Fluorescent dye highlighting nuclei and membranes in a parasite that uses no oxygen Image courtesy of Stephen Douglas Atkinson

Breathing oxygen is seen as a fundamental characteristic of multicellular animals, but we have found at least one that can’t do it.

“It has lost the ability to breathe oxygen,” says Dorothee Huchon at Tel Aviv University in Israel. It remains a mystery how this animal, a parasite that infects salmon, gets the energy it needs without oxygen, she says, but it probably steals it from its host.

All plants and animals were thought to use oxygen to generate a fuel called adenosine triphosphate (ATP), which powers cellular processes. The generation of ATP from oxygen takes places in structures called mitochondria.


Each mitochondrion has its own tiny genome that is separate from the main genome in the cell nucleus. But when Huchon and her colleagues sequenced the DNA of Henneguya salminicola, which is related to jellyfish, they thought they had made a mistake because they found no mitochondrial DNA at all.

Further studies confirmed the finding. When the team stained H. salminicola with a blue fluorescent dye that binds to DNA, no DNA was visible in cells outside the nucleus. By contrast, when they stained a closely related parasite, blue dots corresponding to mitochondrial genomes were visible outside the nucleus.

So while the cells of H. salminicola have structures that look like mitochondria, they can’t make the enzymes needed to use oxygen to produce ATP. “These are not true mitochondria,” says Huchon.

This means H. salminicola is a multicellular animal that can survive entirely without oxygen. “There are plenty that can go for extended periods without, but nothing that can get through the whole life cycle,” says Nick Lane of University College London.

At least, nothing confirmed. In 2010, Roberto Danovaro at the Polytechnic University of Marche in Italy reported that a group of tiny animals called loriciferans that live in sediments in the deep sea had no visible mitochondria when viewed under a microscope, and must rely on other energy sources such as hydrogen sulphide instead.

However, other biologists say genomic studies are needed to confirm that loriciferans have really have lost the ability to respire oxygen. “The genomic analyses are under way,” Danovaro says. “I cannot say more.”

We don’t know why H. salminicola has lost this ability while all of its immediate relatives that we have identified use oxygen. As these parasites move through their life cycle, they may also live inside a worm host where they would have to make do with virtually no oxygen, as well. The worm host of H. salminicola has never been identified, but it too may live in sediments with very low oxygen levels, says Huchon.

Although the parasite is harmless to humans, it is a major problem for fish farmers because it creates unsightly white spots in the flesh of infected fish.

Journal reference: PNAS, DOI: 10.1073/pnas.1909907117