Researchers at Osaka Metropolitan University in Japan have discovered a previously undiscovered behavior in cleaner wrasse (Labroides dimidiatus). When presented with a mirror, the tiny fish not only recognized themselves, but experimented with the mirror themselves, interacting with it using a scrap of food. The results suggest that these social fish can perform a higher level of intelligence known as ‘contingency testing,’ typically seen in intelligent marine mammals like dolphins.

The group led by Specially Appointed Researcher Shumpei Sogawa and Specially Appointed Professor Masanori Kohda at the Graduate School of Science—who previously showed that wrasse can recognize themselves in photographs—observed the behavior during mirror test experiments. These tests are a common way to test cognition in animals and previous studies have shown that cleaner wrasse recognize themselves in the mirror.

First, the research group applied markings that resembled parasites to cleaner wrasse. They found that even wrasse that had never seen their reflection in a mirror before quickly learned to use the mirror to clean off the ‘parasite.’

Even here there were surprising findings as the cleaner fish responded remarkably quickly — in some cases within the first hour of seeing the mirror. The group found that the behavior of attempting to rub off the mark was observed within an average of 82 minutes, much faster than the 4 to 6 days in previous experiments.

“In earlier cleaner wrasse mirror studies, the procedure was typically the fish see a mirror for several days, they habituate to it and stop reacting socially, and a mark is added,” Dr. Sogawa explained. “In this study, the order was reversed, the fish were marked first, then the mirror was introduced for the first time. The fish were likely aware of something unusual on their body, but they couldn’t see it. When the mirror appeared, it immediately provided visual information that matched an existing bodily expectation, hence scraping occurred much faster.”

More unexpected, however, was the next behavior. After several days of mirror exposure, some of the fish were seen picking up a small piece of shrimp from the tank floor, carrying it upward, and deliberately dropping it near the mirror. As the shrimp sank, the fish closely followed its descent along the mirror surface, repeatedly touching the glass with their mouths while watching the shrimp’s descent in the reflection.

The researchers interpreted this as a form of ‘contingency testing,’ an advanced behavior where instead of testing the mirror with their own bodies, an animal tests how external objects behave in mirror space. By dropping shrimp and watching how its movement corresponded with its reflection, the fish were exploring the mirror itself. Similar behaviors have been reported in other marine animals such as manta rays and dolphins that release bubbles and watch them rise in mirrors.

Importantly, this kind of exploratory testing strengthens the case that the mirror-directed actions seen in cleaner wrasse are grounded in flexible, self-referential processing rather than simple confusion or learned associations.

“These findings in cleaner wrasse suggest that self-awareness may not have evolved only in the limited number of species that passed the mirror test but may be more widely prevalent across a broader range of taxonomic groups, including fish,” Dr. Sogawa said. “It is highly likely that mirror self-recognition will be observed in many species where mirror tool use has been reported.”

Going forward, the researchers believe that research on self-awareness across all animals, including invertebrates, will become increasingly important. “The findings from this research will likely influence not only academic issues, such as revising evolutionary theory and constructing concepts of self, but also directly impact matters relevant to our lives, including animal welfare, medical research, and even AI studies,” Professor Kohda added.

The findings were published in Scientific Reports.

Competing interests
The authors declare no competing interests.

###
About OMU
Established in Osaka as one of the largest public universities in Japan, Osaka Metropolitan University is committed to shaping the future of society through “Convergence of Knowledge” and the promotion of world-class research. For more research news, visit https://www.omu.ac.jp/en/ and follow us on social media: X, Facebook, Instagram, LinkedIn.