In 2020, scientists stumbled upon a biological anomaly that rewrote a fundamental rule of life—Henneguya salminicola, a parasite found in salmon, was discovered to survive without oxygen. This tiny creature stunned researchers because until then, oxygen dependency was considered a universal trait of multicellular animals. But here was proof that life could persist, thrive even, in total absence of it.
Henneguya salminicola belongs to the phylum Cnidaria, making it a distant cousin to jellyfish and sea anemones. It is a myxozoan parasite known for forming cysts in fish muscles, notably salmon. What sets it apart is its complete lack of a mitochondrial genome. Mitochondria, often called the “powerhouse of the cell,” are responsible for cellular respiration—a process that transforms oxygen into usable energy. Without mitochondria, aerobic respiration is impossible. So how does H. salminicola survive?
Researchers believe it obtains energy through anaerobic means, possibly by absorbing ATP directly from its host or utilizing an alternative biochemical process. Although anaerobic metabolism is common among single-celled organisms and some fungi, it was previously unknown in multicellular animals. This makes H. salminicola a biological outlier and the first confirmed animal that doesn’t rely on oxygen to stay alive.
How Was Henneguya salminicola Found?
The discovery was made by scientists at Tel Aviv University during genome sequencing of H. salminicola. What they found was surprising—not only did the parasite lack mitochondrial DNA, but it had also lost the ability to perform aerobic respiration altogether. Instead of replacing this vital function with a similar mechanism, it completely adapted to survive through other means. This level of evolutionary regression and adaptation is extremely rare.
This tiny organism pushes the boundaries of how we define “animal life.” It shows that even complex organisms can shed seemingly vital biological features if their environment permits. Living inside salmon muscle tissue—an environment low in oxygen—H. salminicola has evolved to forgo oxygen in favor of stability. Its existence proves that evolutionary pressure doesn’t always demand complexity; sometimes, simplicity wins.
Redefining the Search for Life
Traditional biology textbooks teach that animals need oxygen—it’s the basis of metabolism and energy production. But Henneguya salminicola breaks this mold. It reveals that our understanding of life is not fixed, but evolving. Much like how tardigrades challenged perceptions of survivability by living through radiation and vacuum conditions, H. salminicola calls for a reexamination of our biological rules.
Henneguya salminicola may be invisible to the naked eye, but its impact on science is monumental. By surviving without oxygen, it forces us to reconsider the very essence of biological life. From evolutionary biology to astrobiology, the ripple effect of this discovery spans disciplines and possibilities.it tears down assumptions about life and creates exciting new pathways for research across disciplines.
This tiny parasite tells us that life is capable of extraordinary adaptation. And as science peels back the layers of this mystery, we’re reminded: the universe may be teeming with life—but not necessarily life as we know it.
