Survival mode activated: The incredible fish that can shrink in warm water to survive heat waves

Just like animals on land, those in the oceans have to contend with heatwaves. For those creatures that cannot escape the excessively warm waters for cooler areas this can mean a death sentence as evidenced by coral bleaching.

However, a group of scientists recently observed that clownfish, made famous by the movie ‘Finding Nemo’, have a nifty trick they can employ, shrinking. The researchers from Newcastle University were able to record this surprising discovery in Kimbe Bay, Papua New Guinea during a marine heatwave that began in March 2023 and recently published their findings in Science Advances.

Survival mode activated: Shrinking Nemo

Clownfish live in coral reefs among sea anemone, a type of predatory marine invertebrate, that provide them with protection. They rarely leave their host, lest they risk being eaten by predator.

That means when the water around them gets too hot, which is becoming more and more common as global temperatures rise due to climate change, they have to stick it out. So, they go into survival mode and shrink, which they can do in an incredibly short space of time, as little as a month.

They aren’t getting skinner but actually shorter. The researchers say that this may sound counterintuitive, as they are more at risk of being eaten and they can breed less.

However, this allows them to need less oxygen, which is reduced in warmer waters, and less food. Additionally, they are typically more efficient at foraging in their reduced size the scientists explained.

Furthermore, when conditions improve, they observed that “those fish who shrank by a lot could catch up and grow rapidly.” When this growth flexibility is present in both members of a breeding pair, they “were more likely to survive [a] heatwave than if only one, or neither, fish shrank,” explained the researchers.

Of the fish they observed, none of those that shrank multiple times died and there was a 78% survival rate among those that shrank at least once.

How are clownfish shrinking?

The exact process hasn’t been determined, that would require more research in a laboratory to analyze the physiological changes taking place. However, there are other vertebrates like marine iguanas on the Galapagos Islands that could provide clues.

During El Niño years, when the water temperatures rise in the eastern and central tropical Pacific Ocean, marine iguanas shrink by absorbing part of their bodies explained the scientists.

Shrinking clownfish could tell a bigger story

In an article published in The Conversation by three of the researchers, Dr Theresa Rueger, Melissa Versteeg and Dr Chancey MacDonald, they say that their findings may have wider implications. It has been observed that worldwide many marine fish are getting smaller.

“This could partly be a result of fishing removing larger fish from populations, as well as the warming climate altering the growth or maximum sizes of fish,” they explain. However, if it turns out that “adult fish shrinking in response to environmental stress is more widespread, it could be another reason why fish in the world’s ocean are getting smaller.”

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