Photograph by Joel Sartore, Nat Geo Image Collection
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Desert pupfish have evolved a way to protect themselves from the ravages of heat. But the solution, a remarkable ability to go without oxygen, has some strings attached.

Photograph by Joel Sartore, Nat Geo Image Collection

Desert-Dwelling Fish Can 'Hold Its Breath' for Five Hours

Tiny pupfish have adapted their respiration to go without oxygen for long stretches.

Never go up against a pupfish in a breath-holding contest. This wee swimmer, resident of warm springs near Death Valley in California, can go with almost no oxygen for up to five hours at a time, according to a new study due out this year.

The impressive ability is an adaptation that’s let the desert species hang on despite rapid change to its environment, said researchers this week at the American Physiological Society’s Experimental Biology Meeting.

Too Hot to Handle

Frank van Breukelen and Stanley Hillyard, of the University of Nevada, Las Vegas, wanted to understand how the desert pupfish, a critically endangered species, has coped during the past 10,000 years—the relatively short time it has taken for their home, once a cool, spacious lake, to turn into a series of small pools with temperatures up to 95°F (35°C).

Looking at the evolutionary history of these fish, it’s clear that “they lived in cooler environments not that long ago,” says van Breukelen. It’s just tough luck that things took a turn for the hot: “Sometimes animals live in bad neighborhoods,” he says.

The fish seem to have responded to the change with behavioral shifts, like spending less and less time in the shallower, warmer shelf areas where they breed, the scientists found. Less time making babies surely hasn’t helped the species to thrive. And for the pupfish, which have been hard hit by lost habitat and competition with non-native species, that’s a serious concern.

Bending the Rules

Only because of something called physiological plasticity have the fish survived at all, van Breukelen says. Examples of plasticity in nature include animals that can hibernate or not depending on environmental conditions, or animals that eat their siblings (such as spadefoot toad tadpoles) when there aren’t enough resources for all to survive. It means being flexible according to what the situation requires.

In the case of the desert pupfish, the flexibility shows up in its breathing, or lack thereof, the researchers found.

Breaking down inhaled oxygen is a great way to generate energy. But at high temperatures the process can be dangerous for the fish because it produces a lot of free radicals—chemically reactive molecules that damage proteins, cell membranes, and DNA.

The pupfish mitigate the damage by randomly alternating between oxygen-based, or aerobic, respiration and oxygen-free, or anaerobic, respiration (which is what humans use during stints of heavy exercise, when oxygen is being used up faster than it can reach muscles). Sometimes they go without for five-hour stretches.

During these periods, the fish generate ethanol, which they can then further break down to get energy without taking a whiff of oxygen. This process, the researchers think, is part of what lets the pupfish survive in their extreme environment.

Lesser of Two Evils

But it’s not a simple trade-off. During no-oxygen periods, the pupfish’s metabolism has to work some 15 times harder to produce energy than it does when the fish is breathing O2, the researchers discovered.

“Sometimes organisms have to take the lesser of two evils, but it doesn't necessarily mean this alternative is a great option,” van Breukelen says. “We think this process is really tough on the fish.” While it may cut down on cellular damage, anaerobic respiration may also be linked to the pupfish’s relatively short life span.

It seems the quickly changing climate may have left the pupfish no choice but to tap this better-than-nothing solution, however.

And, brief-lived or not, the little fish have some impressive abilities, van Breukelen says. “Sometimes, we see fish that swim while using this ‘paradoxical anaerobism.’ Imagine being able to swim vigorously while not consuming oxygen! Pretty crazy stuff.”

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