It has fewer than 10 cells, lives within the muscles of salmon and doesn't have mitochondria. It sounds like science fiction, but researchers say they've recently discovered the first-ever "animal" that doesn't require oxygen to live.

A tiny parasite called Henneguya salminicola is the first known multicellular animal that can survive without oxygen, according to a study published Tuesday in the journal Proceedings of the National Academy of Sciences by researchers at Tel Aviv University. In fact, it couldn't breathe oxygen even if it wanted to — it gave up the process as it evolved.

H. salminicola — a relative of jellyfish and corals — lives within the muscle tissue of salmon, but is relatively harmless. It causes "milky flesh" or "tapioca" disease, named for the white fluid-filled cysts it causes in the fish, according to the Alaska Department of Fish and Game.

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Researchers made the discovery accidentally while sequencing the "Henneguya" genome. Mitochondria, aka the powerhouses of the cell, capture oxygen to make energy through aerobic respiration — but researchers were surprised to find that H. salminicola lacks mitochondrial genes.

Henneguya salminicola is a multicellular organism, but it doesn't need oxygen to breathe, scientists say. Tel Aviv University

However, it is still unknown exactly how exactly the parasite produces energy.

"It's not yet clear to us how the parasite generates energy," Tel Aviv University professor and lead author Dorothee Huchon said in a press release. "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 characterizes anaerobic non-animal organisms."

The discovery fundamentally changes the way scientists view the animal kingdom, expanding the definition of what an "animal" can be.

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

Scientists said some single-celled organisms, including fungi and amoebas, have also lost the need to breathe over time, but this is the first time it's been documented in an animal. The parasite lives in an anaerobic environment — one explanation for its peculiar evolution.

"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," Huchon concludes. "But here, right before us, is an animal whose evolutionary process is the opposite. Living in an oxygen-free environment, it has shed unnecessary genes responsible for aerobic respiration and become an even simpler organism."