For species living near their thermal limits – the highest temperatures an animal can tolerate – climate change threatens to push them over the edge. Although zebrafish seem to be everywhere, with hundreds of investigators experimenting on different lab strains, this species may be in trouble in the wild. Zebrafish are a tropical species that live a couple of degrees below the highest temperatures at which they can survive. They live in shallow freshwater that heats up quickly, in India, Bangladesh and Nepal. Climate change scenarios predict that maximum temperatures in India are expected to surpass 44°C by 2100, leaving it unclear whether zebrafish will be able to adapt to these new conditions or if they will perish. With this chilling possibility in mind, Rachael Morgan and a team of colleagues from the Norwegian University of Science and Technology in Trondheim set out to determine whether zebrafish can evolve heat tolerance fast enough to help them survive as temperatures soar.
Receiving zebrafish captured by local fishermen in West Bengal, India, the team allowed the new arrivals to mate back in Norway. They then placed the progeny in a tank and slowly increased the water temperature until the fish lost their balance to identify the highest temperature that each individual could cope with. Next, the team allowed the fish that could stand the heat to breed together, while allowing the least resilient fish to breed with others like themselves. Repeating the process over a total of six generations, the scientists tried to rear a hardy group that were bred to withstand high temperatures and a feebler population that could only cope with cooler conditions. The team also placed a group of the hardy zebrafish in warmer water for 2 weeks before measuring the highest temperature that each fish could tolerate to find out whether these robust animals were capable of adapting to even hotter conditions, which might further increase their heat tolerance and help them deal better with a heatwave.
Having bred at least 20,000 fish in the Herculean series of experiments, Morgan and colleagues found that zebrafish are only able to increase their thermal tolerance at a rate of 0.04°C per generation. In addition, the team found that as zebrafish evolved to endure higher temperatures, they reduced their ability to increase their heat tolerance to the same extent, using physiological changes to deal with thermal stress. These results suggest there is a limit to how much zebrafish can increase their heat tolerance.
The main question is whether the fish can adapt fast enough to keep up with the unprecedented rates of climate change that we are currently seeing and the researchers conclude it is very unlikely. Like many of the things that we have learned from this model species, these zebrafish may be warning us of what is to come for other tropical species living near their upper thermal limits.
