Apr 11, 2021


Even today, new animals are being discovered – and the one just revealed by zoologists at Tel Aviv University (TAU) doesn’t even breathe oxygen. The unexpected finding changes one of science’s assumptions about the animal world.

The tiny creature, having fewer than 10 cells, is a myxozoan parasite called Henneguya salminicola and lives in the muscle tissue of salmon. As it changed, the animal – which is an aquatic animal and a relative of jellyfish and corals – gave up breathing and consuming oxygen to produce energy. 

The study led by Prof. Dorothee Huchon of the School of Zoology at TAU’s Faculty of Life Sciences and the Steinhardt Museum of Natural History was published in the journal Proceedings of the [US] National Academy of Science (PNAS). 

“Aerobic respiration was thought to be ubiquitous in animals, but now we confirmed that this is not the case,” Huchon said. “Our discovery shows that evolution can go in strange directions. Aerobic respiration is a major source of energy, and yet we found an animal that gave up this critical pathway.”  The Israeli zoologist said that the discovery bears enormous significance for evolutionary research. 

“It is generally thought that during evolution, organisms become more and more complex and that simple single-celled or few-celled organisms are the ancestors of complex organisms,” she concluded. “But here, right before us, is an animal whose evolutionary process is the opposite. Living in an oxygen-free environment, it has shed unnecessary genes responsible for aerobic respiration and become an even simpler organism.”

Some other organisms like fungi, amoebas or ciliate lineages in anaerobic environments have lost the ability to breathe over time. The new study demonstrates that the same can happen to an animal –

possibly because the parasite happens to live in an anaerobic environment. 

Its genome was sequenced, along with those of other myxozoan fish parasites, as part of research supported by the US-Israel Binational Science Foundation and conducted with Prof. Paulyn Cartwright of the University of Kansas, and Prof. Jerri Bartholomew and Dr. Stephen Atkinson of Oregon State University.

The parasite’s anaerobic nature was an accidental discovery. While assembling the Henneguya genome, Huchon found that it did not include a mitochondrial genome. The mitochondria is the powerhouse of the cell where oxygen is captured to make energy, so its absence indicated that the animal was not breathing oxygen. 

Until the new discovery, there was debate regarding the possibility that organisms belonging to the animal kingdom could survive without breathing oxygen. The assumption that all animals are breathing oxygen was based, among other things, on the fact that animals are multicellular, highly developed organisms that first appeared on Earth when oxygen levels rose.

“It’s not yet clear to us how the parasite generates energy,” noted Huchon. “It may be drawing it from the surrounding fish cells, or it may have a different type of respiration such as oxygen-free breathing, which typically characterizes anaerobic non-animal organisms.”