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.
Without the ability to breathe, it's a mystery for now how Henneguya salminicola gets its energy.
"It may be drawing it from the surrounding fish cells, or it may have a different type of respiration such as oxygen-free breathing, which typically characterises anaerobic non-animal organisms," which are usually single-celled, said Dr Huchon.
But Henneguya salminicola's strangeness doesn't end there. The scientists also found evidence over the years, it has evolved to become a more simple creature - opposite to the rest of the animal kingdom, which has generally produced more and more complex beings over time.
The structures that would host mitochondria were present, but incapable of producing the necessary enzymes for processing oxygen.
"They have lost their tissue, their nerve cells, their muscles, everything," Dr Huchon told Live Science. "And now we find they have lost their ability to breathe."
That simplicity allows Henneguya salminicola to reproduce very quickly however, an advantage for parasitic species.
"It is generally thought that during evolution, organisms become more and more complex, and that simple single-celled or few-celled organisms are the ancestors of complex organisms," said Dr Huchon.
"But here, right before us, is an animal whose evolutionary process is the opposite."
The findings were published in journal Proceedings of the National Academy of Sciences.