'Finding Nemo’ just got more difficult because clownfish are shrinking. The colourful fish, made famous by the animated film, can quickly reduce their size as a way to deal with heat stress, a study in Science Advances suggests. The finding could help to explain why fish all over the world’s oceans are also getting smaller.
Researchers studied 67 pairs of clownfish living in reefs off Kimbe Bay in Papua New Guinea, during the marine heatwave of 2023. They measured the length of each fish every month for five months, and found, to their surprise, that three quarters of them shrank during this time.
Adult clownfish grow to be around 8 centimetres long, but these animals lost a couple of millimetres. “This is not just about getting skinnier under stressful conditions, these fish are actually getting shorter,” says Melissa Versteeg from Newcastle University, who studied the fish.
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It’s well known that animals can evolve to become smaller over generations. Island dwarfism, for example, occurs when limited resources on island settings lead to the evolution of ‘mini’ species, such as dwarf elephants. But these changes occur across hundreds of years.
Clownfish now join the list of animals known to shrink in their own lifetime. This includes, European moles, which can shrink by over 10 per cent, and marine iguanas, which can shrink by up to 20 per cent, both to save energy during harsh conditions.
This is the first time that a coral reef fish has been shown to reduce the length of their body in response to environmental conditions. Exactly how they do this is unclear, but they may be reabsorbing fat and bone, as has been seen in other animals.
This rapid response is actually good for the fish. Shrinking increased individuals’ chances of surviving the heatwave by as much as 78%, the team found.
As the world warms, marine heatwaves are becoming more common. We don’t yet know if individuals belonging to other fish species are also shrinking during heatwaves, but if they are it could help to explain a bigger trend; that over the last few decades, many of the world’s fish are indeed getting smaller.
Other explanations for this include pollution, habitat degradation, overfishing and disruptions to the oceans’ food webs. “If individual shrinking were widespread, it could provide a plausible possible alternative hypothesis for why the size of many fish species is declining,” says Theresa Rueger, who led the study.
Main image: clownfish. Credit: Newcastle University
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