Every animal needs oxygen to live—except, we now know, for at least one.
Oxygen not required
A team of researchers led by Dorothee Huchon, an associate professor in the Department of Zoology at Tel Aviv University, set out to discover how the multicellular parasite Henneguya salminicola manages to survive buried deep in the flesh of its host creature, salmon, where there is virtually no oxygen. Using deep sequencing and fluorescence microscopy, the researchers were surprised to find that H. salminicola lacks the mitochondrial genome that is necessary for aerobic respiration. In fact, the parasite also lacks the nuclear genes responsible for transcribing and duplicating the mitochondrial genome. In other words, the animal evolved in such a way that it doesn’t need to breath at all. Because it gets essentially all of what it needs to survive from its host, the parasite over time saved energy by no longer copying genes for structures and functions it no longer needed.
What is Henneguya salminicola?
H. salminicola is a ten-celled myxozoan cnidarian and belongs to the same phylum as jellyfish and anemones. The parasite likely traces its ancestry to a free-living jellyfish creature and, through a process of genetic simplification, evolved into its present form. As one of the research team’s members, Stephen Atkinson of Oregon State University’s Department of Microbiology put it, the H. salminicola is a good example that “sometimes, less is more.” Very little of the jellyfish genome has survived in H. salminicola’s genetic profile, though it does still have a structure that seems derived from the jellyfish’s stinging cells. It uses this structure, not to sting, but to cling to the salmon in which it lives.
How does it survive?
The discovery raises more questions, of course. For one thing, how does H. salminicola survive? While there’s no firm answer for this yet, one theory is that the parasite absorbs from the salmon’s flesh the organic chemical adenosine triphosphate (ATP), which provides the energy that drives most processes in living cells. Infusions of ATP from the host could drive the parasite’s muscle contractions, nerve impulse propagation, and chemical synthesis—all functions also driven by oxygen in creatures that breath.
Why does it matter that it doesn’t breath oxygen?
The discovery reveals that adapting to be able to survive without breathing, in an environment without oxygen, is something that a multicellular animal can do just as well as a single-celled creature, the research team concluded in their study, published in 2020 in the journal Proceedings of the National Academy of Sciences of the United States of America. Their discovery about H. salminicola thus gives scientists the chance to learn more about how creatures can evolve away from aerobic forms of life and become purely anaerobic.
One of the fundamental aspects of how we have traditionally explained what an “animal” is centers on the idea that it engages in respiration. As Atkinson points out, though, H. salminicola means that we need now to expand our definition of what an “animal” can be. And he notes that it likely means that there are many other oxygen-free animals out there, waiting to be discovered.
In addition, the example of H. salminicola poses some new questions about the nature of evolution itself. “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,” Huchon stated. H. salminicola, however, has done the opposite, shedding genes and evolving into a simpler organism.
Changing how we define what it means to be an “animal”. Challenging how we understand evolution operates. That’s a pretty big impact for such a small creature.