When scientists set out to see if fish can survive in warm waters, they never expected to discover a surprising superpower. Not only are these fish adapted to heat – they are also more resistant to cold temperatures, potentially with unexpected advantages in our rapidly changing climates.
The study, published Wednesday in Natural Climate Change, tracked zebrafish through seven generations of selective breeding, responding to how aquatic species respond to our warming world.
Surprising dual adaptation challenges scientific expectations
“What we usually expect is that if we are more accustomed to warm wild species, it is that they have lower cooling tolerance. Then we see this higher cooling tolerance, which is surprising,” said Anna Andreassen, a PhD researcher at the Norwegian University of Science and Technology (NTNU).
The unique experiment began with wild zebrafish collected from India in 2016. The researchers divided them into three different groups:
- Fish selective breeding to increase heat tolerance
- Allow natural reproduction of the control group
- Choose fish that reduces heat resistance
After seven generations, heat-adapted fish not only improves their ability to withstand warm waters, but also surprisingly expands its overall temperature range, thereby increasing tolerance to cold. This adaptation does not have obvious tradeoffs to challenge the conventional understanding of evolution.
No tradeoffs: Unexpected distortion of climate adaptation
Perhaps most important is the lack of obvious drawbacks. Scientists have tested oxygen consumption, swimming ability, reproduction and growth – traditional measurements of fish fitness and have essentially no trade-offs among heat-adapted fish.
“Seldom research groups have been able to test how fish develop in the face of climate change, as it requires thousands of fish and years of careful experimentation,” said Fredrik Jutfelt, head of the Jutfelt Fish Biophysiology Laboratory at Paper Paper and NTNU. “This makes this work very important because we can eventually start to understand how evolution can help fish adapt to warming water.”
Fish on the treadmill show that oxygen is not a limiting factor
To test the oxygen efficiency, the researchers placed tiny 2 cm of fish in a swimming tunnel (essentially a fish treadmill) where they could measure their intake of oxygen while controlling the speed of the water.
“How much oxygen can we take out of the water when we swim at maximum capacity,” Andreassen explained. “It’s like putting them on a small fish treadmill.”
Contrary to the main assumptions in climate biology, the results suggest that oxygen limitations may not be the reason for determining heat tolerance.
“A very big assumption is that warming itself may not limit animals, but oxygen is limited at high temperatures. But we don’t see that,” Andresen pointed out. “This is important in our field.”
Can wild fish adapt in time?
Although the lab findings suggest that fish may be more adaptable than previously thought, researchers warn that natural evolution may not be enough to keep pace with rapidly warming water.
“Even if zebrafish do not show tolerance for higher water temperatures, climate change will continue to pose unexpected and dangerous challenges to all life on Earth,” Jutfelt stressed.
The study provides important insights to predict which populations may survive global warm oceans and freshwater systems. For fish facing rapidly changing climates, this unexpected adaptability may be their greatest hope for survival.
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