Animals that can live without oxygen. Perhaps a new paradigm shift for SETI!
The sediments of a deep-sea hypersaline and sulfidic Mediterranean basin have yielded an unexpected discovery, the first multicellular animals living entirely without oxygen. Reported by Danovaro et al in BMC Biology, these three new species of Loricifera add a new and remarkable dimension to anoxic ecosystems previously thought to support only unicellular life.
Loricifera are small (< 1mm), exclusively marine meiofauna that belong to a relatively recently described marine phylum. Although there are only 22 described species, they have been recorded from a broad range of depths and settings ranging from shallow, coastal waters to methane seeps and
hydrothermal vents, to the Izu-Ogasawara trench off Japan [7, 8]. Loriciferans are an unlikely candidate for the honor of being the first anoxy-philic metazoan. Low-oxygen sediments have been studied extensively and loriciferans are rarely reported . Whether they were overlooked or are exceedingly rare and thus not sampled is unclear. Perhaps scientists have been looking for them in all the wrong places. In the L’Atalante Basin the loriciferans were sampled with other metazoans (copepods and nematodes) but Danovaro et al. , using a protein binding stain, fluorogenic probes, and radiolabel uptake experiments, determined that only the loriciferans were alive and metabolically active at the time of collection. They found not one, but three new species of Loricifera, living at the highest densities ever reported for this group. Some of the individuals contained a single large oocyte, and empty moults were also present, providing strong evidence that these populations are reproducing in place under anoxic conditions.
Why is this finding important? These metazoans have had to cope with multiple physiological stresses – extreme salinity, toxic sulfide levels and the absence of oxygen. Evolving adaptations to any one of these would be a challenge. Clues to the success of the Loricifera found by Danovaro et al.  may lie in part with the ultrastructure of their cells. Their mitochondria have been replaced with hydrogenosome-like structures, organelles that apparently evolved from mitochondria and are only previously known from protozoans inhabiting anaerobic environments. The additional presence of rod-shaped structures in
close proximity to the hydrogenosome-like organelles raises questions about microbial symbioses, perhaps evolved from associations with Archaea. That Archaea might play a role in adaptation to anoxia would not be surprising. They are the masters at harnessing energy in anoxic environments  and apparently act in association with hydrogenosomes inside ciliates , which themselves may be symbionts . If such an association occurred directly in the cells of marine metazoans, without a protozoan intermediary, this would be very exciting indeed!
Lisa A Levin
Read the full abstract here
BMC Biology 2010, 8:31doi:10.1186/1741-7007-8-31
Published: 6 April 2010