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This Alcohol-Producing Fish Can Give Up Oxygen

By animal standards, the desert pupfish's physiology is utterly alien.
​The desert pupfish that could no longer afford to breathe. Image: Frank van Breukelen

Whether you're a marathon runner, a century cyclist, or training to climb Mount Everest, please check your pride at this desert pupfish's doorstep. He may look a bit scrappy, but this tiny creature, which hails from Death Valley, California, is rewriting our understanding of physiology by showing that it's possible for an animal to live for a significant stretch of time without oxygen.

That astounding ability is the discovery of Frank van Breukelen and Stanley Hillyard from the University of Nevada, Las Vegas, who've spent years studying the hardy desert pupfish, which measures just two inches in length. By animal standards, its physiology is utterly alien: For up to five hours at a time, the fish can metabolize without oxygen, like a bacterium.

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As Hillyard explained this week at the American Physiological Society's Experimental Biology Meeting, forsaking oxygen was probably a desperate evolutionary survival move precipitated by extreme environmental changes over the past 10,000 years.

The rise of oxygen in Earth's atmosphere was a critical step towards the evolution of complex life. While certain microbes base their respiration around other chemicals, including sulfur, iron and methane, oxygen gives you the most bang for your buck when it comes to generating cellular energy. Without it, animals and our energy-intensive lifestyles may have never evolved.

Yet throughout evolutionary history, animals have also adapted to dramatically conserve their oxygen use. The elephant seal, for instance, can go hours without taking a breath by decreasing its heart rate and employing special proteins that tightly bind oxygen. Other marine mammals, and some aquatic birds, have evolved similar mechanisms for improved O2 efficiency. But like us, when these animals do run out of oxygen, they need to take another breath—and fast.

The desert pupfish is different. It cycles between periods of oxygen-based and anaerobic (oxygen-free) metabolism. During its oxygen-free stints, the fish's cells must work fifteen times harder to produce enough energy to stay alive. It's a perplexing way to live, especially since the pupfish's environment is not lacking for O2. But rapid climate change seems to have left the creature no other option.

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A Death Valley hotsprings, the desert pupfish's only habitat. Image: Frank van Breukelen

"The entire Southwest of the USA was much wetter in its recent past," van Breukelen said in a statement. "Just 10,000 years ago, Death Valley was inundated by 100 meters of water and was called Lake Manley. When those waters receded, pupfish were isolated to their current habitats."

That is, an animal that once thrived in temperate lakes found itself uncomfortably thrust into steam baths where temperatures hover around 95 degrees Fahrenheit. And when its world grew hotter, oxygen became much more dangerous.

As temperatures rise, oxygen produces more free radicals—chemically reactive molecules that can damage cells and erode DNA. Fish, being ectotherms, are unable to regulate their internal temperature. And so, the desert pupfish was likely faced with a dramatic choice: Cut back on oxygen, or die.

"Our data suggest the fish are shutting down their mitochondria in order to avoid production of free radicals," van Breukelen said.

Instead of using its mitochondria—the cellular powerhouse where electrons and oxygen generate energy—the pupfish will subsist for long periods through anaerobic respiration. A byproduct of this anaerobic metabolism of sugars, which is similar to yeast-powered fermentation, is ethanol, which van Breukelen says helps further decrease the fish's oxygen needs.

"Our work emphasizes the notion that there are multiple 'currencies' for animals in describing the cost of living in a given environment," van Breukelen said. "Despite living in a resource-poor environment, these fish use the very inefficient processes of anaerobic metabolism. But by doing so, they're able to prevent deadly cellular damage."

Which is to say, what sounds like evolutionary suicide may actually be ingenious, given the right circumstances. And when breathing becomes a death sentence, that puts everything in perspective.