These Tiny Fish Will Assess Themselves in a Mirror Before Taking on a Foe

Bluestreak cleaner wrasse are small, territorial fish that aggressively fend off intruders. But when they have access to a mirror, the fish size themselves up before deciding whether or not to fight.

About the size of a human finger, bluestreak cleaner wrasse are tiny, blue, black and white fish that live on Indo-Pacific coral reefs. Bluestreak cleaner wrasse set up “cleaning stations” on the reefs, then wait for other fish to show up so they can eat the parasites off their bodies, per the Georgia Aquarium. These diminutive swimmers are industrious, inspecting up to 2,000 fish each day. They also have good memories and can recognize more than 100 different “clients.”

Scientists already knew bluestreak cleaner wrasse (Labroides dimidiatus) were savvy creatures. In 2018, they became the first fish to pass what’s known as the mirror test, an experiment used to gauge self-awareness by assessing whether or not an animal recognizes its own reflection. Other creatures that have passed the mirror test include bottlenose dolphins, chimpanzees and Asian elephants.

Last year, researchers also showed that bluestreak cleaner wrasse could recognize themselves in photos after looking at their reflection in a mirror.

Bluestreak cleaner wrasse found to recognize self in photograph after passing mirror test

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Scientists wanted to explore the bluestreak cleaner wrasse’s self-awareness on an even deeper level, so they set up a series of new laboratory experiments. They shared their findings in a new paper published this week in the journal Scientific Reports.

In the first phase, researchers placed a bluestreak cleaner wrasse inside a clear fish tank. Then, they held photos against the glass showing bluestreak cleaner wrasses of varying sizes—some that were 10 percent larger than the fish in the tank, and some that were 10 percent smaller. No matter which photo the scientists showed, the bluestreak cleaner wrasse inside the tank tried to attack it.

Next, the team repeated the same experiment but added a mirror to the tank. The fish checked out their own reflection before deciding whether to fight—and they would only battle photos of smaller intruders, not larger ones.

This fish sizes it itself up in a mirror before it decides to fight

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To scientists, this suggests that bluestreak cleaner wrasse are capable of understanding their own body size, as well as how their body size stacks up against a rival.

“This was unexpected because we had an image that this fish always shows aggression against rivals, regardless of size,” says study co-author Taiga Kobayashi, a scientist at Osaka Metropolitan University in Japan, to New Scientist’s Corryn Wetzel.

There are no mirrors in the wild, so the findings also suggest that bluestreak cleaner wrasse adapted and learned to use the mirror as a self-preservation tool. This discovery can “help clarify the similarities between human and non-human animal self-awareness and provide important clues to elucidate how self-awareness has evolved," Kobayashi says in a statement.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.