Wednesday October 15, 2025
bioGraphic —
Humans keep cranking the ocean’s thermostat, making marine heatwaves longer, hotter, and more frequent. For marine life, heatwaves can be deadly. After a record-setting heatwave called the Blob struck the North Pacific starting in 2014, creatures from krill and sponges to sea lions and pollock died in droves. In the short term, heatwaves are a dire threat. But how will marine animals and their descendants cope with such blazing conditions in the long run?
Evolutionary biologist Lisa Shama, of the Alfred Wegener Institute in Germany, set out to test whether fish could better withstand more extreme marine heatwaves if they are deliberately subjected first to a shorter, less intense one.
“It’s this idea that when you put a little bit of stress on something, it builds up tolerance to that stress,” Shama says.
Strikingly, Shama and her colleagues found that not only could they prompt three-spined sticklebacks (Gasterosteus aculeatus) to develop more resilience to extreme heat, but that these benefits—somehow—get passed on to their offspring.
For their research, Shama and her colleagues pulled 285 wild three-spined sticklebacks, a widespread fish roughly the size of a large paperclip, from northern Germany’s temperate waters. They plopped them into saltwater tanks set to mimic marine heatwaves of varying durations and intensities.
One group got to relax in a tank modeled after the naturally varying temperatures of their home waters, around 15 to 17 degrees Celsius (59 to 62.6 degrees Fahrenheit). A second group of fish weathered a major 14-day-long extreme heatwave, with the temperature rising a sweltering 4.1 degrees Celsius (7.4 degrees Fahrenheit) above average.
The last group faced a double heatwave—first a minor one, then a major one. The fish spent six days in water that reached an uncomfortable 2.6 degrees Celsius (4.7 degrees Fahrenheit) hotter than usual; then, after a short a break, they endured 14 more days in the same extreme heat as the second group. Next, the researchers bred the fish, repeated the heatwave experiment on their offspring, and bred the offspring to spawn a third generation.
For those in the double heatwave group, Shama and her team found that first experiencing minor heat stress primes sticklebacks to withstand worse heat later—an adaptation that subsequent generations inherit as a sort of biological memory.
Suffering through a heatwave “takes its toll, of course,” Shama says. “Their metabolism is working overtime just to keep up.” In the first generation, heatwave-stressed fish laid fewer, smaller eggs. By the second generation, however, the team found that fish that had weathered the scientists’ double heatwave, or had a parent that had lived through one, went on to lay more eggs than those whose ancestors hadn’t had a chance to acclimate to such heat. Additionally, once fish whose parents had endured a double heatwave hit the first heatwave in their own lives, something unexpected happened: they grew larger than their peers.
It’s that result that stood out so starkly to Bayden Russell, a marine ecologist at the University of Hong Kong who was not involved in the study. Usually, Russell says, stress causes animals to grow more slowly. But these fish, from heatwave-weary parents, grew quickly.
It’s a counterintuitive finding, but Russell notes that it does parallel his own research on the coping mechanisms of heat-afflicted urchins. In that work, he showed that when urchins are exposed to high water temperatures their offspring go on to mature more quickly. The crucial caveat, he says, is that if an intense heatwave lasts too long creatures may use up all of their energy and die. That is, after all, what eventually happened when he subjected his urchins to a particularly severe and persistent artificial heatwave.
What Shama and her team found offers a nuanced, but hopeful, picture of the future of three-spined sticklebacks.
Shama can only speculate about the mechanism that passes this resilience on from stickleback parents to their children. Her favorite explanation has to do with epigenetic marks—the idea that the sticklebacks are accumulating changes to their own DNA that then affect how their offspring’s genes are expressed. It’s a “super exciting” prospect, Shama says, that “theoretically shouldn’t happen.” But she’s also ready with a few other possible explanations. The sticklebacks could be delivering heat-optimized mitochondria, hormones, or a protein boost to their offspring, she says. She hopes to identify the specifics in future work. Shama also wants to study whether other species, from algae to bivalves, experience a similar capacity for inherited resilience.
Given the challenges of studying how animals weather marine heatwaves over multiple generations, especially for longer-lived species, Russell applauds Shama and her team’s rigor. Still, he would have liked to see the sticklebacks exposed to even more extreme heat to see what the limits are to the fish’s capacity to acclimate. Inherited resilience will only carry sticklebacks so far, he says.
To give this and other species the best chance for survival in the long run, he emphasizes, “We have to do something about climate change—and the sooner the better.”