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Oxygen is so essential for animals that every multicellular species we’ve ever studied has the ability to use oxygen to create energy... except one.

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There are a few things in life that we, as animals, absolutely cannot live without. One of those things is oxygen.

It's so essential that every multicellular species we've ever studied has the ability to use oxygen to create energy. That is, every species except one. Because in 2020, biologists reported that they'd found an animal that not only doesn't breathe oxygen — as far as they could tell, it can't.

This species is called Henneguya salminicola, because it infects salmon during a couple of stages of its development. It's a type of animal called a myxozoan, making it a sort of tiny parasitic jellyfish with a complex life cycle. We know it spends part of its time in salmon, but not where it goes after that.

And it doesn't seem to have much in the way of mitochondria. Say it with me, now: Mitochondria are the powerhouses of the cell. They take sugars and oxygen and turn them into the molecules that carry energy.

All eukaryotic life -- everything with a nucleus, both single-celled and multicellular -- has these in its evolutionary history. Even life forms that have adapted to oxygen-poor environments tend to hold onto some version of their mitochondria. And all animals have some capacity to use their mitochondria to metabolize oxygen.

Even though they live inside other cells, mitochondria have their very own genomes -- called mitochondrial or mtDNA. In this study, published in the journal PNAS, researchers were trying to compare the mitochondrial genomes of H. salminicola and another, closely-related species. Except they discovered that… they couldn't.

Because H. salminicola didn't seem to have mtDNA something we've never confirmed in a multicellular organism before. DNA, both mitochondrial and regular, is what tells a cell how to make different proteins. And without mtDNA, this parasite is missing the instructions to make proteins it would need to turn oxygen into energy.

The team was understandably intrigued, and they decided to look at its regular DNA, too. Specifically, they were looking for genes that we know help mitochondria turn oxygen into energy, some of which are in the regular genome rather than the mitochondrial one. And H. salminicola only had 7, while other myxozoans usually have somewhere between 18 and 25.

These guys still have structures similar to mitochondria, called mitochondria-related organelles. And these structures probably do help with energy production. Just… not from oxygen.

So why'd our salmon-infecting friend toss its ability to use oxygen out the window? It lives in an oxygen-poor environment anyway, such as… inside a salmon. So it must get its energy through less efficient, oxygen-independent means specifically, by breaking down sugars without oxygen.

Life that does breathe oxygen, like us, can also break down sugars this way, but you get much less energy out of it. We know that when genes go unused for many generations, they can simply… be lost. In the case of this parasite, losing its mtDNA could be an evolutionary adaptation.

It takes energy to maintain genes. And especially in a creature with a tiny genome like H. salminicola, losing DNA that it wasn't using anyway could have saved energy it could otherwise use to survive. Or something similar could have happened as a fluke.

Natural selection can drive genetic change, but so can pure random chance. The team points out that besides revealing the first known animal to not breathe oxygen, this discovery could also be useful on a practical level. This myxozoan parasite can actually be a big problem for salmon farmers, and knowing that is doesn't breathe oxygen could help us make drugs that are better at targeting it.

So maybe it wasn't that great a trick, after all. Thanks for watching this episode of SciShow, which is produced by Complexly. If you like learning about super small stuff, we think you'll really like our sister channel, Journey to the Microcosmos.

The show's whole MO is slow, calming descriptions of microscopic life, paired with fantastic microscopy and soothing music. It's what we all deserve right now. Check it out at the link in the description! [♩OUTRO].