Deep in the Mediterranean, scientists have discovered the first complex animals known to live without oxygen.

It was previously thought that only viruses and single-celled microbes could survive without oxygen long-term.

But three new species of multicellular animals found during recent research expeditions live comfortably in oxygen-free depths, said team leader Roberto Danovaro of Italy's Polytechnic University of Marche.

"The bodies of multicellular animals have previously been discovered [in oxygen-free zones] but were thought to have sunk there from upper, oxygenated waters," Danovaro said in a statement. "Our results indicate that the animals we recovered were alive. Some, in fact, also contained eggs."

The new animals are microscopic—each measuring less than a millimeter across—and they resemble tiny jellyfish. (Read about giant mucus blobs on the rise in the Mediterranean.)

The creatures seem to thrive in the extremely salty sediments of the Mediterranean seafloor, one of the most extreme environments on Earth (see map).

Oxygen-Free Ancestor

Most multicellar organisms, including humans, have structures inside their cells called mitochondria, which use oxygen to convert nutrients into energy molecules known as ATP. (See an overview of how our cells work.)

The new animals appear to have modified versions of mitochondria called hydrogenosomes, which can produce ATP without oxygen. Hydrogenosomes were previously known only in single-celled organisms.

It's possible that a common ancestor of these animals from hundreds of millions of years ago had the ability to live without oxygen at least part of the time, said Marek Mentel, a biochemist at Comenius University in Slovakia who was not part of the expedition team.

As oxygen became more abundant, first in the atmosphere and then in the ocean, two lineages formed, with most—but not all—animals adapting to their new oxygen-rich environments.

When the oceans became oxygenated, there was an abundance of the energy-making chemical available to animals, said Mentel, who co-authored a commentary on the new study. "They grew to larger sizes, and later on they adapted to land, and so on."

Life As We Don't Know It

The study also "has strong implications for the search for life in the universe," Abel Méndez, an astrobiologist at the University of Puerto Rico at Arecibo, said in an email.

"Few people consider the possibility of finding complex life in other planetary bodies of the solar system," said Méndez, who was not involved in the new research, which was published in the April 6 issue of the journal BMC Biology.