Clownfish in Kimbe Bay, Papua New Guinea, were observed shrinking in size during an extreme marine heat wave in 2023, according to a new study published in Science Advances. Researchers measured 134 clownfish and found that 101 individuals reduced their body length at least once due to heat stress. Rising ocean temperatures, driven by climate change, are causing more frequent and intense heat waves. These conditions damage coral reefs and sea anemones, which serve as shelter for clownfish, forcing the fish to adapt to survive.

The researchers suggested that shrinking may reduce the clownfish’s energy needs, as smaller animals require less food in stressful environments. One theory is that the fish reabsorb bone tissue to reduce size. Some breeding pairs were also seen adjusting their body size together, with females maintaining a larger size than males to preserve the species’ social order. This behavior had not been previously recorded in coral reef fish, though it is known in other species such as marine iguanas in the Galápagos Islands. The team reported that the clownfish were able to grow again after temperatures returned to normal, showing a form of biological flexibility. However, ocean ecologists warned that this strategy might not be sustainable if heat waves become longer and more intense in the future. The findings demonstrate a new survival method used by marine animals in response to a changing climate, but also raise concerns about long-term resilience in warming oceans.