Scientists searching the floor of the Mediterranean say they have found the first multi-cellular creatures that live their lives entirely without oxygen.

A group of Italian and Danish researchers found three new species of Loricifera — a phylum of tiny bottom-dwelling animals that live in marine sediments. A light microscopy image of new species of Loricifera. This specimen is stained red. (Courtesy BMC Biology)

The organisms, which are less than a millimetre long, were found in the Mediterranean's L'Atalante basin at a depth of more than 3,000 metres.

Researchers used electron and infrared microscopy, biochemical analysis and radioactive tracers to confirm that the sample organisms they retrieved were metabolically active.

They also found the organisms showed specific adaptations to their extreme environment, such as the lack of mitochondria — cellular structures that are responsible for converting sugar and oxygen into energy.

Two of the species also contained eggs that scientists were able to hatch in a completely oxygen-free environment.

Implications of find far-reaching

"All of these findings provide the first evidence that the anoxic sediments of the L'Atalante basin are colonized by natural populations of loriciferans, and that these metazoans are metabolically active and able to reproduce," the researchers said.

Until this discovery, it was thought that only single-celled organisms like bacteria, could live and reproduce without oxygen.

The findings also open the door to the possibility that such creatures could exist on other planets that lack oxygen in their environments.

The expedition that found the creatures was the third in the last 10 years to explore for living organisms in the oxygen-starved salty sediments of the Mediterranean's L'Atalante basin. The basin is 200 kilometres off the coast of the Greek island of Crete.

The research was published this week in the journal BMC Biology.