Animals change their behavior and bodies in response to human-induced climate change.

In recent years, some birds have developed larger beaks to help them dissipate excess body heat. Animals that migrate with the seasons have been observed seeking higher latitudes.

Now, scientists have found that clownfish, the saltwater fish featured in the hit Disney movie Finding Nemo, have been observed to temporarily shrink in response to heat stress.

Melissa Versteeg, a PhD student in marine science at Newcastle University and co-author of the article recently published in the journal Science Advances, said she “freaked out a little” when she noticed the shrinkage.

“It's generally accepted that growth is relatively unidirectional, so it's beneficial to be a little bigger. And until now, we haven't seen that they actually have this capacity or ability to shrink,” Versteeg said.

Versteeg's team measured 134 clownfish on reefs in Papua New Guinea every month for five months and monitored the water temperature during a marine heatwave.

They found that some clownfish, which are only about 7.6 cm long on average, shrink as the water temperature rises. Those that shrink increase their chances of surviving heat stress by as much as 78%.

“When they shrink, it's about 1 or 2% of their body size. So, really small reductions in length — only between one and two millimeters,” Versteeg said.

Why fish might shrink

Versteeg explained that it is still unclear how fish shrink or why they are significantly more likely to survive a heatwave. But the research team speculates that the literally smaller size of the fish – by reducing their body size – means they need less food, which makes their metabolism more efficient.

“Something about these shrinking fish gave them a better chance of surviving the heatwave than those that continued to increase in size,” the scientist said. She added that the fish grew back to their normal size when water conditions improved.

Clownfish usually live among coral reefs, which are also affected by warming ocean waters.

Researchers have also noticed that clownfish in breeding pairs shrink together.

“This is a beautiful example of real cooperation and teamwork in that sense, to try to make sure you come out the other side unscathed,” Versteeg said.

She emphasized that the discovery means that if clownfish can shrink, it is likely that many other fish in coral reefs can do so to survive. | BGNES