Clownfish shrink to survive maritime heatwaves.

• The 2023-2024 coral bleaching event affected 84% of the world’s reefs, linked to ocean warming and marine heatwaves.

• Adult clownfish can shrink, regrow, and synchronize these changes with their partners, using body size as a tool to survive hot water stress.

• Researchers from the UK and the US tracked 67 breeding pairs of wild clownfish in Kimbe Bay, Papua New Guinea for 180 days.

• The study found 71% of females and 79% of males shrank at least once during the heatwave, with 41% shranking over multiple months.

• Shrinkage thresholds differed by status, with females shrinking only once they exceeded about 80 mm and males above 61 mm.

• Higher heat in the current month encouraged growth, while heat last month promoted shrinkage, suggesting a delayed stress response.

• Shrinking became less likely when males and females were already close in size, but more likely when their size gap was large.

• Only 11 fish died, with a single shrinking episode cutting death risk by 78%.

• The shrinking could help reef-fish populations survive more frequent marine heatwaves, but may also lower future reproduction.