A human can’t shrink away from the threats of climate change. A clownfish, however, can. In a new paper published today in Science Advances, a team of researchers revealed that these tiny “Finding Nemo” fish can actually shrink to survive heat stress, allowing them to overcome the threat of heatwaves.
“We were so surprised to see shrinking in these fish,” said Melissa Versteeg, a study author and a Ph.D. student at Newcastle University in the U.K., according to a press release. “In the end, we discovered it was very common in this population.”
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Clownfish Shrink in Size
A clown anemonefish. (Image Credit: Morgan Bennett-Smith)
Climate change has transformed terrestrial and marine habitats and continues to transform them, with heatwaves — or periods of abnormal warmth — having one of the most significant impacts on animals. Studies show, for example, that increasing temperatures have a strong influence on the dimensions of terrestrial and marine species, shaping their size and size variability and contributing to their overall reduction in size over time.
But what, exactly, is the effect of marine heatwaves on the clownfish, also known as the clown anemonefish (Amphiprion percula)?
Setting out to study how heatwaves transform these fish over time, Versteeg and a team of researchers turned to the wild clownfish population in Kimbe Bay in Papua New Guinea, where heatwaves caused temperatures to sit around 4 degrees Celcius above average over the course of the study. Measuring the water temperatures and the size of the clownfish there from February 2023 to August 2023, the team found that individual clownfish shrank over time.
“We measured each fish individual repeatedly over a period of five months,” Versteeg said in the release. “During our study, 100 fish shrank out of the 134 fish that we studied.”
Rather than getting slimmer, these clownfish shrank by getting shorter, with the degree of their reduction depending on the individual’s initial size and social rank. According to the researchers, the results reveal that clownfish reduce their size in response to heat stress, which, in turn, increases their chances of surviving a heatwave by 78 percent.
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Clownfish Survival Improves
According to the researchers, some clownfish shrank one time, and some clownfish shrank multiple times, with all of the fish that shrank multiple times surviving throughout the course of the study. Intriguingly, the chances of clownfish survival were also improved if a clownfish shrank alongside its breeding partner.
“We witnessed how flexibly they regulated their size, as individuals and as breeding pairs, in response to heat stress as a successful technique to help them survive.” Versteeg said in the release. “It was a surprise to see how rapidly clownfish can adapt to a changing environment.”
Similar shrinking abilities are seen in other animals, including marine iguanas. And while clownfish are the first coral reef fish that researchers have shown to shorten in response to heat stress, they may not be the last. In fact, the results could have implications for other coral reef fish, and for other fish overall.
According to the researchers, fish on the whole are much smaller today than they once were. A 2023 study in Science found, for instance, that fish, in particular, are driving a decrease in size in the world’s animal populations. One possible explanation for this is that smaller species of fish are surviving over larger species of fish. Another is that fish species of all sizes are shrinking over time, with the smaller individuals of each species surviving (and thus procreating) more than the larger individuals of each species.
It is possible, however, that there are other factors contributing to the smaller size of fish today, too, including the ability to shrink in size in times of stress.
“If individual shrinking were widespread and happening among different species of fish, it could provide a plausible alternative hypothesis for why the size many fish species is declining,” said Theresa Rueger, the senior study author and a lecturer at Newcastle University, according to the press release. “Further studies are needed in this area.”
Though the fish themselves are small and becoming smaller, the researchers say that their results raise big questions about animal size and about animal shrinking, more specifically. “We don’t know yet exactly how they do it,” Versteeg said. “But we do know that a few other animals can do this too.”
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Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
