Comment: Clownfish shrink during marine heatwaves Published on: 22 May 2025 Writing for The Conversation, Dr Theresa Rueger, Melissa Versteeg and Dr Chancey MacDonald discuss a new study they led which found that clownfish shrink in response to heat stress. Clownfish that shrank during heatwaves were more likely to survive them. Morgan Bennett-Smith , ; , , and , As the world contemplates dealing with more extreme temperatures, one coral reef fish has found a novel way to beat the heat: . Wanting to know how clownfish cope with changes to their environment, we repeatedly measured 134 wild fish in Kimbe Bay, Papua New Guinea, during a marine heatwave that started in March 2023 and is part of an ongoing . Clownfish have unique markings, which make it easy to identify and measure them underwater. To our complete surprise, we found that 100 of the fish we measured shrank during our study from February to August 2023. Those that shrank had a better chance of surviving the heatwave. The clownfish, Amphiprion percula, lives in small social groups within anemones on coral reefs. As the movie Finding Nemo indicated, clownfish rarely, if ever, leave their host anemone because the anemone from predators. Sadly, this also means that clownfish cannot move to cooler areas as marine heatwaves due to rising global temperatures. Clownfish need other strategies to survive the heat. Get your news from actual experts, straight to your inbox. to receive all The Conversation UK鈥檚 latest coverage of news and research, from politics and business to the arts and sciences. This is the first time that coral reef fish have been shown to shrink in response to heat stress. And by shrink, we don鈥檛 mean getting skinnier 鈥� we mean getting shorter. This is surprising because growth in vertebrates (animals with backbones, like us) is generally considered to be a one-way street. You get larger over time and you might stop growing if stressed or as you reach your maximum length, but it is rare to find vertebrates shrinking, especially over periods as short as a month, and in response to environmental conditions. It may also seem counter-intuitive to shrink. After all, smaller individuals are more prone to being eaten and they . Here, however, being smaller increased the chances of survival for clownfish, possibly because smaller fish and are typically more efficient at foraging and , which is scarcer in hot water. Orange clownfish in a bleached anemone during the 2023 heatwave in Kimbe Bay, Papua New Guinea. Morgan Bennett-Smith If you shrink, I shrink We found that there is a social component to shrinking and surviving a heatwave. A remarkable feature of clownfish social groups is that they maintain strict hierarchies based on size. This means growth 鈥� and shrinking 鈥� don鈥檛 just affect the individual in question, but also risks conflict within the group that could force a fish to be evicted, which usually leads to death. So, shrinking is a risky proposition. On each anemone the biggest clownfish is female, the second biggest is male, and together they form a breeding pair. To avoid fights in the pair, males control their growth to keep a fixed size ratio between the two. In our study, breeding pairs in which both fish shrank were more likely to survive the heatwave than if only one, or neither, fish shrank. We also found that those fish who shrank by a lot could catch up and grow rapidly when conditions improved. That means that it鈥檚 not just the shrinking that helps, but being able to shrink and grow flexibly to meet your needs. A breeding pair of clownfish. The large female is on the right and the smaller male on the left. Theresa Rueger While not all fish beat the heat and survived, none of the fish that shrank multiple times in our study died, and even shrinking once increased a clownfish鈥檚 survival probability during the heatwave by 78%. Our research didn鈥檛 investigate how clownfish do this, but studies on other vertebrates might give us clues. Marine iguanas on the Gal谩pagos Islands for example , when water temperatures in the eastern and central tropical Pacific Ocean warm. This reduces the and prompts the reptiles to shrink by absorbing part of their bones. The average size of many marine fish species around the globe is according to long-term surveys. This could partly be a result of from populations, as well as the warming climate of fish. If our finding of adult fish shrinking in response to environmental stress is more widespread, it could be another reason why fish in the world鈥檚 ocean are getting smaller. Don鈥檛 have time to read about climate change as much as you鈥檇 like? Every Wednesday, The Conversation鈥檚 environment editor writes Imagine, a short email that goes a little deeper into just one climate issue. , Senior Lecturer in Tropical Marine Biology, ; , NERC Independent Research Fellow in Coral Reef Ecology, , and , PhD Candidate in Marine Sciences, This article is republished from under a Creative Commons license. Read the . 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