An evolutionary paradox in salmon
Deep within the muscle tissue of Pacific salmon, a biological revolution is underway. The perpetrator is Henneguya salminicola, a microscopic parasite that challenges the very definition of what it means to be an animal. In a discovery published in 2020, scientists found that this creature completely lacks a mitochondrial genome. This is significant because all other known animals have mitochondria, the cellular powerhouses that use oxygen to generate energy. H. salminicola has abandoned aerobic respiration, the oxygen-breathing process that powers nearly all animal life.
This organism is a myxozoan, a group of highly simplified parasitic animals that are relatives of jellyfish, corals, and sea anemones. For years, myxozoans were so strange they were classified as protozoans, but DNA evidence confirmed they are cnidarians that have undergone extreme evolutionary changes. As they adapted to a parasitic lifestyle, they shed many complex features, resulting in some of the smallest genomes in the animal kingdom. H. salminicola represents the most extreme example of this reduction. It lost its mitochondria and nearly all the nuclear genes required for their replication and function. However, it retains structures that appear to be mitochondrion-related organelles (MROs), though they lack the genes for respiration.