Stephen D. Atkinson

Oxygen, I'm sure you'd agree, is pretty important for life on Earth. We breathe it in, our cells survive on it and without it, we're pretty much doomed. Basically all multicellular life on Earth evolved over millions of years to utilize oxygen.

But take a deep breath, because we need to talk about Henneguya salminicola, a tiny parasite containing fewer than 10 cells that lives within the muscle tissue of salmon. The alien-tadpole-looking parasite does not busy itself with such trifling matters as breathing oxygen. Nope, it seems H. salminicola is the first multicellular animal we've found that survives without the stuff.

"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 Israel, in a press release.

Huchon and a team of international researchers examined and sequenced all of H. salminicola's genes in their study, published Monday in the journal Proceedings of the National Academy of Sciences. They found that the parasite, which is closely related to jellyfish, lacks the DNA machinery necessary to "breathe" -- it doesn't have mitochondria, often called the "powerhouse" of the cell, because they use oxygen to make energy. It's like a little factory inside (almost) all cells, and DNA sleuths can find mitochondrial genes during sequencing.

The discovery came as something of an accident for the team, who were probing the genome of the species. When they went searching for the mitochondrial genes, they came up empty-handed.

"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."

Some single-celled lifeforms have evolved the ability to survive without oxygen and do not possess mitochondria, either. Plenty of bacteria and protozoans can exist without oxygen at all, like those at the bottom of the ocean near hydrothermal vents and like some human pathogens that live inside the body. Instead of oxygen, they get their energy from fermentation or use other molecules like mercury or iron.

But how H. salminicola generates energy is still unclear. Huchon suggests it could draw oxygen from the cells in salmon or it could have evolved similar methods to those of unicellular organisms scientists have documented in the past.