Fish Evolve Superhero Powers in Climate Crisis: Adapt to Both Hot and Cold

When scientists set out to see if fish could evolve to survive in warmer waters, they never expected to discover a surprising superpower. Not only did these fish adapt to heat—they became more resistant to cold temperatures too, potentially giving them an unexpected edge in our rapidly changing climate.

The study, published Wednesday in Nature Climate Change, tracked zebrafish through seven generations of selective breeding, offering a comprehensive look at how aquatic species might respond to our warming world.

Surprising Double Adaptation Challenges Scientific Expectations

“What we normally expect, if we compare species in the wild that are warm adapted, is that they have lower cooling tolerance. And then we saw this higher cooling tolerance, which was surprising,” said Anna Andreassen, the study’s lead author who conducted the research as a PhD fellow at the Norwegian University of Science and Technology (NTNU).

The unique experiment began with wild zebrafish collected from India in 2016. Researchers separated them into three distinct groups:

• Fish selectively bred for increased heat tolerance
• A control group allowed to reproduce naturally
• Fish selected for decreased heat tolerance

After seven generations, the heat-adapted fish had not only improved their ability to withstand warmer waters but had surprisingly expanded their overall temperature range—showing improved tolerance to cold as well. This adaptation without apparent tradeoffs challenges conventional understanding of evolution.

No Trade-offs: The Unexpected Twist in Climate Adaptation

Perhaps most significant was the absence of apparent downsides. The scientists tested oxygen consumption, swimming ability, reproduction, and growth—traditional measures of fish fitness—and found essentially no tradeoffs in the heat-adapted fish.

“Very few research groups have been able to test how fish can evolve when facing climate change, because it takes thousands of fish and many years of careful experiments,” said Fredrik Jutfelt, senior author of the paper and head of the Jutfelt Fish Ecophysiology Lab at NTNU. “That makes this work very important because we can finally start to understand how evolution may help fish adapt to warming waters.”

Fish on Treadmills Reveal Oxygen Isn’t the Limiting Factor

To test oxygen efficiency, researchers placed the tiny 2-cm fish in a swim tunnel—essentially a fish treadmill—where they could measure oxygen uptake while controlling water speed.

“We can measure how much oxygen they take up from the water while they’re swimming at their maximum capacity,” Andreassen explained. “It’s like putting them on little fish treadmills.”

Contrary to a major hypothesis in climate biology, the results suggest oxygen limitation may not be what determines heat tolerance.

“One very big hypothesis is that warming itself might not limit the animals, but that oxygen becomes limited at high temperatures. But we didn’t see that,” Andreassen noted. “And that is big in our field.”

Can Wild Fish Adapt in Time?

While the laboratory findings suggest fish may have more capacity to adapt than previously thought, the researchers caution that natural evolution might not proceed quickly enough to keep pace with rapidly warming waters.

Jutfelt emphasized, “Even though the zebrafish didn’t show any adverse effects from developing a tolerance to higher water temperatures, climate change will nevertheless continue to pose unanticipated and dangerous challenges to all life on Earth.”

The study offers vital insights for predicting which populations might survive in warming oceans and freshwater systems worldwide. For fish facing a rapidly changing climate, this unexpected adaptability might be their best hope for survival.