The diversity of life on Earth is sometimes amazing to behold. Until recently biologists have assumed, based on existing evidence, that only one-celled organisms such as bacteria were able to survive in oxygen-depleted places. Multicellular animals were known to pass through or hunker down temporarily in environments without oxygen, but only temporarily. That assumption may now be invalid.
It was recently reported that three species of loriciferan (http://en.wikipedia.org/wiki/Loriciferan) appear to spend their entire life without oxygen. The loriciferans were located in a briny, sulfurous environment 3.5 kilometers below the surface of the Mediterranean Sea. A variety of tests convinced the researchers that the loriciferans were not temporary, but permanent, residents of the locale. Once again the diversity of life on Earth never ceases to amaze.
The report can be found at http://www.biomedcentral.com/1741-7007/8/30.