A salmon parasite can survive without oxygen, without mitochondria, and with a lot of mystery.

Science News reports on the humble jellyfish-relative that lives in the bodies of Pacific salmon and undersea worms, and gets along fine without any mitochondria – the part of a cell that provides energy:

While a few single-celled eukaryotes have adapted to low-oxygen environments by ditching their mitochondrial genomes, rendering their mitochondria useless, scientists had assumed that more complex animals couldn’t get by without them. But a parasitic cnidarian can, researchers report February 24 in PNAS. This cnidarian — a group of animals that includes jellyfish and coral polyps — may challenge biologists basic assumptions about what animals can do.

Dorothée Huchon, an evolutionary biologist at Tel Aviv University in Israel, and colleagues analyzed the genomes of members of a large and peculiar group of microscopic, parasitic cnidarians called Myxozoa, and found that one species’s mitochondrial genome was missing. Microscopy revealed mitochondria-like structures within Henneguya salminicola, though the researchers doubt they are capable of aerobic respiration.

The loss may be an adaptation to H. salminicola’s low-oxygen environment. Like other Myxozoa, it jumps during its life cycle between two hosts — fish, specifically salmon, and annelid worms. In addition to shelter, the parasite also may be able to rely on its hosts for energy, instead of its own mitochondria. Shedding unnecessary and cumbersome DNA through evolution might have helped the parasite save energy, giving H. salminicola a leg up over its mitochondria-filled Myxozoan cousins.

If you click the link, you’ll see an image of these little guys, looking for all the world like a cross between a tadpole and a grey alien.