Pretty much everyone knows that oxygen is a fundamental requirement for life as we know it, but oddly enough that’s not entirely right. There are many simple life forms, such as some kinds of bacteria and archea, that don’t have a major metabolic pathway that needs oxygen. There are even a few complex one-celled organisms and fungi that don’t have the oxygen-requiring pathway. While that may be true for some living things, one would think that all animals need oxygen.
Well, that’s what biologists used to think. Animals are made up of many complex cells. Each animal cell usually has a metabolic pathway that uses oxygen to extract chemical energy from substances derived from food. This set of biochemical reactions occurs in parts of the cell called mitochondria. It is one of the most important metabolic pathways in most complex cells.
But researchers no longer believe that all animals have it. In 2020 an international team of researchers reported the surprising finding that they had found a simple animal whose cells lack the metabolic pathway for using oxygen. While sequencing the animal’s genome, the biologists found that it lacks the genes that encode the proteins needed for the metabolic pathway.This animal is a tiny parasite that lives in the muscles of salmon. The creature is part of a large group of parasitic animals that are closely related to jellyfish, but much simpler. It consists of only about ten cells. The cyst-like nodules that it creates in the fish’s muscle lack oxygen. Since parasites often evolve to become simpler, the animal probably lost an oxygen-using pathway possessed by its ancestors.