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Damselfish in Distress Release Chemicals That Summon More Enemies

In the finale Jurassic World—and spoiler warning for anyone who hasn’t seen it—Bryce Dallas Howard’s character uses a flare to summon a Tyrannosaurus, which then battles the film’s main antagonist Indominus rex. As the two predators duke it out, the humans escape.

A similar strategy plays out in the Great Barrier Reef. When threatened by predators, the lemon damselfish sheds a chemical alarm—a substance housed in its skin that says, “I’m being attacked!” This message isn’t intended for other damselfish. It’s not a warning or a call for help. Instead, it attracts even more predators.

Many aquatic animals release alarm chemicals from their skin when they are injured. Corals, sea urchins, crayfish, mosquito larvae, tadpoles, goldfish, trout, and salmon all do it. For any eavesdropping passers-by, these substances provide valuable intelligence about nearby danger. But how does the injured individual benefit? Why would it produce and release these chemicals in the first place?

One idea is that the chemicals are not alarms but disinfectants, which help to keep wounds free from infections if the victims should escape. But Oona Lonnstedt from Uppsala University and Mark McCormick from James Cook University have another explanation. They think that these alarm substances help by attracting secondary predators that compete with the one currently attacking. While the two threats fight each other, their prey can use their distraction to escape.

Recently, Lonnstedt showed that the dusky dottyback, a medium-sized coral reef predator, was indeed drawn to the scent of an injured lemon damselfish, and could even discern the smell of big and healthy targets. These were lab experiments, but Lonnstedt has now found that wild dottybacks behave in the same way.

First, she created sniffing samples of Eau de Injured Damselfish by cutting the skin of dead individuals and immersing them in bags full of seawater. She then extracted the water with a syringe and injected them into plastic tubes, which she fixed in the territories of wild dottybacks. As before, the dottybacks were attracted to the scents—but so were other predators, including rock-cods, coral trout, snappers, and even sharks. And when the different predators met, they often clashed.

Back in her lab, Lonnstedt showed that these conflicts can be life-savers. A dottyback will almost invariably capture and kill a damselfish that’s released into the same tank. But when Lonnstedt also introduced a second predator, the interloper would often try to snatch the poor damselfish straight out of the dottyback’s mouth. To deal with these nuisances, the dottyback would typically drop the damselfish, which then sped straight into the shelter of nearby coral. Around 40 percent of the time, it escaped with its life.

Together, these experiments provide the first evidence that animals can escape from predators by using alarm cues to summon even more predators—a risky gambit, but clearly an effective one.

This strategy is probably a common one, and not just in fish. Many animals scream in alarm when captured or menaced by predators, and these calls neither seem to startle the predator nor warn eavesdropping individuals. And in many cases, the individuals do better if they make these calls: for example, when Belding’s ground squirrels spot birds of prey, they’re more likely to survive if they make alarm calls than if they stay silent. Lonnstedt’s work helps to explain why.

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