Climate change: Clownfish's survival strategy for hotter weathers

Common clownfish (Amphiprion ocellaris), with their whimsical bright stripes, are among the most recognisable residents of coral reefs — made famous by the movie Finding Nemo. But climate change is transforming the world, heating the seas and oceans, bleaching vast stretches of coral reefs, and leaving them lifeless. Rising temperatures not only threaten their homes but also directly affect the clownfish themselves.

Two recent studies reveal how clownfish cope with rising sea temperatures. One study, conducted under controlled laboratory conditions at the Okinawa Institute of Science and Technology Graduate University (OIST) in Japan, investigated how key biological processes respond to thermal stress. The other, carried out in the wild by researchers at Newcastle University in the UK, tracked how clownfish responded during a real marine heatwave. Together, the findings highlight two survival strategies- a rewiring of metabolism and the resizing of bodies.

Simulating climate change in the lab

In a study published in the journal iScience, researchers at OIST raised young clownfish in labs under two conditions: one at the normal reef temperature of 28°C and another at 31°C. With climate change expected to raise ocean temperatures by as much as 4 °C over the next 75 years, the experiment recreated a likely future scenario. This allowed the researchers to observe, in real time, how clownfish may adapt to the warmer seas of the future. 

Clownfish are small, breed readily in captivity, and have a relatively long lifespan. These traits make them ideal experimental models for studying physiological and behavioural responses to environmental changes, such as rising sea temperatures.

At first glance, the clownfish raised in both temperatures appeared identical, with no apparent differences in size. Closer observation revealed that fish in warmer water were burning more energy—measured as Routine Metabolic Rate (RMR)—to maintain basic functions like breathing and circulation.

However, RMR rose only when clownfish were suddenly exposed to higher temperatures. Those raised in warmer water since an early age were able to acclimatise, and their RMR was found to be the same as that of the clownfish raised at cooler temperatures. While their metabolism had normalised, they exhibited changes in physiology across different tissues. The most prominent and significant changes were noticed in the liver and pancreas.

The liver plays a central role in energy storage and release. In clownfish raised at higher temperatures, genes linked to energy production became more active. This boosted biological processes that are responsible for making glucose, breaking it down for energy, generating new glucose when needed, transporting it out of liver cells, and converting stored glycogen into glucose.

The pancreas, which produces insulin to regulate blood sugar, showed the opposite trend. At higher temperatures, genes controlling insulin secretion become less active, sending fewer signals to cells, which results in less glucose being stored and more circulating in the blood. 

Together, these changes show a coordinated response. The liver boosts energy availability, while the pancreas reduces glucose storage. As a result, more glucose stays in the bloodstream, helping clownfish maintain stable metabolic rates under heat stress. However, because sugar storage and utilisation are key to maintaining energy balance, prolonged disruptions from temperature changes could impact growth and reproduction.

“We’ve found mechanisms for heat acclimation; these biological changes may have other long-term negative impacts on fish health, so we need to expand our studies... But knowing that developmental exposure to these high temperatures can support heat acclimation, we can move forward with hope for the future of our tropical fish," says Prof. Timothy Ravasi, head of the Marine Climate Change Unit at OIST and an author of this paper. 

Observing real-world impact

Scientists from Newcastle University, UK, studied the same fish in their natural habitat in Papua New Guinea during an ongoing marine heatwave and coral bleaching event. Their findings were published in the journal Science Advances.

They tracked 134 clownfish from 67 breeding pairs over a period of several months. What they discovered surprised them. Clownfish that were more flexible in reducing their body size had a higher survival rate. During the heatwave, 79% of dominant males and 71% of dominant females shrank in size. This wasn’t just a loss of weight from stress — it involved an actual shortening of body length. 

Some individuals shrank more than once during the study period. Those that shrank were 78% more likely to survive. 

"We don't know yet exactly how they do it, but we do know that a few other animals can do this too”, explains Melissa Versteeg, a PhD researcher at Newcastle University and lead of the study. He suspected it might be due to tissue reabsorption and hormonal changes.

Clownfish are common across coral reefs in the Indo-Pacific region. Their societies are strictly size-based: the largest fish is the dominant female, the second largest is her male mate, and all the rest are much smaller, non-breeding juveniles. In these groups, growth is tightly controlled to maintain the social hierarchy. Subordinate fish grow slowly to remain smaller than their superiors; otherwise, they risk being attacked or driven out of their anemone home. Therefore, any changes in body size in clownfish can affect their social hierarchy.

To solve this, breeding pairs were seen to coordinate shrinking in size while still maintaining the size ratio between them. Keeping this gap intact was crucial because if the male grew too close in size to the female, conflict could erupt and destabilise the breeding pair. By shrinking in sync, pairs preserved their hierarchy and boosted their chances of survival.

"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. 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," comments Dr Theresa Rueger, senior author of the Newcastle University research team.

Both studies suggest that clownfish cope with heat by reprogramming their metabolism and adjusting their body shape. While the OIST study shows that young clownfish can acclimatise to gradual rises in temperature but struggle with sudden spikes, the Newcastle University study highlights how clownfish may adjust during extreme heatwaves, shedding light on their potential strategies in increasingly variable oceans. These may be quick fixes that help them survive in the short term, but they come at a longer-term cost — stressed organs and reduced reproduction. The findings also raise broader questions: What are the long-term consequences of these changes? How might they reshape clownfish populations in the future? Do other fish species employ similar tactics to endure a warming ocean?

(The writer is a science communicator)