‘Shrinking Nemo’: Study finds clownfish can shrink to survive higher sea temperatures

While finishing off a wall dive near Bunaken, Indonesia, Christian Bachmann came across a fast-moving clownfish. He says “patience and a little luck” were required to get this photo. (Christian Bachmann / Scuba Diving Magazine)

Clownfish, a small orange and white species made famous by the “Finding Nemo” movies, have been found to shrink in order to boost their chances of surviving marine heat waves, according to a new study.

Working at a conservation center in Papua New Guinea, a team led by scientists from Newcastle University, England, monitored 134 clownfish over a period of five months during a marine heat wave in 2023, according to a statement from the university published Wednesday.

Study lead author Melissa Versteeg, a PhD student at Newcastle University, measured the length of each fish every month, as well as taking the water temperature every 4-6 days.

Versteeg found that the fish would get shorter as temperatures rose, the first time that a coral reef fish has been found to shrink when environmental conditions change.

“I was very surprised by the findings,” Versteeg told CNN on Thursday, adding that the clownfish displayed “incredible growth plasticity.”

“We’re seeing that they have a great capacity to respond to what the environment throws at them,” she said.

This is particularly relevant because marine heat waves are becoming increasingly common as climate change intensifies, with serious impacts on coral reefs and other marine life.

Of the 134 clownfish studied, 100 got shorter, the team found, and this shrinking increased their chances of surviving the heat stress by up to 78 per cent.

Senior study author Theresa Rueger, a marine ecologist at Newcastle University, told CNN that shrinking is not necessarily a good thing as small fish reproduce less, which could be bad for the population.

“But our study also found that because they can shrink and they have this great growth plasticity, they survive better during marine heatwaves,” she said.

“That could be a very positive thing, that they have that capacity and can adapt to their circumstances in that way.”

A small number of other animals are also capable of shrinking, including marine iguanas, which are able to reabsorb bone material to become smaller during times of environmental stress, said Versteeg in the statement.

The team also found that clownfish which shrank at the same time as their breeding partner had a higher chance of survival.

This is because of the balance of power between the female, which is the larger and more dominant, and the male, they explained.

If a female starts to shrink, the male will also shrink in order to prevent social conflict and reduce the possibility of a confrontation that he would always lose.

This is important because clownfish live symbiotically with either of two sea anemone species – Heteractis magnifica and Stichodactyla gigantea.

The anemones provide protection for the clownfish which are “bad swimmers,” said Rueger.

“If they leave the anemone they have very little time before they get eaten, frankly,” she added, which means it is important for them not to fight with their partner and risk being forced out into the open sea.

The symbiotic relationship with their home anemone is another reason why clownfish are so fascinating, said Versteeg.

“They don’t move, that’s where they’re at. You’re able to just track them over time, and they’re uniquely marked and they’re relatively easy to catch,” she said.

“You really know who you’re dealing with, and you can really follow them over extensive periods of time, which is unique,” added Versteeg.

Next up, the team plans to investigate the mechanisms behind this capacity to grow and shrink depending on environmental conditions, as well as looking into whether other fish species are also able to do so.

“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,” said Rueger in the statement.

The study was published in the journal Science Advances.

Article by Jack Guy.