It’s a microscopic Cnidarian parasite of salmon:
An international team of researchers has found a multicellular animal with no mitochondrial DNA, making it the only known animal to exist without the need to breathe oxygen. In their paper published in Proceedings of the National Academy of Sciences, the group describes their study of Henneguya salminicola, a microscopic, parasitic member of the group Myxozoa and its unique physiology.
One of the common characteristics of all multicellular animals on Earth is mitochondrial respiration—the process by which oxygen is used to generate adenosine triphosphate—the fuel used to power cellular processes. The process takes place in mitochondria, which has both its own genome and the main genome found in the rest of the body’s cells. But now, there is a known exception: Henneguya salminicola.
In their work, the researchers sequenced the DNA of H. salminicola tissue and found no mitochondrial DNA at all. Believing they had made an error, the team repeated their work and once again found no sign of mitochondrial DNA. Confused, they sequenced the DNA of close relatives of H. salminicola and found evidence of the expected mitochondrial genomes. H. salminicola did have structures that resembled mitochondria but they were not capable of producing the enzymes needed for respiration, a finding that suggested the creature was capable of surviving without oxygen—a first. The presence of structures that resemble mitochondrial DNA suggests that the tiny parasites have undergone a process of de-evolution. In addition to losing the apparatus to create ATP, they also have lost tissue, nerve cells and muscles.
The researchers did not find any other mechanism for producing the fuel cells in H. salminicola would need to survive, but suggest they likely steal energy from their host using some type of proteins.Bob Yirka, “Henneguya salminicola: Microscopic parasite has no mitochondrial DNA” at Phys.org
Paper. (open access)
Devolution, of which this is an example, may be more common than we suppose and will probably have precisely the effect of creating “exceptions” like this. Note that we are told, “they likely steal energy from their host using some type of proteins.” It makes sense that many devolved creatures are parasites. They can afford to throw away equipment if they are using the host’s toolbox anyway.
See also: Devolution: Getting back to the simple life