An extreme ocean heat wave did something remarkable to these fish

Vox

23-05-2025

is an environmental correspondent at Vox, covering biodiversity loss and climate change. Before joining Vox, he was a senior energy reporter at Business Insider. Benji previously worked as a wildlife researcher.
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-Smith
When it's good to be small
Last 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.
Related This is how many animals could go extinct from climate change
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-Smith
Regardless 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.