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Finding Shrinking Nemos

5th June 2025

As World Environment Day rolls around, scientists have uncovered some surprising findings about the well-loved “Nemo” fish!

By Ashley Hayes, Account Executive.

Finding Shrinking Nemos

An image of 2 nemo fish hiding coral

In a recent study published in Scientific Advances, a team of researchers set out to investigate how environmental conditions affected Amphiprion percula clownfish – a species that we’ve all come to know and love from Disney’s hit film Finding Nemo.

During their observations, a marine heatwave hit the bay, pushing temperatures 4°C above average. This unexpected event gave the team a rare opportunity to study how clownfish respond to marine heatwaves, which are becoming increasingly frequent, intense, and prolonged due to climate change.

What did the researchers find?

The team measured the length of the clownfish every month over a 5-month period – and the results were quite bewildering! 71% of dominant females and 79% of breeding males shrank at least once during the study. These reductions in size had a major impact on the population, with just one single shrinking event increasing the chance of survival by 78%. And to make this phenomenon even more fascinating, when both partners in a breeding pair shrank, their chances of survival increased even further.

When it comes to clownfish, size matters

To understand why clownfish shrink under elevated temperatures, you might be surprised that social dynamics could be at play. This fish species lives in size-based hierarchies, with the largest and most dominant fish being the breeding female. The second highest-ranking fish is the breeding male, who measures up to be slightly smaller. The rest of the group is made up of smaller non-breeding juveniles.

Clownfish can adapt their size to their social environment. If a more dominant fish dies, a lower-ranking fish can grow to take its place. However, there are limits – if a subordinate grows too large, the breeding pair may exile them from the anemone, diminishing their chances of survival. To avoid this harsh fate and maintain harmony within the group, subordinate clownfish will suppress their growth.

Finding Shrinking Nemos

A school of clown fish swimming amongst the coral

Why do clownfish shrink at hotter temperatures?

While it was already known that clownfish can modulate their size in response to social dynamics, the recent study in Scientific Advances is the first to report growth changes in response to their environment. This novel finding raised the question: why does shrinking improve survival during a marine heatwave?   

The researchers proposed a few possible explanations. One is that higher ocean temperatures reduce oxygen availability and food resources, making a smaller body size more metabolically efficient. From the results of the study, it was also proposed that clownfish social cues continue to play a role even under environmental stress.

Size adaptation might be a double-edged sword

A key take-home message from this study is how useful growth plasticity can be in marine species, like clownfish, that can’t escape deteriorating environmental conditions. However, while size adaptability is impressive, it could come with consequences. As clownfish females become smaller, the birth rates are expected to decrease, potentially leading to declines in the population. As global warming continues to affect our ocean temperatures, it’s clear that further studies like this one are crucial to predict how marine species will adapt.

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