In a surprise finding that appears to show that the brain power of fish has been considerably underestimated, scientists said small tropical reef fish were able to recognize themselves in a mirror.

The scientists conducted a study, published on Feb. 7 in the open-access journal PLOS Biology, involving experiments in which the bluestreak cleaner wrasse, a small saltwater fish, was subject to a mirror self-recognition test, a technique developed in 1970 for gauging animal self-awareness.

Researchers from the Max Planck Institute for Ornithology (MPIO) and Osaka City University (OCU) applied a brown-colored mark on the body of the fish in places that could be seen only in a mirror’s reflection. They found that the fish tried to remove the marks by scraping their bodies on hard surfaces after seeing themselves in a mirror, but never tried to remove the marks without a mirror present, indicating that they understood the reflection was of them.

When a transparent, rather than brown, mark was applied, the fish never tried to remove it, the researchers said.

“The behaviors we observe leave little doubt that this fish behaviorally fulfills all criteria of the mirror test as originally laid out,” Dr. Alex Jordan, senior author on the study, said in a press 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.”


Alex Jordan, senior author of the study, administered the mirror test on cleaner wrasse in order to test the cognitive capacity of social fish.


Some scientists, however, have disputed the study, and question whether the fish really recognized themselves.

University at Albany evolutionary psychologist Gordon Gallup, who pioneered the mirror test, called the new study “not methodologically sound” and faulted the researchers for a “zeal to undermine the integrity” of the technique to appraise animal self-awareness,according to the Guardian.

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

The test has been passed by chimpanzees, bonobos, gorillas, orangutans, as well as dolphins, killer whales, an elephant and a magpie species, but failed by some other animals. Humans pass it at around 18 months old.

Jordan told the Guardian some of the criticisms were prompted by people’s preconceptions about fish, rather than scientific objectivity. “When it’s a fucking elephant and one of two elephants passes the test, everyone’s like ‘Yeah cool’,” he said. “When it’s a fish they’re like, ‘Ooh you need a conspecific control and a control for empathy and a control for this and that … the fish are not doing this’.”



PLOS Biology also recognized the study’s potential for controversy, and commissioned an accompanying commentary from Professor Frans de Waal, a leading primatologist at Emory University who has studied mirror self-recognition in mammals.

While de Waal finds the fish study intriguing, he urges caution in interpreting it. In doing so, he calls for less black-and-white approach to animal self-awareness. “What if self-awareness develops like an onion, building layer upon layer, rather than appearing all at once?” he asked. “To explore self-awareness further, we should stop looking at responses to the mirror as its litmus test. Only with a richer theory of the self and a larger test battery will we be able to determine all of the various levels of self-awareness, including where exactly fish fit in.”