Clownfish shrink to survive heatwaves, study finds

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As the marine world heats up, clownfish are showing an unsuspected talent for adapting to increasingly extreme conditions. Faced with heat waves exacerbated by climate change, these little fish, made famous by the movie “Finding Nemo,” adopt a surprising strategy to ensure their survival.

In Papua New Guinea’s Kimbe Bay, researchers from the Universities of Newcastle, Leeds, and Boston, in collaboration with the Mahonia Na Dari Center, observed 134 clownfish (Amphiprion percula) over a five-month period.

Each fish was individually identified and measured each month using a non-invasive capture device. Water temperature was recorded every four to six days using submersible sensors installed on site, enabling continuous monitoring of temperature changes during a marine heatwave.

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Their aim was to understand how these fish react to a sudden rise in water temperature. And the results were striking: 100 of the 134 clownfish decreased in size during the study period—a reaction that could increase their chances of survival by 78% during a marine heatwave.

This discovery, published in the journal Science Advances, is a first. Never before has a coral reef fish been observed to shorten its body in response to environmental and social stress. Indeed, it’s not just a matter of slimming down.

“This is not just about getting skinnier under stressful conditions—these fish are actually getting shorter,” says Melissa Versteeg, a PhD researcher at Newcastle University’s School of Natural and Environmental Sciences, who led the study, quoted in a news release.

“We don’t know yet exactly how they do it, but we do know that a few other animals can do this too. For example, marine iguanas can reabsorb some of their bone material to also shrink during times of environmental stress,” Versteeg added.

Shrinking together to survive together

The researchers took care to measure each clownfish individual repeatedly, month after month. The extent of the phenomenon surprised them. This ability to shrink even seems to be coordinated between breeding partners, further increasing their chances of withstanding the heat together. This form of synchronization underlines the importance of social dynamics in the survival of the species.

The morphological flexibility of these fish raises a broader question: What if this kind of shrinking explains why fish are becoming smaller overall in the world’s oceans?

For Theresa Rueger, a specialist in tropical marine sciences and coauthor of the study, this hypothesis merits further investigation.

“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,” Rueger said.

“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,” she explains.