Clownfish, the brightly colored reef fish made famous by Finding Nemo, have revealed a surprising survival strategy in the face of climate change: they shrink.
New research led by Newcastle University shows that these fish can reduce their body length in response to marine heatwaves, improving their chances of survival and maintaining social harmony.
“This is not just about getting skinnier under stressful conditions, these fish are actually getting shorter,” said Melissa Versteeg, a PhD researcher at Newcastle University’s School of Natural and Environmental Sciences, who led the study.
“We don’t know yet exactly how they do it, but we do know that a few other animals can do this too. For example, marine iguanas can reabsorb some of their bone material to also shrink during times of environmental stress.”
Shrinking as a heat stress response
In a five-month study conducted in Kimbe Bay, Papua New Guinea, researchers monitored 134 clownfish through a marine heatwave. Water temperatures were logged every 4–6 days, and fish length was recorded monthly.
The study found that 100 out of the 134 fish studied physically shrank during the heatwave — a response previously undocumented in coral reef fish.
This ability to get shorter (not just slimmer) boosted survival odds by up to 78%. While the exact biological mechanism is still unknown, parallels exist in the animal kingdom: marine iguanas, for instance, are known to shrink by reabsorbing bone during stressful periods.
“We were so surprised to see shrinking in these fish that, to be sure, we measured each fish individual repeatedly over a period of five months. In the end, we discovered it was very common in this population. During our study, 100 fish shrank out of the 134 fish that we studied.”
“It was a surprise to see how rapidly clownfish can adapt to a changing environment and we witnessed how flexibly they regulated their size, as individuals and as breeding pairs, in response to heat stress as a successful technique to help them survive.”
Social synchrony enhances survival
Clownfish live in strict social hierarchies, often as monogamous breeding pairs. The study found that survival was higher when both members of a breeding pair shrank in coordination.
This social dimension suggests that size reduction is not only a response to environmental pressure but also a strategy to maintain social balance, possibly reducing conflict within the pair or group.
This is the first scientific evidence showing that coral reef fish can shrink in response to environmental and social cues. As marine heatwaves become more frequent, these findings highlight both the resilience and vulnerability of coral reef ecosystems.
Dr Theresa Rueger, Senior Lecturer in Tropical Marine Sciences and senior author of the study, added: “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 many fish species is declining and further studies are needed in this area.”
The study has been published in Science Advances.
