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The Pupfish Paradox

UNLV researchers discover the pupfish's unique ability to go without oxygen. Unfortunately, it comes at a cost to the endangered species.

Research  |  Apr 23, 2015  |  By Shane Bevell
Pupfish swimming

Pupfish are able to survive without breathing oxygen for long periods of time. (Courtesy photo)

Editor's Note: 

Early research concerning Nevada's pupfish population was conducted by UNLV Distinguished Professor emeritus James Deacon, who died Feb. 23. Read more about one of UNLV's first biology professors.

UNLV researchers have discovered that pupfish, which are found only in the desert Southwest, can go for up to five hours with nearly no oxygen, and they do this even though oxygen is available.

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.

Stanley Hillyard in diving gear

As they conducted their testing, van Breukelen and Hillyard 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," said 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."

Why do these fish, which are up to two inches in length and found in warm springs and spring outflows near Death Valley, go for long stretches of time without consuming oxygen? To better understand, researchers looked at the historical climate conditions of the area.

Some 10,000 years ago, the desert Southwest was a very different environment than it is today. There were numerous lakes and Death Valley was filled with water 100 meters deep then. Even as recently as 2,000 years ago, the area was filled 10 meters deep with water. Those lakes were colder, under 20? Celsius (68? F). Somehow, as the lakes dried out, the fish ended up in the area's warm springs, which can be up to 35? Celsius (95? F).

"The question is whether the pupfish adapted to live in those warm springs, because after all, they spent all of their evolutionary history in cooler waters and there hasn't been much time to evolve," van Breukelen said.

"What's really neat is how pupfish go into this paradoxical anaerobism," he said. "They are producing some ethanol, and we think that ethanol closes down a channel in the mitochondria, where they use oxygen. We think this paradoxical anaerobism is almost a mistake."

Ventilation in fish is driven by oxygen levels. When the oxygen level is high and fish produce ethanol, it closes off the mitochondria. This means the fish are not consuming the oxygen they are bringing in, and so there is no need to increase ventilation, which is why they end up dropping into this paradoxical anaerobism.

Hillyard and van Breukelen think the reason pupfish do this is because the mitochondria produce reactive oxygen species, and these reactive oxygen species damage proteins, DNA, and membranes. They hypothesize that the fish are turning off the mitochondria in order to avoid the reactive oxygen species production to avoid this damage.

However, going without oxygen does have negative side effects. Researchers discovered damage to some of the pupfish similar to the effects of chronic alcoholism. "There is a protein that is normally expressed in smooth muscle called alpha actin," said van Breukelen. "Much like what we'd find in an alcoholic's liver, we find this smooth muscle actin being expressed in the fish's livers."

This can lead to premature death. The pupfish that live in a very warm environment typically live six to nine months; their counterparts in cooler environments can live a couple of years.

Last month Hillyard presented the paper, "Paradoxical Anaerobism in Desert Pupfish; When Oxidative Metabolism Costs More Than Anaerobic Metabolism" at the 2015 Experimental Biology conference in Boston.