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Clownfish survive heat waves by shrinking, study reveals

Clownfish have been shown to shrink in order to survive heat stress and avoid social conflict, Newcastle University research reveals.

The fish, recognizable from the starring role in the film "Finding Nemo," were studied by academics from the universities of Newcastle, Leeds and Boston, U.S. The work was conducted in collaboration with Mahonia Na Dari Conservation and Research Center, in Kimbe Bay, Papua New Guinea.

The team measured the length of 134 clownfish every month for five months, and monitored the water temperature every four to six days, during marine heat waves which are becoming increasingly common as a result of climate change.

Publishing in the journal Science Advances, they reveal the remarkable ability of clownfish to shrink, i.e. they can get shorter, in response to heat stress. Shrinking increased individuals' chances of surviving the heat stress event by as much as 78%.

In a heart-warming discovery, they also show that coordination is important for clownfish, as they have a higher chance of surviving heat waves 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 Ph.D. researcher at Newcastle University's School of Natural and Environmental Sciences, 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. 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."

The authors also highlight that individual shrinking might explain the rapidly declining size of fish in the world's oceans.

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."