminicola causes what fishers call tapioca disease, because they look like tiny white cysts in the fish's flesh.