When it comes time to build an animal, a few crucial ingredients come to mind: carbon-containing molecules, to build a body; water, to slosh nutrients in and waste out; and oxygen, to power each cell’s busy work. Across millennia, from bumblebees to blue whales, this recipe has held true—until now.
For the first time, researchers have identified an animal that gets by without breathing oxygen: a parasite relative of jellyfish that appears to acquire its energy from another, still mysterious, source. Unlike all other known multicellular organisms, this lollipop-shaped creature, called Henneguya salminicola, lacks mitochondria, subcellular structures that turn the vital gas into units of energy that power a dizzying array of essential functions.
“There are plenty [of animals] that can go for extended periods without [oxygen], but nothing [else] that can get through the whole life cycle,” Nick Lane, a biologist at University College London who wasn’t involved in the study, tells Michael Le Page at New Scientist.
To call the finding a surprise would be an understatement. In an interview with the New York Times’ Veronique Greenwood, biologist Dorothee Huchon of Tel Aviv University in Israel recalls thinking, “OK, something went wrong” when she first made the discovery, published last week in the Proceedings of the National Academy of Sciences.
Mitochondria, an evolutionary relic left over from when a bacterium was engulfed by a larger cell, have their own genomes, separate from the DNA housed in the nucleus. Huchon and her colleagues had hoped to analyze H. salminicola’s mitochondrial genome, and were baffled to see that it simply didn’t exist. While the team found mitochondria-like structures in the cell, they were empty shells devoid of genetic material—and thus couldn’t possibly function. “These are not true mitochondria,” Huchon tells New Scientist.
When the researchers next inspected the DNA in the creature’s nucleus, they found that certain genes that normally support mitochondrial development were also missing or mutated, further supporting the idea that H. salminicola had mostly discarded the oxygen-processing structure.
The team suspects this odd trait is a product of H. salminicola’s extreme lifestyle, which involves alternation between two hosts—fish and worms—both environments lacking in an abundance of fresh air. Some single-celled organisms living in low-oxygen environments appear to have lost their mitochondria too, but H. salminicola is the first multicellular creature confirmed to manage the feat.
Still mysterious is where H. salminicola gets its energy. As Jonathan Lambert reports for Science News, the parasite may simply steal it from its hosts, relieving it of the need to manufacture energy on its own.
At least one other study hints that H. salminicola may soon have company in its oxygen-free existence. In 2010, a team of researchers in Italy reported that loriciferans—tiny animals that live in deep sea sediments—appeared to lack mitochondria when viewed under a microscope. But the find has yet to be genetically confirmed, as H. salminicola’s was, according to New Scientist. Either way, researchers may need to rethink the requirements for complex life.