May 16, 2024

Why some clownfish are growing up too quickly

This article was originally featured on Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at

For clownfish, life begins with an adventure. In 2003’s Finding Nemo, young Nemo takes a dizzying journey from coral reef to captivity and back again. In real life, it’s a different kind of quest: soon after hatching, tiny translucent clownfish larvae swim for 10 to 15 days, traveling up to 35 kilometers through open ocean. It’s the biggest trip they’ll ever take. After this brief excursion, young clownfish develop their iconic orange and white coloring and settle down on an anemone, where they dwell for the rest of their days.

But recent research suggests that climate change could disrupt this delicate life stage. In laboratory experiments, graduate student Billy Moore at Japan’s Okinawa Institute of Science and Technology (OIST) and colleagues found that clownfish larvae raised in water 3 °C warmer than normal zoom through early development. After 18 days, fish raised at 31 °C instead of 28 °C had bodies 16 percent longer, on average. The fish raised in warmer water also grew complete fins and pelvic fin spines—a key stage of clownfish development—two days faster than the fish raised under cooler conditions.

Timothy Ravasi, study coauthor and marine scientist at OIST, says that faster growth in a warming world could become a problem for wild clownfish. If climate change causes clownfish larvae to develop too quickly, they might arrive on an anemone when there’s not enough food to go around. Or fish that grow faster might not swim as far—if they settle close to home and mate with nearby fish, clownfish genetic diversity could suffer.

But the fish’s quicker growth could have benefits. Emily Fobert, a marine ecologist at the University of Melbourne in Australia who was not involved in the study, suggests that faster maturing clownfish larvae may spend less time in the open ocean where they are vulnerable to predators.

Either way, clownfish are a prime choice for studying the consequences of climate change because, unlike many coral reef fishes, they are easy to breed in captivity. This gives researchers the chance to study their entire life cycle up close, and probe questions about how warming water might affect wild fish at each stage of their development. Plus, Ravasi jokes, “everyone loves Nemo.”

The clownfish that Moore raised in warmer water also had faster metabolisms, which the scientists determined by measuring how much oxygen the clownfish consumed in a tiny swim tunnel. This squares with previous research on older clownfish, as well as Ravasi’s not-yet-published research on juvenile grouper.

The researchers based the warmer temperature in their study on the projection of future climate change if carbon dioxide emissions double by the year 2100. Although the Intergovernmental Panel on Climate Change predicts a 3 °C increase in average ocean temperatures by 2100 under that scenario, temperature spikes are already common during ocean heatwaves. This year, ocean temperatures have broken records around the world, with the North Atlantic more than 1 °C warmer than normal, on average. Some spots are seeing even higher temperatures, like the 10 °C jump near coastal Newfoundland in July.

“The temperature is going to increase, marine heatwaves are going to increase, so we do need to understand how these fish will respond,” says Moore.

This article first appeared in Hakai Magazine and is republished here with permission.

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