Can survive for long periods minus oxygen.

• Position: Pupfish

Pupfish, which are found exclusively in the desert Southwest, can go for up to five hours with nearly no oxygen, and they do this even though oxygen is available, it has been discovered by researchers from the University of Nevada, Las Vegas.

Life scientists Frank van Breukelen and Stanley Hillyard originally wanted to measure how much oxygen the pupfish used to better understand the energy needs of the endangered species. When conducting their testing, they discovered the phenomenon they call paradoxical anaerobism.

"The pupfish's extended time not consuming oxygen made no sense, since oxygen is so much more efficient, and these pupfish live in an incredibly energy-deprived environment," relates van Breukelen. "Some of the pupfish were able to go without breathing oxygen for up to five hours with only a couple of brief interruptions of oxygen use. The longest continuous period of paradoxical anaerobism we observed was 149 minutes. What we didn't see is compensatory oxygen consumption. The fish aren't simply holding their breath; instead, they are going into this anaerobic metabolism."

Some 10,000 years ago, the desert Southwest was a very different environment...