Animals, anoxic environments, and reasons to go deep

One of the classic questions in the early evolution of eukaryotic life concerns the role of oxygen. Many unicellular eukaryotes are strict anaerobes and many animals have long anoxic phases in their life cycle. But are there also animals that can complete their life cycle without oxygen? In an ongoing debate in BMC Biology, Danovaro and colleagues say “yes” while Bernhard and colleagues say “no”. The debate concerns reports of anoxic metazoans in deep sea anaerobic habitats. In a Correspondence contribution to BMC Biology, Danovaro and colleagues [1] weigh in to defend the view that the loriciferans they reported in 2010 from anoxic sediments beneath the Mediterranean brine were living at the time of collection [2]. That view has recently been challenged by Bernhard et al. [3], who say that the loriciferans in question were not alive, but were dead and merely well preserved. The issue at hand is whether metazoans exist that can complete their entire life cycles without oxygen. This is important, but what is at stake