Deep under the Mediterranean Sea, small animals have been discovered that live their entire lives without oxygen and surrounded by 'poisonous' sulphides. Researchers writing in the open access journal BMC Biology report the existence of multicellular organisms (new members of the group Loricifera), showing that they are alive, metabolically active, and apparently reproducing in spite of a complete absence of oxygen.

Roberto Danovaro, from the Polytechnic University of Marche, Ancona, Italy, worked with a team of researchers to retrieve sediment samples from a deep hypersaline anoxic basin (DHABs) of the Mediterranean Sea and studied them for signs of life. "These extreme environments," said Danovaro, "have been thought to be exclusively inhabited by viruses, Bacteria and Archaea. The bodies of multicellular animals have previously been discovered, but were thought to have sunk there from upper, oxygenated, waters. Our results indicate that the animals we recovered were alive. Some, in fact, also contained eggs."

Electronmicroscopy shows that instead of aerobic mitochondria, these animals possess organelles resembling the hydrogenosomes found previously in unicellular organisms (protozoans) that inhabit anaerobic environments.

The implications of this finding may reach far beyond the darker parts of the Mediterranean Sea floor, according to Lisa Levin of the Scripps Institution of Oceanography. In one of two commentaries accompanying this piece of research, she said, "The finding by Danovaro et al. offers the tantalizing promise of metazoan life in other anoxic settings, for example in the subsurface ocean beneath hydrothermal vents or subduction zones or in other anoxic basins."

In the second commentary Marek Mentel and William Martin, from Comenius and Dusseldorf Universities look at the incidence of anaerobic mitochondria and hydrogenosomes in other organisms and focus on the evolutionary significance of the new findings. "The discovery of metazoan life in a permanently anoxic and sulfidic environment provides a glimpse of what a good part of Earth's past ecology might have been like in 'Canfield oceans', before the rise of deep marine oxygen levels and the appearance of the first large animals in the fossil record roughly 550-600 million years ago."