Can fish recognize themselves in a mirror? Study reveals surprising results

Even tiny species of fish may have greater intelligence than you might think. An international group of scientists identified a type of fish that may be able to recognize itself in the mirror.

The mirror test is often used by biologists to test self-awareness in different animals. In this latest study, researchers found that the cleaner wrasse — a species known for “cleaning” parasites off of other fish — responded to itself and attempted to rid itself of marks on its own body after apparently seeing its reflection in the mirror. This finding suggests that fish may have higher cognitive abilities than previously thought.

Cleaner wrasse
A cleaner wrasse interacts with its reflection in a mirror placed on the outside of the aquarium glass. Note that the mirror itself cannot be seen in this photo because the aquarium glass itself becomes reflective at the viewing angle of the camera, according to Snell’s law. This is not the case for the fish itself, which sees the aquarium glass as transparent because of its direct viewing angle. (Photo credit: Alex Jordan)

To be sure, the authors placed a mark on the fish and then put a mirrored divider in the the tank. When a transparent mark was placed on the fish, they did not react with the mirror present. But if they were given a colored mark, the researchers found the fish would attempt to scrape its body on hard surfaces as if to try and remove the mark.

When the mirror was removed, the fish didn’t pay mind to the mark. Unmarked fish also didn’t react when a clear divider was placed in the tank and a marked fish was placed on the other side.

Though their results provide evidence of animal behavior that appears to pass all parts of the mirror test, the authors say questions remain. If the cleaner wrasse passes the mirror test, does it possess self-awareness, which was previously thought to be unique to primates and some other mammals, or does passing the mirror test involve a different cognitive process than previously thought?

“The behaviors we observe leave little doubt that this fish behaviorally fulfills all criteria of the mirror test as originally laid out,” says Dr. Alex Jordan, senior author on the study, in a media release. “What is less clear is whether these behaviors should be considered as evidence that fish are self-aware — even though in the past these same behaviors have been interpreted as self-awareness in so many other animals.”

Jordan continues: “Depending on your position, you might reject the interpretation that these behaviours in a fish satisfy passing the test at all. But on what objective basis can you do this when the behaviours they show are so functionally similar to those of other species that have passed the test?”

The research group included scientists from the Max Planck Institute for Ornithology and Osaka City University.

The study was published in the open-access journal PLOS Biology.