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.<\/p>\n
Last week, the researchers reported their findings in Science Advances, concluding that the fish got shorter \u2014 on the scale of a few millimeters, or a small, single-digit percent of their length \u2014 in response to the heat wave.<\/p>\n
\u201cWe 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,\u201d 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. \u201cIn the end, we discovered [that downsizing] was very common in this population.\u201d<\/p>\n
Versteeg and her colleagues don\u2019t know how, exactly, the fish are shrinking \u2014 one untested idea is that the fish might be reabsorbing some of their bone material or tissue. But getting smaller isn\u2019t a problem. In fact, the study found, it may be an adaptation to help clownfish survive hotter ocean temperatures.<\/p>\n
Last year, the planet was about 2.65 degrees F (1.47 degrees C) 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.<\/p>\n
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.<\/p>\n
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 F (4 degrees C) above average.<\/p>\n
Being tiny has its advantages in a hot climate: Warm-blooded animals, like mammals, shed heat more easily when they\u2019re small and this helps them cool down. The benefits for cold-blooded creatures, such as clownfish, aren\u2019t as clear, though researchers think they may have an easier time meeting their bodies\u2019 energy requirements when they\u2019re small.<\/p>\n
Regardless of the reason, being small seems to help clownfish when it\u2019s hot. The fish that shrank, the study found, had a much higher chance of surviving.<\/p>\n
\u201cIt was a surprise to see how rapidly clownfish can adapt to a changing environment,\u201d Versteeg said. \u201cWe 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.\u201d<\/p>\n
The study adds a layer of complexity to what is otherwise a depressing tale about the world\u2019s oceans. Heat waves linked to climate change, like the one that occurred during this study, are utterly devastating coral reefs \u2014 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.<\/p>\n
Amid that loss, animals are proving highly resilient. They\u2019re trying hard to hold on. Yet if warming continues, even the best adaptations may not be enough.<\/p>\n