Light microscopy image of the species of Spinoloricus, which was believed to be adapted to an anoxic environment. Danovaro et al.

In 2010, an international team of researchers discovered what was considered the first animal to live and thrive without oxygen. The discovery is now being questioned by a new piece of research conducted in the same area.

A team of researchers, led by Joan Bernhard from the Woods Hole Oceanographic Institution, thinks that the discovered creatures were dead and that their bodies had been taken over by “bodysnatcher” bacteria, which gave the impression that the creatures were alive. These findings are published in the journal BMC Biology.

At the bottom of the Mediterranean Sea, there are some areas in which brine lakes form. These zones, called deep hypersaline anoxic basins (DHAB), are found on the seafloor, 3 kilometers (2 miles) from the surface. They are so dense that they remain separated from the above seawater and are completely devoid of oxygen.

In 2010, it was discovered that these DHAB had metazoans, small multicellular animals, in them. The researchers, led by Roberto Danovaro, also discovered a new species called loriciferans that dwelled in these brine pools. In the study, they interpreted the finding of the metazoans as “the result of a rain of cadavers that sunk to the anoxic zone from adjacent oxygenated areas,” but they discovered that the loriciferans were metabolically active.

"We have known for a long time that some metazoans inhabit extreme anoxic habitats on a periodic or even semi-permanent basis," Bernhard said in a statement.

"But scientists have thought that metazoan's high-energy activities, such as reproduction, would require oxygen. If these loriciferans spend their whole lives and reproduce in a zero-oxygen environment, we would have to reconsider our concepts of animal metabolism. It was important to revisit the DHABs to confirm and understand those previous remarkable findings."

They revisited the DHABs in 2011 and collected new samples. They discovered that the metazoans in the brine lakes showed no indication of being alive or reproducing. They did found loriciferans living in normal (control) soil, but not in the DHABs. They think that the difference between the two environments is too extreme for these metazoans to have adapted to live in both environments.

"One alternative scenario," the authors wrote in the paper, "is that remnant metazoa bodies were inhabited by [living] anaerobic bacteria and/or archaea," which they have colloquially called "bodysnatchers."

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