However, when there is a rule, there is usually an exception, and a group of scientists have discovered a small, parasitic cnidarian — a relative of a jellyfish — that apparently doesn't use oxygen to respire. They published their findings in the Feb. 24, 2020 issue of the Proceedings of the National Academy of Sciences.
This animal, Henneguya salminicola, is a tiny alien-headed parasite with a long tail that feeds on the muscle tissue of salmon and other fish. It's a eukaryote — a member of a broad group of organisms that includes most of the living things you can see with your naked eye: animals, plants, fungi, etc. The cells of eukaryotes contain all sorts of fancy organelles that their more primitive counterparts, the prokaryotes, don't have. One of those organelles is the mitochondria, a structure that has a tiny genome all its own, separate from the rest of the organism, and which eukaryotic cells use to produce energy, with the help of oxygen.
But within the larger groups we call "eukaryotes," there are a few single-celled, non-animal species that are anaerobic. They don't have mitochondria, but have something scientists call "mitochondrion-related organelles." Henneguya salminicola is the first animal to have this feature.
It's all very strange, but how did they get this way?