Shrink or Die

Some animals cope with heat by burrowing underground. Others take to the water or even roll in mud. Clownfish (Amphiprion percula), however, have a more unusual way of dealing with increasingly high temperatures: They shrink themselves.

That was the result of a new study on how this coral reef species copes with environmental changes. As part of the research, scientists repeatedly measured 134 wild fish in Kimbe Bay, Papua New Guinea, during a marine heatwave that started in March 2023, which is part of an ongoing global mass bleaching phenomenon, the Conversation explained.

They found that 100 of the fish in the sample shrank from February to August 2023. Additionally, the fish that shrank had a better chance of making it out of the heatwave alive.

“This is not just about getting skinnier under stressful conditions, these fish are actually getting shorter,” said lead study author Melissa Versteeg in a statement.

Clownfish live in social groups within anemones on coral reefs, and they rarely leave their “home” as it protects them from predators. However, never leaving the anemone also means that these fish can’t escape to cooler regions as marine heatwaves on coral reefs become increasingly common due to rising global temperatures.

Considering that clownfish are vertebrates, scientists were surprised to discover that they can shrink in response to heat stress because growth in vertebrate beings is generally considered unidirectional.

Getting smaller might also seem counterintuitive, as it makes animals more vulnerable to predators and less likely to reproduce.

For clownfish during heatwaves, however, becoming smaller turned out to be more beneficial for survival because they needed less food, and became more efficient at foraging for food and also at using oxygen, which exists in lower quantities in hot water.

Clownfish social groups are based on strict hierarchies based on size, so shrinking is a tricky move and risks throwing off the balance of the group, which could lead to a fish being evicted – which usually results in death.

On each anemone, the largest clownfish is female and the second largest is male, and together they make up a breeding pair. To prevent fights, males adjust their growth to keep the size ratio fixed.

The study results showed that the breeding pairs where both clownfish shrank were more likely to survive the heatwave compared with those where only one, or neither, did.

While some of the fish in the sample didn’t survive the heat, all of the fish that shrank multiple times did, and the results showed that shrinking just once increased the survival probability during the heatwave by 78 percent.

“We don’t know yet exactly how they do it, but we do know that a few other animals can do this too,” said Versteeg. “For example, marine iguanas can reabsorb some of their bone material to also shrink during times of environmental stress.”