In the pinkish muscle of some Pacific salmon lives a distant cousin of jellyfish that thrives without working mitochondria, the energy-producing part of cells thought to be a cornerstone of animal life, a study suggests.

About 2 billion years ago, the ancestor of all eukaryotes — the large group of organisms with complex cells that includes everything from maple trees to manatees — engulfed a bacterium, striking up a mutually beneficial relationship (SN: 2/14/20). Eventually, this bacterium evolved into mitochondria, the cellular machine that converts food and oxygen into energy, a process called aerobic respiration. Mitochondria retain many of the instructions for aerobic respiration in their own genome, separate from an organism’s DNA housed in a cell’s nucleus.

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.

While biologists think that mitochondria are the essential powerhouses behind eukaryotes’ more complicated lifestyles, Huchon says this study shows that things may not be so simple. “Evolution can take life in funny directions,” she says.