Planet Earth

Can Fish Recognize Themselves? These Cleaner Wrasses Passed the Mirror Test

Mirror tests are used to assess an animal’s response to its reflection. Explore how fish react and learn if they possess self-awareness.

By Avery HurtAug 18, 2023 10:00 AM
Bluestreak cleaner wrasse swimming
Bluestreak cleaner wrasse - Labroides dimidiatus(Credit:Vojce/Shutterstock)


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In February, a team of researchers published a paper showing that the cleaner wrasse, a small fish known for cleaning parasites from bigger fish, had passed the mirror test.

The mirror test — more formally known as the mirror self-recognition test — has long been considered a gauge of whether or not an animal has self-awareness.

Mirror Testing Fish

It usually works like this: Researchers test the animal by exposing it to a mirror long enough to get used to it. Then, while the animal is anesthetized, researchers place a mark on the animal's body in some place it can't see without looking in the mirror — typically the forehead for primates and most other mammals.

When the animal wakes up and looks in the mirror, if it touches or tries to remove the mark, seemingly knowing that the mark is on its own body, it is said to have "passed" the mirror test. To give the mirror test to the cleaner wrasse, researchers placed a mark — a bit of brown dye that looked similar to the parasites they nibble off other fish — on the throats of the fish.

When the fish saw themselves in the mirror, they rushed to the bottom of the tank, where they rubbed against rocks in what appeared to be an attempt to scrape off the dye. But wait; there's more.

Read More: Do Fish Feel Pain?

(Credit:Masanori Kohda, Osaka Metropolitan University) Cleaner fish photograph models used in experiment 1. (Cleaner fish photograph models used in experiment 1. (A) Faces of focal fish, and (B) an example of the photographic models used in the experiment. SS, self; UU, unknown fish; self-face/unknown body, SU; and unknown face/self-body, US.

Photo Recognition Experiment

After the cleaner wrasse passed the mirror test, the researchers showed each fish four photographs: a photo of itself; a photo of an unfamiliar cleaner wrasse; a photo of its own face superimposed on an unfamiliar fish's body; and a photo of an unfamiliar cleaner wrasse's face on its body.

This might be enough to send a human into an existential crisis. The cleaner fish, however, didn't miss a beat. Though cleaner wrasse typically attack other cleaner fish that stray into their territory, they didn't attack photos of their own faces. They did, however, attack the photos that showed the faces of unfamiliar cleaner fish.

And get this: The researchers took a group of fish that had become familiar with the mirror but had not been marked. They showed each fish a photo of its face with a mark on the throat, and the fish promptly tried to scrape the mark off. When the fish was shown a photo of another fish with a similar mark, it did not try to remove the mark from its own throat. 

Clearly, we've been underestimating fish.

In a press release announcing the research, the study's lead author, Masanori Kohda, said, "This study is the first to demonstrate that fish have an internal sense of self. Since the target animal is a fish, this finding suggests that nearly all social vertebrates also have this higher sense of self." 

Read MoreYes, Fish Can Communicate Acoustically

Insights from Animal Mirror Tests

The little cleaner wrasse seems to have joined a fairly selective group. Not many animals have passed the mirror test, much less the photograph test. Chimpanzees, orangutans, and bonobos have; some say dolphins and magpies have; maybe penguins have and maybe ants. Humans generally don't pull this off until they are around two years old, even older in some cultures.

However, not everyone is convinced that tests like these prove much of anything about an animal's inner life or about its sense of self. In a recent paper reviewing research on the mirror test, Canadian researcher Yanyu Lei points out that the mirror test is biased toward animals, like us, for whom vision is a critical sense. 

Dogs, for example, might be more likely to recognize their scent than their picture on Mom's Facebook page. In addition, Lei's analysis found that the animals who have passed the test are exclusively social animals. (Cleaner wrasse are very social fish.)

Read MoreZebrafish Could Be Key To Healing A Broken Heart

Are Fish Self Aware?

Perhaps more interestingly, Lei asks if failing the mirror test indicates a lack of self-awareness. "It is possible," Lei writes, "that some self-aware animals simply do not care about the mark but do possess awareness of their private mental thoughts and an understanding of self."

The tiny cleaner wrasse has definitely done something impressive, at least by human standards. However, the question of whether the mirror test is a reliable indicator of self-awareness remains unanswered.

Read More: Reefs Are So Damaged That Fish Have Begun to Use Each Other As Cover

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