For the endangered desert pupfish, however, the ability to break down oxygen by alternating types of respiration has proven essential to the survival of the species in California's Death Valley, National Geographic reports.
Two researchers at the University of Nevada, Las Vegas, sought to understand how pupfish managed to endure the past 10,000 years. In that span of time, their habitat transformed from a cool lake to a series of hot pools with temperatures reaching 95 degrees Fahrenheit.
The evolutionary history of pupfish suggests that the species flourished in cooler environments not too long ago. Their survival through dramatic environmental change is a product of behavioral shifts and physiological plasticity, according to the researchers. It turns out their biggest adaptation was developing the ability to suspend their breathing.
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 periods of anaerobic respiration, pupfish generate ethanol that can be further broken down to create energy without oxygen. The trade-off, however, is that this process requires their metabolism to work about 15 times harder than aerobic respiration.
The researchers' study is due out later this year after they presented their findings at the American Physiological Society's Experimental Biology Meeting.
Here's what pupfish look like in the shallow pools of Death Valley.