During a severe heat wave in 2023, scientists scuba diving off the coast of Papua New Guinea captured clownfish to measure their bodies. Between February and August, they calculated the length of 134 of these iconic, orange and white fish once a month, taking a total of six measurements for each fish.Those measurements revealed something peculiar: Most of the fish shrank.This week, the researchers reported their findings in Science Advances, concluding that the fish got shorter — on the scale of a few millimeters, or a small, single-digit percent of their length — in response to the heat wave.Morgan Bennett-Smith“We were so surprised to see shrinking in these fish that, to be sure, we measured each fish individual repeatedly over a period of five months,” said Melissa Versteeg, a doctoral researcher at Newcastle University, who led the study in collaboration with Mahonia Na Dari, an environmental organization, and Walindi Resort. “In the end, we discovered [that downsizing] was very common in this population.”Versteeg and her colleagues don’t know how, exactly, the fish are shrinking — one untested idea is that the fish might be reabsorbing some of their bone material or tissue. But getting smaller isn’t a problem. In fact, the study found, it may be an adaptation to help clownfish survive hotter ocean temperatures.Morgan Bennett-SmithWhen it’s good to be smallLast year, the planet was about 2.65 degrees warmer than it was in the late 1800s. This level of warming impacts wild animals in a number of strange, mostly bad, ways, from fueling koala-killing wildfires to causing corals to bleach and then starve.But rising temperatures also appear to be making many species smaller. One especially striking study, published in 2019, found that birds shrank by an average of about 2.6 percent between 1978 and 2016. More recent analyses have linked rising temperatures to a reduction in body size of small mammals in North America and marine fish. Most of these existing studies report that animals, on average, are simply not growing as large.The new study on clownfish, however, suggests individual fish are shrinking over mere weeks in response to a heat wave, which, in the case of the Papua New Guinea event, pushed temperatures in the bay about 7 degrees (4 degrees Celsius) above average.Why do they do this?Being tiny has its advantages in a hot climate: Warm-blooded animals, like mammals, shed heat more easily when they’re small and this helps them cool down. The benefits for cold-blooded creatures, such as clownfish, aren’t as clear, though researchers think they may have an easier time meeting their bodies’ energy requirements when they’re small.Morgan Bennett-SmithRegardless of the reason, being small seems to help clownfish when it’s hot. The fish that shrank, the study found, had a much higher chance of surviving.“It was a surprise to see how rapidly clownfish can adapt to a changing environment,” Versteeg said. “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.”The study adds a layer of complexity to what is otherwise a depressing tale about the world’s oceans. Heat waves linked to climate change, like the one that occurred during this study, are utterly devastating coral reefs — and in severe cases, are nearly wiping out entire reef sections. These colorful ecosystems are home to countless marine animals, including those we eat, like snappers, and clownfish.Amid that loss, animals are proving highly resilient. They’re trying hard to hold on. Yet if warming continues, even the best adaptations may not be enough.See More:

Clownfish Shrink Down Their Bodies to Survive Ocean Heat Waves, New Study Suggests The adaptation appears to help the fish cope with high temperatures, since individuals and breeding pairs that shrank improved their survival odds Clownfish seem to become shorter during heat waves, according to the new study. Morgan Bennett-Smith A new study reveals that clownfish use a surprising strategy to adapt their bodies to ocean heat waves: They shrink. “[Clownfish] have these amazing abilities that we still don’t know all that much about,” says study co-author Theresa Rueger, a tropical marine ecologist at Newcastle University in England, to the Washington Post’s Dino Grandoni. The findings offer some hope for fish in the face of climate change, she adds. “There’s potential that maybe some other species will adapt in a way that will allow them to hang on longer than we think.” Rueger and her team didn’t initially plan to study a heat wave. They were monitoring how freshwater runoff might affect breeding clownfish in Papua New Guinea’s Kimbe Bay, when temperatures dramatically rose and warmed the water to 7.2 degrees Fahrenheit above average. But these conditions, they realized, offered a key opportunity for research. The scientists measured 134 clownfish in Kimbe Bay every month during the ocean heat wave, which spanned from February to August 2023. Astoundingly, 100 of those fish shrank. The researchers found that 71 percent of the dominant females and 79 percent of the breeding males reduced in size at least once over the study period. Their findings were published in the journal Science Advances on Wednesday. At first, lead author Melissa Versteeg, a PhD researcher at England’s Newcastle University, thought she was making a mistake in her measurements. She kept trying again. And again. “She had several people measuring them at the same time to really make sure that we’re confident with the numbers,” Rueger says to Melissa Hobson at National Geographic. But after these repeated attempts, she concluded the measurements were correct. The fish that shrank increased their chances of surviving the heat wave by 78 percent, according to the study. Some of the clownfish even shrank in pairs, reducing their size alongside their breeding partner—a move that also boosted their chance of survival. The study marks the first time a coral reef-dwelling fish has been documented to shrink in response to environmental and social cues, according to a statement from Newcastle University. A pair of clownfish swims near an anemone. When the studied fish became smaller, females maintained a larger size than males. Morgan Bennett-Smith Clownfish aren’t the only animals shifting their size because of heat. Fish around the world are adapting to warmer temperatures by downsizing their bodies. “This is another tool in the toolbox that fish are going to use to deal with a changing world,” says Simon Thorrold, an ocean ecologist at Woods Hole Oceanographic Institution who was not involved in the new work, to Adithi Ramakrishnan at the Associated Press. But these clownfish stand out from the rest. “Until now, when talking about shrinking fish, nearly all studies do not mean that fish literally shrink but that they grow to smaller sizes,” explains Asta Audzijonyte, a senior lecturer at the University of Tasmania in Australia who was not involved in the work, to the Washington Post. “This study, in contrast, reports observations of [clownfish] actually shrinking by a few percent of their total length over the course of a month.” Previous research has found that other animals, like birds and rodents, appear to have gotten smaller because of climate change. And marine iguanas will shrink in response to warmer water temperatures during El Niño years. The researchers don’t yet know how the clownfish are pulling off their shrinking act. One hypothesis is that the fish are reabsorbing their own bone matter, reports the Associated Press. They’re also not sure why, exactly, changing size is so advantageous to the clownfish. But it could be that a smaller size makes it easier to maintain oxygen levels or get by with less food available. “If you’re small, you obviously need less food, and you’re also more efficient in foraging a lot of the time,” explains Rueger to National Geographic. Still, this adaptation method can only go so far. The heat wave exacerbated coral bleaching, which decreases available reef habitat, and subsequent heat waves ultimately killed many of the fish the researchers studied. “We’ve lost many of those fish,” Rueger says to the Washington Post. Get the latest stories in your inbox every weekday.

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.”Clownfish Shrink in SizeA 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.Read More: How Volunteers Are Helping Keep Coral Reefs AliveClownfish Survival ImprovesAccording 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.”Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article: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.

shrinking Nemo Incredible shrinking clownfish beats the heat Shrinking down to size boosted clownfish survival rates up to 78 percent during heat waves. Jennifer Ouellette – May 21, 2025 2:00 pm | 7 Credit: Morgan Bennett-Smith Credit: Morgan Bennett-Smith Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only   Learn more Pixar's Finding Nemo immortalized the colorful clownfish, with its distinctive orange body and white stripes, in the popular imagination. Clownfish, like many other species, are feeling the stress of rising temperatures and other environmental stressors. Fortunately, they have a superpower to cope: They can shrink their body size during dangerous heat waves to substantially boost their odds of survival, according to a new paper published in the journal Science Advances. “This is not just about getting skinnier under stressful conditions; these fish are actually getting shorter," said co-author Melissa Versteeg, a graduate student at Newcastle University. "We don’t know yet exactly how they do it, but we do know that a few other animals can do this too." Many vertebrates have shown growth decline in response to environmental stressors, especially higher temperatures. Marine iguanas, for example, reabsorb some of their bone material to shrink when their watery habitat gets warmer, while young salmon have been known to shrink at winter's onset. This can also happen when there is less food available. And social factors can also influence growth. When female meerkats, for example, are dominant, they have growth spurts, while a disruption in their social status can cause stunted growth in male cichlids What has been lacking in prior research is an investigation into how environmental and social factors interact to influence growth rates, according to Versteeg et al. They thought clownfish were the best species to study to fill that gap, since they've been extensively studied and are well understood. The fish live on Indo-Pacific coral reefs where heat stress has been increasing and has become more severe—an environment that is close to the thermal tolerance limits of clownfish. Clownfish also live in social groups: They form breeding pairs with a dominant female and subdominant male, sometimes adding subordinate non-breeding fish. The dominants tend to grow to match the size of the host anemone, while the subordinates only grow to a size that ensures there are sufficient resources—otherwise they risk being evicted and likely dying. Let’s get small The team observed 67 breeding pairs of wild clownfish—briefly caught and photographed for distinctive markings and measured before being returned to the water—living on single anemones in Kimbe Bay, Papua New Guinea, between February and August 2023. This happened to coincide with the world's fourth global bleaching event. They measured the body size of the fish once a month and measured the temperature around the individual anemones every four to six days. Then the team analyzed the collected data. "Individual fish can shrink in response to heat stress." Credit: Morgan Bennett-Smith The results: Over the course of those months, 101 of the 134 clownfish shrank at least once in response to heat stress, and doing so boosted their likelihood of survival up to 78 percent compared to the 33 fish that did not shrink. And between breeding pairs, there were distinctive growth ratios between the dominant and subordinate fish; those pairs that shrank together were also more likely to survive the heat waves. “We were so surprised to see shrinking in these fish that, to be sure, we measured each fish individual repeatedly over a period of five months," said Versteeg. "In the end, we discovered it was very common in this population. It was a surprise to see how rapidly clownfish can adapt to a changing environment, and 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 et al. have not yet identified a possible mechanism for the shrinkage, but suggest the triggering of neuroendocrine pathways via thyroid hormones might play a role, since those hormones regulate growth. The adaptive strategy could also be a means of adjusting to changing metabolic needs. But there are trade-offs: While shrinking in response to heat waves ensures greater survivability, there can also be a corresponding decrease in birth rates. "Our findings show that individual fish can shrink in response to heat stress, which is further impacted by social conflict, and that shrinking can lead to improving their chances of survival," said senior author Theresa Rueger, also of Newcastle University. "If individual shrinking were widespread and happening among different species of fish, it could provide a plausible alternative hypothesis for why the size of many fish species is declining, and further studies are needed in this area.” Science Advances, 2025. DOI: 10.1126/sciadv.adt7079  (About DOIs). Jennifer Ouellette Senior Writer Jennifer Ouellette Senior Writer Jennifer is a senior writer at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. Jennifer lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban. 7 Comments