Small tropical reef fish were able to recognize themselves in a mirror, scientists said Thursday, in a finding that raises provocative questions about assessing self-awareness and cognitive abilities in animals.

The study involved experiments in which the bluestreak cleaner wrasse was subject to a mirror self-recognition test, a technique developed in 1970 for gauging animal self-awareness.

In aquarium experiments at Osaka City University, researchers applied a brown-colored mark on the body of the fish in places that could be seen only in a mirror’s reflection.

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

The four-inch-long (10-centimeter) species consumes parasites and dead tissue off skin of other reef fish in a relationship benefiting both. The color of the brown marks resembled that of the parasites.

The fish show “behaviors during the mirror test that are accepted as evidence for self-awareness in many other species,” said evolutionary biologist Alex Jordan of the Max Planck Institute for Ornithology in Germany, leader of the study that was published in the journal PLOS Biology.

Jordan, however, questioned whether the test represents a reliable measure of animal cognitive abilities.

“I don’t claim that fish lack self-awareness, but rather that the minimal required explanation for the behaviors we observe in the mirror test does not require invocation of self-awareness, self-consciousness, or theory of mind,” Jordan said.

The test has been passed by great apes including chimpanzees, bonobos, gorillas and 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.

“I consider that there is a spectrum of animal consciousness, with some animals, likely primates, showing abilities closer to human consciousness,” Jordan said. “My point with this paper is not that fish are as smart as chimpanzees, but that the way we ask that very question across taxa (animal groups) needs to be re-evaluated.”

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.

Emory University primatologist Frans de Waal, who has studied mirror self-recognition in mammals, called the findings “interesting and provocative.”

“The hope is that this study will throw open the discussion about self-awareness in animals. Instead of the black-and-white distinction we have had thus far, that some animals have it and most of them don’t, we need to develop a more gradualist perspective,” de Waal said.

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