Pisces assess themselves in the mirror to decide if they can win a fight

Before deciding whether or not to fight another fish, cleaner wrasses examine their own reflection and assess themselves.

Blue-striped cleaner wrasse (Labroides dimidiatus) are remarkably bright. These finger-sized coral reef fish were the first fish to pass the mirror test, a common test to determine whether an animal recognizes that the reflection shows its own body and not another animal. Researchers have now found that these wrasses use their similarity to create a mental image of their body size that they can compare with others.

First, Taiga Kobayashi from Osaka Metropolitan University in Japan and his colleagues tested the fish's readiness to attack. They held a picture of another wrasse that was either 10 percent larger or smaller than the real fish to the glass wall of the aquarium. Regardless of the size of the model fish in the photo, the territorial wrasses started a fight.

They then repeated the test, but added a mirror and the fish looked at their reflection. But when the researchers held a photo of a larger or smaller cleaner wrasse up to the glass, the fish chose to fight only against smaller rivals.

“This was unexpected because we had the image that this fish always shows aggression towards rivals, regardless of size,” says Kobayashi.

The tank was divided in such a way that the wrasse could not see its own reflection and the photo of its rival at the same time. Scientists therefore believe that the fish compares the image with a mental approximation of its own body size.

Cleaner wrasse evolved in an environment without a mirror, so how did they develop this ability? Whether in the lab or in the wild, it is beneficial for a fish to know its size before starting a fight, says Kobayashi. In other words, the fish were smart enough to use the mirror to help them make decisions.