Clownfish – the species most recognisable for their starring role in the film Finding Nemo – have been shown to shrink in order to survive heat stress and avoid social conflict, a new study from Newcastle University has revealed – giving plausibility to just how Nemo evaded detection for so long in the first place.

It’s a survival method that scientists suggest could just explain the rapidly declining size of fish in the world’s ocean as marine heatwaves intensify with greater persistence than previously recorded.

Academics from the universities of Newcastle, Leeds, and Boston in the USA worked alongside the Mahonia Na Dari Conservation and Research Centre in Kimbe Bay, Papua New Guinea, to measure the length of some 134 clownfish every month for five months, while monitoring the water temperature every four to six days.

Results published this week in the journal, Science Advances reveal the remarkable ability of clownfish to shrink, i.e. growing shorter in response to heat stress. It was concluded that shrinking increased an individual’s chances of surviving the stress of a marine heatwave event by as much as 78%.

In a more heart-warming discovery, it was also discovered that individuals aren’t shrinking alone, indicating that coordination is important for clownfish. It was found that they had a higher chance of surviving heatwaves when they shrink alongside their breeding partner. This is the first time that a coral reef fish has been shown to reduce the length of their body in response to environmental and social conditions.

Melissa Versteeg, a PhD researcher at Newcastle University’s School of Natural and Environmental Science led the study. Of this newly-discovered ability for the clownfish to shrink, she said: “This is not just about getting skinnier under stressful conditions, these fish are actually getting shorter.

“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.”

“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,” she added. 

During the study, ‘Individual clown anemonefish shrink to survive heat stress and social conflict’ the researchers noted that 100 fish shrank out of the 134 fish monitored.

“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,” said Versteeg.

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 of many fish species is declining and further studies are needed in this area.”

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