In the muck of the deep Mediterranean seafloor, scientists have found the first multicellular animals capable of surviving in an entirely oxygen-free environment.

Some types of bacteria and other single-celled organisms can live without oxygen, but nothing as complex had been found as these three species of Loricifera, a group of marine-sediment dwellers who inhabit one of Earth’s most extreme and little-known environments.

“The discovery of these life forms opens new perspectives for the study of metazoan life in habitats lacking molecular oxygen,” wrote researchers led by Roberto Danovaro, a marine biologist at Italy’s Polytechnic University of Marche, in a study published April 6 in BMC Biology.

Like other Loricifera, the new species are sub-millimeter–long, Lovecraftian tangles of tentacle and shell, with their closest taxonomical relatives found among mud dragons and penis worms.

The new species, however, don’t have the mitochondria found in almost every other animal cell, converting oxygen and nutrients into chemical energy.

Even the few parasite species once thought to be mitochondria-free seem to have had them at some point in history, and possess mitochondrial remnants that perform the same essential functions.

Instead the new Loricifera species have structures called hydrogenosomes, which are found in some single-celled organisms and require no oxygen to produce chemical energy.

The evolutionary history of these creatures is not known, but they live in an environment reminiscent of Earth’s oceans some 600 million years ago, before the deep seas were oxygenated and large animals evolved, wrote Comenius University (Slovakia) biochemist Marek Mentel and Düsseldorf University (Germany) biologist William Martin in an accompanying commentary.

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