A bluestreak cleaner wrasse attacks its reflection in a mirror, then turns upside down, apparently to determine if the reflection is its own. After acquiring self-awareness, it attempts to remove a colored mark researchers applied to its body. (Provided by Osaka City University)

Fish can recognize themselves in a mirror, proving they are self-aware, Osaka City University researchers found.

The cognitive abilities of chimpanzees, dolphins and other species have been widely reported, but this is the first evidence of the trait in fish.

The findings of the team, headed by Masanori Koda, a professor of behavioral ecology and animal ecology, were posted in Plos Biology, a U.S. biological science journal, on Feb. 8.

The researchers conducted a self-recognition test on bluestreak cleaner wrasses, a small tropical reef fish that inhabit coastal areas in southern Japan, using specimens raised in an aquarium.

At first, immediately after a wrasse was shown a mirror, it moved to attack the “invader,” providing proof that it took its own reflection to be another fish.

Later, however, it apparently tested that impression, doing things such as turning upside down. On the experiment's fifth day and after, it reacted in this way less and less.

Koda's team then applied a brown mark on the bodies of four fish that had gone through the first experiment.

After looking at the mirror, three wrasses scraped their bodies along the bottom of the fish tank.

The researchers concluded from this reaction that the fish knew the image in the mirror was their own, because they tried to dislodge the mark, which resembled a parasitic insect.