A school of fish might seem like a sea of identical faces, but at least one species has no problem telling its comrades—and even strangers—apart, new research says.
To human eyes, which cannot see ultraviolet light, the Ambon damselfish (Pomacentrus amboinensis), appears yellow with a few spots.
But to damselfish, which can see ultraviolet wavelengths, their fellow species sport a complex array of facial patterns that are unique to each individual.
"The idea is that these patterns help the fish communicate secretly—without attracting the attention of predators, which, like us, are UV blind," says experiment leader Ulrike Siebeck of the University of Queensland, Australia. (See photo gallery: "Masters of Undersea Camouflage.")
Scientists could see these patterns when they viewed the fish, native to coral reefs in the Western Pacific Ocean, through a camera with a filter that blocks all wavelengths of light except ultraviolet.
In experiments, Siebeck and colleagues found the 3.5-inch (9-centimeter) long damselfish can discriminate between individual faces of their own species and those of a similar species, the lemon damselfish. They could also tell the difference between different individuals of their own species.
"I was amazed at how well these fish could discriminate facial patterns that were almost identical to my eyes," says Siebeck, who presented her preliminary results at the recent Behaviour 2015 conference held in Cairns, Australia.
For the experiments, Siebeck and her colleagues trained captive damselfish to associate a particular facial pattern, say, a stripe on the forehead, with a food reward.
"The fish quickly learn to select rewarded targets placed in or behind their aquarium by swimming up to them," says Siebeck. "Initially we used printed and laminated pictures, but recently we moved on to presenting [the facial patterns] on a computer monitor located behind the aquarium." (Also see "Wasps Can Recognize Faces.")
The team then tested the fish's recognition ability by presenting them with two patterns: one familiar face for which they were rewarded, and one new face.
It turns out the fish could discriminate almost any pair of fish faces the team showed them, whether the faces belonged to their own species of the similar-looking lemon damselfish.
To create more of a recognition challenge, the team morphed, or blended, two or more photographs of fish faces to create a series of unique images the fish hadn't seen before, and repeated the same experiments. Again, the fish could identify the faces and passed the tests with flying colors.
Since ambon damselfish are social animals that live in groups, facial recognition may help the animals discriminate between individuals and thus develop and maintain bonds.
The researchers suspect that a fish's facial pattern may also contain information about its health or social status, but haven't yet tested this idea.
Of Fish and Humans
Jochen Zeil, a biologist at Australian National University in Canberra, says the experiments are "a wonderful example" of how animals can recognize—and use—patterns.
"We know too little about the importance of color and brightness patterns in individual recognition in animals," says Zeil, who wasn't involved in the research. (See "It's Thanks to Evolution That No Two Faces Are Alike, Study Finds.")
"Two aspects of this experiment were surprising: The fact that these patterns are only in salient in UV, and the fine pattern detail these fish attend to."
Experiment leader Siebeck also did not expect to discover how face discrimination in fish is so similar to how people recognize faces—even though fish lack the parts of the brain in humans that are associated with that skill.
For instance, in the morphed facial pattern experiment, her team found evidence for categorical perception in their fishy subjects.
This means the fish could perceive differences in similar-looking photographs in the same way that humans do.
"Categorical perception is thought to allow animals to make quick decisions about an image or stimulus," says Siebeck. (Can you always remember faces? You may be a super-recognizer.)
"In nature, this could be the vital decision about whether an approaching animal is classed as a predator or a harmless animal."
It's unknown how damselfish can use categorical perception without the part of the brain that's known to process it.
Many fish have distinctive facial patterns, especially in UV, so it is likely that other species can tell each other apart, she adds.
Maybe that means they can finally find Nemo.