This week of SciShow is supported by Brilliant!

To learn more, go to Brilliant.org/SciShow. [ ♪ Intro ]. I don't know if you've noticed, but animals kind of need oxygen.

That's because animals generally get their energy from cellular structures called mitochondria, and those processes require oxygen to work. So if somebody stole all of the Earth's O2, things would end pretty quickly around here. Except, as it turns out, there are at least some animals that would be perfectly fine.

Because in 2010, scientists published a paper announcing that they'd found three species of them that straight-up don't need oxygen! Now, to be clear, not all life needs oxygen. There are plenty of single-celled microbes that are anaerobic, meaning they can survive just fine without the stuff.

Instead of oxygen, these organisms can use other molecules like sulfate or nitrate. But for years, scientists thought a system like that wouldn't work for animals, since their complex, multicellular bodies have higher energy requirements. Instead, they thought animals needed the more efficient energy production that takes place in mitochondria.

And then came that 2010 paper. This discovery happened in the L'Atalante basin, three thousand meters below the surface of the Mediterranean Sea. L'Atalante is a deep hypersaline anoxic basin, meaning it's super salty and completely devoid of oxygen.

It's the kind of place you wouldn't expect to find animals. And indeed, when a research team visited three times between 1998 and 2008, that's generally what they saw. They did find a lot of single-celled organisms living in the basin, but most of the animals they saw were dead, the result of a so-called “rain of cadavers” from oxygenated waters above.

Most of the animals, but not all of them. Because the team also found an unusually high abundance of tiny, sediment-dwelling animals called loriciferans, and they were seemingly very alive. Loriciferans are pretty weird creatures to begin with.

Their heads are covered in spines, and their bodies are typically encased in a vase-like shell called a lorica. But finding them in an oxygen-free basin was a whole new level of weird. The researchers observed that the loriciferans were still taking up nutrients, and that some had recently molted.

Some even had developing offspring inside them. So these animals apparently spend their lives buried in this sediment, with no oxygen, not only surviving, but thriving. Part of this incredible survival might be down to their size.

At less than one millimeter long, loriciferans have pretty low energy needs. But they also seem to have some unique adaptations. For one thing, they don't have mitochondria!

Instead, they have cellular structures that look a lot like hydrogenosomes. These are organelles that some microbes use to produce energy, and they use hydrogen ions in place of oxygen. Alongside these structures, the researchers also noticed shapes that might be microbes living inside the loriciferans' cells.

That's intriguing because some anaerobic, single-celled organisms also have symbiotic microbes that live alongside their hydrogenosomes. All in all, it looks like these loriciferans have developed similar cellular structures to anaerobic microbes for living in the same way, although it's not clear how they did this. One option is that they retained these adaptations from an earlier ancestor more similar to anaerobic microbes.

But it's also possible that their ancestors swiped genes from their microbial neighbors, allowing them to use the same cellular tricks for survival. Of course, this is an extraordinary claim, and some researchers have doubts. For example, a study published in 2015 looked in the same basin and was unable to find independent evidence of living loriciferans.

The researchers of the original study are still confident in their results, but it may take more confirmation to convince everyone. If these results do hold up, though, it could change how we understand the requirements of complex life. It would have implications for the diversity of animal life in the world today, for scientists interested in how life got started on an oxygen-deficient early Earth, and maybe even for scientists looking for life elsewhere in the solar system.

Ultimately, life is so adaptable and endlessly diverse, that we wouldn't be surprised if there are more surprises to be found. Critical thinking in science is obviously a great thing, though, and that's why it's so important for researchers to check each other's work. And if you want to brush up on your critical thinking skills, or just learn some really cool stuff, the Daily Challenges from Brilliant are a great way to do that.

Every day, Brilliant has new challenge questions about math and science topics. They're short, fun, and you can access them every day of the week for free! Like, I just completed one about black body radiation in a real life setting: a campfire!

If you become a Premium member, you'll also get access to the whole archive of questions. Best of all, the first 200 people to sign up at Brilliant.org/SciShow, will get 20% off an annual Premium subscription. And if you check it out, well, hey, thanks for supporting us! [ ♪ Outro ].