Henneguya salminicola is a parasite living inside the muscle tissue of salmon (Stephen Douglas Atkinson)

Oxygen makes up around 20% of the atmosphere of Earth and it’s also kind of a big deal when it comes to cellular respiration.

But, apparently, not for everything.

Scientists in Tel Aviv have discovered a parasite called Henneguya salminicola which appears to be the first ever multicellular animal ever found that doesn’t require oxygen to breathe.

It’s a small animal, made up of only about 10 cells. It looks a bit like a tadpole and is closely related to a jellyfish. The team of researchers found it inside the muscle tissue of a salmon.

The discovery that it doesn’t need oxygen was a bit of an accident. The researchers were studying the genome of the species and found that it doesn’t have any mitochondria – the gene that converts oxygen to energy.

‘Aerobic respiration was thought to be ubiquitous in animals, but now we confirmed that this is not the case,’ said Dorothee Huchon, a zoologist at Tel Aviv University in a press release announcing the find.

H. salminicola lacks the mitochondria gene to convert oxygen into energy (Stephen Douglas Atkinson)

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

It’s not yet clear how H. salminicola generates energy for itself, but Prof Huchon says this discovery has enormous significance in the field of evolutionary research.

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‘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,’ she 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.’