Three new species of anaerobic multicellular organisms have been discovered deep under the Mediterranean Sea. The small animals, new members of the group Loricifera, live their entire lives without oxygen and surrounded by 'poisonous' sulphides.

Researchers writing in BMC Biology report the existence of multicellular organisms, showing that they are alive, metabolically active, and apparently reproducing in spite of a complete absence of oxygen. Sediment samples from a deep hypersaline anoxic basin (DHABs) of the Mediterranean Sea revealed evidence for the new organisms' existence.

"These extreme environments have been thought to be exclusively inhabited by viruses, Bacteria and Archaea," said Roberto Danovaro, from the Polytechnic University of Marche, Ancona, Italy. "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. "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," said Lisa Levin of the Scripps Institution of Oceanography.

Citation: Danovaro et al., 'The first metazoa living in permanently anoxic conditions', BMC Biology, April 2010, 8(30); doi:10.1186/1741-7007-8-30