scientists find an animal that doesn't need oxygen to survive

We were pretty sure that all complex life on our world needs oxygen. Now, we're not so sure after discovering a small, harmless parasite that doesn't seem to need it.

Here's how multicellular living things on this world work from a biochemical standpoint. Using water as a solvent for chemical reactions, they absorb or actively breathe in oxygen, then use the mitochondria -- which is the powerhouse of the cell -- to create adenosine triphosphate, or ATP, which is then used to enable every other metabolic process in their cells. We think this is the end result of a symbiotic relationship between two single celled organisms one of which evolved to be a power plant for the other since the mitochondria has structures and genetic signatures indicating that it was once independent. But no matter exactly how this happened, the end result is that all complex life on Earth needs oxygen to function.

Or so we thought until a group of scientists looked at Henneguya salminicola, a simple salmon parasite that looks like the popular minimalistic depiction of a gray alien's face grew a tail and started swimming around. Its ancestor was a jellyfish that took in oxygen, used its mitochondria to make ATP, and powered itself the same way we do. But the resulting cnidarian -- the term for a creature related to jellyfish and sea anemones -- seems to have simplified and not only stopped using mitochondria to power its cellular processes, it lost the organelles altogether. Not only does this mean that the parasite doesn't need oxygen to survive, it means this odd creature doesn't even know what to do with it anymore.

Hold on a minute, you might say. If multicellular life needs an energy molecule produced by mitochondria to function, how exactly does this organism survive? Well, we really don't know. It may produce something different or leech ATP from its host. The study just looked at the overall composition of its genetic information. What we do now know is that not only does unicellular life not require oxygen to exist and function, it may be optional for multicellular life as well, and animals could evolve with the capacity to use oxygen, then lose that ability, and maybe never even acquire it at all. And that means we could look for complex life on other worlds without worrying about oxygen being a requirement.

See: Yahalomi, D., et al, (2020) A cnidarian parasite of salmon lacks a mitochondrial genome, PNAS, DOI: 10.1073/pnas.1909907117