For sport-fishing enthusiasts, sea marlins are the dream catch. The muscular beasts, up to four meters long and weighing 220 kilograms, attract anglers the world over to the Pacific, where epic tournaments are held every year. With peak speeds of 110 kilometers per hour, rare stamina and a sword-shaped bill, the animal offers its human predators a real challenge. Are you horrified? Just know that the organizers swear that their catches are systematically returned to the sea.

Prey, then, but also a predator. A top predator even, perched at the tip of the marine food chain. Off the coasts of Mexico and Australia, or in the seas around Hawaii, tuna, octopus, mackerel and sardines all pay the price. The marlin's onslaughts are all the more fearsome because it hunts in packs. Groups of up to 30 individuals gather around schools of sardines.

But according to which rules, following which trajectories, with what efficiency? This is what a team of German and British researchers has been studying for several years on the coast of Baja California, Mexico. An almost impossible task on water, given the speed of movement of both prey and predators. So the biologists turned to a tool of choice seldomly used in marine research: drones. The result is what Alicia Burns, a postdoctoral researcher at Berlin's Humboldt University calls "the bonus." A discovery as unexpected as it has been spectacular, it was the subject of a publication in the February 26 issue of Current Biology.

During these successive assaults, which see the fish attack in turn, the main assailant "suddenly changes color," said the article. In a matter of seconds, the light-colored stripes to which the species owes its name, and its fins, suddenly begin to glow for about 20 seconds. The animal then pounces on its prey, catching one or more. Then it "turns off the light" and moves away. On to the next.

For Burns, "it's probably a signal to other marlins." A "motivational indicator" to avoid collisions between fellow marlins. It must be said that at such speeds and with such weapons, a collision would have dire consequences. Another explanation is that the luminous stripes could disrupt the collective behavior of the sardines. "When a marlin attacks a solitary sardine, it doesn't change color," Burns pointed out. Unless the luminous stripes could perform both functions simultaneously.

The fish's formula for entering into shiny mode has already been described in detail. In the melanophore cells of its stripes, but also in its fins and crest, pigment granules are redistributed and crystalloid blades reflect sunlight. However, the conditions of this color change remain unknown.

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