H. salminicola is quite a common parasite that infects the muscle tissue of salmon, reproducing in the muscle tissues of the fish with unbelievably high speed and causing tapioca disease (also known as milky flesh) in fish. These are white cysts in the salmon's flesh, which do appear quite unsightly but are generally harmless to both humans that eat the fish and the salmon itself.
Since the animal resides within the flesh of the fish, and like many parasites, it needs to reproduce rapidly, h. salminicola is quite a simple organism that doesn’t have a nervous system, muscles, or a digestive tract. The only occasion the animal appears to exhibit any complexity is when it’s in the spore stage, seeking to attach to its host. During this stage of development, the animal looks similar to a human sperm cell with two “alien eyes” that are actually stinger cells that help the animal attach itself to the host.
The DNA of h. salminicola has been sequenced in a recent study at Tel Aviv University, in which researchers tried to understand how the parasite can possibly survive in an oxygen-free environment. What they found mystified them, as the animal turned out to be so simple, in fact, that it appears to have shed an ability previously believed essential for animals - cellular respiration.
The process of cellular respiration is when oxygen is used by the cell to generate power to sustain itself. Cellular respiration occurs in the mitochondria, a unique organelle that has its own DNA. H. salminicola appears to lack this entire system and mitochondrial DNA altogether. Hence, it is the first-ever animal species that doesn't require oxygen to survive.
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The scientists compared the DNA of h. salminicola with that of similar organisms to make sure their findings weren't a mistake, and the results turned out to be genuine. All of the close relatives of the parasite from the Myxozoa group had mitochondrial DNA. The researchers further point out that the genome of the animal suggests that it once was capable of respiration, but gradually lost the ability in an attempt to simplify its structure and be able to survive and multiply in its host very fast.
This is a great example of a somewhat counterintuitive evolutionary adaptation where the structure of a once more complex animal simplifies to its core features in order to suit its surroundings. As one of the authors, Dorothée Huchon, mentioned in an interview with LiveScience, "Here, we see an organism that goes completely the opposite way. They have evolved to be almost unicellular."
As of now, the way h. salminicola actually gets energy to sustain itself remains a scientific mystery. The researchers believe that the animal may be retrieving energy directly from the host's surrounding cells similarly to other anaerobic bacteria, but they can't confirm this theory as of now. Whatever the case may be, we can confidently say that the very existence of this microscopic animal is unique and absolutely mystifying.
Bonus! Video About h. salminicola