Striped marlin are some of the fastest animals on the planet and one of the ocean’s top predators. When hunting in groups, individual marlin will take turns attacking schools of prey fish one at a time. Now a new study (Current Biology) explains how they might coordinate this turn-taking style of attack on their prey to avoid injuring each other. The key is rapid colour changes.

Studying the way rapid colour change as groups of marlin hunted schools of sardines, the authors found that the attacking marlin ‘lit up’ and became far brighter than others as it made its attack and the colour rapidly returned to its ‘non-bright’ colouration after the attack. The researchers used drones to study this phenomenon.

The video footage revealed that the stripes on individual marlins got far brighter as a fish moved in for an attack, and dimmed once they swam away. To explore if the changing colours was to communicate with one another, they analysed 12 high-resolution video clips, each containing two separate attacks on a school of sardines by two different marlins. They also quantified the contrast of the stripes on the two attacking marlins compared to a randomly chosen marlin that was not attacking. Their analysis confirms that the predatory fish rapidly change colour, suggesting that the colour change might serve as a reliable signal of an individual’s motivation to go in for an attack.

“Colour change in predators is rare, but especially so in group-hunting predators,” Dr. Alicia Burns of Humboldt University, Berlin, Germany and the corresponding author of the paper said in a release. “Although it is known that marlin can change colour, this is the first time it’s been linked to hunting or any social behaviour.”

The discovery suggests that marlins have more complicated communication channels than had been suspected. The researchers propose that the colour changes might even serve a dual purpose of confusing their prey.

They now hope to explore this idea, alongside other questions. For example, they want to find out whether marlins use their colour-changing abilities in other contexts. The authors are curious to know whether they still change colour when hunting solo and how the changes affect their prey. They are also looking into similar colour changes in other predatory species of fish.

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