As the world contemplates dealing with more extreme temperatures, one coral reef fish has found a novel way to beat the heat: shrinking.

Wanting to know how clownfish cope with changes to their environment, we repeatedly measured 134 wild fish in Kimbe Bay, Papua New Guinea, during a marine heatwave that started in March 2023 and is part of an ongoing global mass coral bleaching event. Clownfish have unique markings, which make it easy to identify and measure them underwater.

To our complete surprise, we found that 100 of the fish we measured shrank during our study from February to August 2023. Those that shrank had a better chance of surviving the heatwave.

The clownfish, Amphiprion percula, lives in small social groups within anemones on coral reefs. As the movie Finding Nemo indicated, clownfish rarely, if ever, leave their host anemone because the anemone offers them protection from predators.

Sadly, this also means that clownfish cannot move to cooler areas as marine heatwaves become more common on coral reefs due to rising global temperatures. Clownfish need other strategies to survive the heat.

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