Some facts about the universe and our experience of it seem unlikely to change. The sky has lifted. Gravity sucks. Nothing can go faster than light. Multicellular life needs oxygen to exist. But we may need to rethink that last one.

In 2020, scientists discovered a jellyfish-like parasite that lacks a mitochondrial genome, becoming the first multicellular organism to do so. This means he is not breathing; In fact, he lives his life completely free of oxygen dependence. This discovery not only changes our understanding of how life might operate here on Earth, but could also have implications for the search for extraterrestrial life.

Life began to develop the ability to metabolize oxygen, that is, to breathe, about 1.45 billion years ago. The larger archaea engulfed the smaller bacteria, and somehow the bacteria’s new home proved beneficial to both parties, and they stayed together. This symbiotic relationship led to the two organisms evolving together, and eventually the bacteria that settled within them evolved into organelles called mitochondria. Every cell in your body, except red blood cells, contains many mitochondria, and they are essential for the respiratory process.

They break down oxygen to form a molecule called adenosine triphosphate, which multicellular organisms use to fuel cellular processes. We know that there are adaptations that allow some organisms to thrive in conditions of low oxygen or hypoxia. Some single-celled organisms have evolved mitochondria-related organelles for anaerobic metabolism; however, the possibility of the existence of exclusively anaerobic multicellular organisms has been the subject of some scientific debate.

That was until a group of researchers led by Diana Yahalomi of Tel Aviv University in Israel decided to take another look at a common salmon parasite called Henneguya salminicola. It is a cnidarian belonging to the same phylum as corals, jellyfish, and anemones. Although the cysts formed on the fish’s body are unsightly, the parasites are not harmful and will live with the salmon throughout its life cycle.

Hiding inside its host, the tiny cnidaria can survive in highly hypoxic conditions. But it’s hard to know exactly how this is done without looking at the creature’s DNA. So that’s exactly what researchers did. They used deep sequencing and fluorescence microscopy to study this H. salminicola and found that it had lost its mitochondrial genome. It also lost aerobic respiratory capacity and almost all of the nuclear genes involved in mitochondrial transcription and replication.

Like single-celled organisms, mitochondria-related organelles have evolved, but they are also unusual; There are folds in their inner membranes that cannot normally be seen. The same sequencing and microscopic techniques for a closely related cnidarian fish parasite, Myxobolus squamaliswas used as a control and clearly showed the mitochondrial genome.

These results showed that a multicellular organism had finally emerged that did not need oxygen to survive.

Despite H. salminicola remains a mystery, the loss is quite consistent with the genetic simplification that is the general tendency of these creatures. Over many years, they evolved from the free-living ancestor of the jellyfish into the much simpler parasite we see today.

They lost most of the original jellyfish genome, but retained a complex structure oddly reminiscent of the jellyfish’s stinging cells. They use them not to sting, but to cling to their hosts: an evolutionary adaptation from the needs of a free-living jellyfish to the needs of a parasite. You can see them in the picture above; These are things that look like eyes.

This discovery could help fisheries adapt strategies to control the parasite; Even though it’s harmless to humans, no one wants to buy salmon full of tiny, weird jellyfish. But it’s also an incredible discovery that helps us understand how life works.

“Our discovery confirms that adaptation to anaerobic environments is common not only in unicellular eukaryotes but also in multicellular animal parasites,” the researchers explained in their paper published in February 2020.

“For this reason, H. salminicola It makes it possible to understand the evolutionary transition from aerobic metabolism to exclusively anaerobic metabolism.” The study was published on: PNAS.