Imagine this: you're sitting outside your high school biology class cramming for an exam. The topic is aerobic respiration. "The process of using oxygen to create energy that powers the living cell," you mutter to yourself. Aerobic respiration takes place in the mitochondria, and it is why animals — including us — need oxygen. Whether we breathe it in, extract it from water through gills, or absorb it through the skin, we all need it.

Or so we thought. But brand new research has upended that assumption. Scientists recently discovered a parasite that lacks mitochondria and so cannot use oxygen to power their cells.

Let me introduce you to Henneguya salminicola. H. salminicola is a member of the phylum Cnidaria. You might be familiar with a few other Cnidarians, such as sea jellies and corals. However, H. salminicola belongs to a group of parasitic animals called Myxozoa. Their hosts include salmonid fishes, such as salmon, trout, and other freshwater fish.

When the researchers examined the parasite's genome, they didn't find any evidence of mitochondria. And when they looked at its cells under a microscope, they found cristae, a structure that resembles the inner folds of mitochondria, but doesn't perform the same way. This suggests that the lack of a mitochondrial genome in H. salminicola is not a very old trait. Instead, they speculate that the loss of mitochondrial DNA may be a recent event in this organism's evolutionary history. 

Although this discovery upends what we knew about animal biology, it also makes sense. Salmon muscles are low-oxygen environments, and so over time H. salmonicola apparently lost its ability to power itself with oxygen. But what makes this discovery so cool is the proof of a possibility. Multicellular life can exist without oxygen. Where else on Earth and in space could they be thriving?