Scientists have discovered an unusual species of parasite hiding the muscles of salmon. The tiny species, comprised of just ten cells, is unlike all other animals known to science. The species, Henneguya salminicola, doesn't breathe oxygen.

Over the course of its evolution, the parasite abandoned breathing and consuming oxygen in order to produce more energy.

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

Several fungi species, as well as amoeba or ciliate lineages, have lost the ability to breathe oxygen over long periods of evolution.

When scientists sequenced the genome of the myxozoan species, a relative of jellyfish and corals, they found its mitochondrial genome was missing. The mitochondria is responsible for collecting oxygen and converting it into energy.

Because the parasite is without mitochondria, scientists determined Henneguya salminicola no longer breathes oxygen. The parasite -- described this week in the journal PNAS -- provides proof that animals can survive anaerobic environments.

The first large and diverse group of complex, multicellular life forms emerged around the time that oxygen levels on Earth rose dramatically. Scientists have long assumed that aerobic respiration is ubiquitous in the animal kingdom.

There is still a lot scientists don't know about the unusual parasite.

"It's not yet clear to us how the parasite generates energy," Huchon said. "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."

According to Huchon, the new parasite species undermines another scientific assumption, a principle of evolution. Organisms are supposed to get more complex as they evolve. Simple organisms are interpreted as the ancestors of more modern, complex species.

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

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