These tiny reef fish may be far more self-aware than anyone imagined.
Cleaner fish have been found to exhibit mammal-like cognitive abilities in the presence of their own reflection. Credit: Osaka Metropolitan University
Scientists at Osaka Metropolitan University in Japan have identified a newly documented behavior in cleaner wrasse (Labroides dimidiatus). When the small reef fish were given access to a mirror, they did more than simply recognize their reflections. They also began experimenting with the mirror using bits of food. The findings suggest these highly social fish are capable of a sophisticated cognitive process known as ‘contingency testing,’ a level of intelligence more commonly associated with marine mammals such as dolphins.
The research team was led by Specially Appointed Researcher Shumpei Sogawa and Specially Appointed Professor Masanori Kohda at the Graduate School of Science. In earlier work, the team demonstrated that cleaner wrasse could recognize themselves in photographs. This new behavior emerged during mirror test experiments, a widely used method for studying animal cognition. Previous studies had already shown that cleaner wrasse can identify themselves in a mirror.
Mirror Test With Parasite Markings
To begin the experiment, researchers placed marks on the fish that resembled parasites. Even individuals that had never encountered a mirror before quickly learned to use their reflection to locate and try to remove the artificial ‘parasite.’
The speed of their response surprised the scientists. In some cases, the fish attempted to rub off the mark within the first hour of seeing a mirror. On average, scraping behavior appeared after just 82 minutes. In earlier studies, similar responses typically took 4 to 6 days to occur.
"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."
Fish Use Shrimp to Test the Mirror
An even more unexpected behavior appeared after the fish had several days of exposure to the mirror. Some individuals picked up a small piece of shrimp from the tank floor, carried it upward, and intentionally released it near the mirror. As the shrimp drifted downward, the fish tracked its movement in the reflection and repeatedly touched the glass while watching the mirrored image fall.
The researchers describe this behavior as ‘contingency testing.’ Rather than testing the mirror with their own bodies, the fish used an outside object to observe how it behaved in reflected space. By dropping the shrimp and comparing its real movement with the reflected movement, the wrasse were effectively investigating how the mirror worked. Similar actions have been documented in manta rays and dolphins, which release bubbles and observe them rising in a mirror.
Expanding the Debate on Animal Self Awareness
This type of deliberate exploration strengthens the argument that the cleaner wrasse’s mirror behaviors reflect flexible, self-related processing rather than confusion or simple conditioning.
"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."
Looking ahead, the team believes that studying self-awareness across a wide range of 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 study was published in Scientific Reports.