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This innocuous-looking reef fish is actually a vicious killer

Brown versus yellow dottyback. Dottybacks change color to imitate other fish, then kill them and eat them. (N Justin Marshall/Courtesy of University of Basel)
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Behold the dusky dottyback, con artist of the coral reef.

Bright, lithe and no longer than your index finger, these Australian reef-dwelling fish may look innocent. But they are actually lethal predators and masters of the art of disguise. Capable of changing color to resemble the types of fish they prey on, they hide themselves among a school of their prospective victims and then swoop in to devour the unsuspecting young.

“It’s the classic wolf in sheep’s clothing scenario,” said Cambridge University research fellow William Feeney, who studies the dottyback. “They’re really good at looking like something harmless so they can exploit it.”

The species that the dottybacks prey on are called damselfish — yep, like damsel in distress — because apparently the whole scenario wasn’t Shakespearean enough. That’s life in the wild for you.

The discovery about the dottyback, which was published in the journal Current Biology on Thursday, is unique in the world of zoology, according to Feeney. Unlike other animal mimics, dusky dottybacks are capable of changing color to imitate two different types of fish, yellow damselfish and brown damselfish, depending on the kind of school that surrounds them.

“This is the first time that anyone has been able to show … a species that is capable of mimicking multiple things, looking like one fish one day, looking like another fish next week,” said Feeney, a co-author of the Current Biology study.

Fabio Cortesi, an evolutionary biologist at the University of Basel in Switzerland and the lead-author of the study, said that dottybacks can change their coloring thanks to special pigment-containing cells called “chromatophores” in their skin. Each fish has a supply of both black and yellow cells, and can rotate in the appropriate color when needed.

To test what causes dottybacks to shift color, Cortesi and Feeney created several mock habitats filled with live coral, rubble and groups of either brown or yellow damselfish. When they added dottybacks into the mix, they found that the fish were able to change coloring within two weeks, becoming nearly indistinguishable from the harmless damselfish that surrounded them. Their new form allowed them to sneak up on defenseless juveniles, who apparently weren’t smart enough to be suspicious of strangers.

Their disguises made the dottybacks particularly lethal, Cortesi said: A dottyback that was the same color as the fish around it was two to three times more successful at hunting, consuming as many as 30 juvenile damselfish a day.

Mimicry — the tactic of taking on the form of another species in order to hide from predators or sneak up on prey — is a common phenomenon in the animal world, according to Feeney. But it comes with a major drawback: Species can’t use it too often, or else their victims will wise up and the ability becomes useless.

“Mimics have to be rare to be successful generally, and then you’ve got this neat evolutionary conundrum that things don’t want to be rare,” he said. “Dottybacks have found a unique way of getting around that problem by mimicking multiple things. … So perhaps when things start to clue onto them they can change and hang out with other fish.”

The dottybacks’ color changing ability has the secondary benefit of helping them blend in with their surroundings — yellow dottybacks camouflage into living coral, whereas brown ones are well hidden among the rubble of dead reefs. This makes the dottyback as good at hiding when it’s being hunted as it is when it’s doing the hunting.

“It really is a crafty little fish,” Feeney said.