Cleaner Wrasse Shows Remarkable Self-Awareness in Mirror Test

A small fish has displayed a striking degree of intelligence in mirror test trials. It not only appeared to recognise itself in a reflection, but also used food to investigate the way a mirror functions.

Mirror mark tests are widely used scientific experiments for examining self-recognition in animals, helping researchers assess how closely the intelligence of other species may resemble our own.

Seeing your reflection and correcting smudged eyeliner is an obvious example of self-recognition. You understand that the mark on your face does not belong there and can use the reflection as a guide to locate and remove it.

In principle, an animal responding to an unfamiliar mark on its reflected body is taken as evidence of self-recognition. Chimpanzees, elephants and dolphins are among the species to have passed this mark test, which many see as an indication of intelligence comparable with our own.

Cleaner wrasse and the mirror test

The cleaner wrasse (Labroides dimidiatus) is a marine fish about the size of a finger. It gets its name from feeding on parasites and dead tissue found on the bodies of larger fish. Researchers first reported that it had passed the mirror test in 2018.

The cleaner wrasse seems an obvious species to examine. Since its usual role involves removing material from other fish, a parasite-like mark on its own body might prompt it to use a mirror to ‘freshen up’ in the same way.

Although the 2018 research concluded that the fish had passed, Gordon Gallup, the evolutionary psychologist who created the mirror mark test, told New Scientist that he thought the fish were interpreting marks on their own bodies as parasites on other fish.

Researchers from Osaka Metropolitan University in Japan and the University of Neuchâtel in Switzerland have now adjusted the experiment to probe the wrasse’s self-awareness more closely.

"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," animal behavioralist Shumpei Sogawa from Osaka Metropolitan University in Japan explains.

"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."

Though it is not an ideal control, the altered approach partly addresses limitations in the original design by allowing the fish to detect the ‘parasite’ on their bodies before they see a reflection.

The researchers were taken aback by the speed of the fish’s response in these latest trials. On average, they attempted to scrape away the ‘parasite’ after roughly 82 minutes, which they say points to self-awareness before mirror exposure.

Fish use shrimp to investigate reflections

Once the fish had spent several days becoming familiar with the mirror, the scientists observed an unexpected action in some individuals. They collected a small piece of shrimp from the tank floor, took it to the mirror and released it.

The mirrored shrimp dropped in synchrony with the actual piece, while the fish watched the reflection intently and touched the mirror with their mouths.

Sogawa and his colleagues think the behaviour allows the fish to investigate a mirror’s properties, using an object distinct from their own bodies to gain a clearer understanding of reflected images.

Similar ‘contingency testing’ and the use of mirrors as tools have been documented in several species that did not pass the mark-based mirror test, including pigs, rhesus monkeys, manta rays and corvids.

"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," says biologist Masanori Kohda, who contributed to both the new study and the original cleaner wrasse self-recognition paper.

The team says the results indicate that self-awareness, previously considered unique to great apes, may instead be an ability that emerged in a far broader variety of animals, fish included.

"Our results suggest that self-awareness evolved at a minimum with the bony fishes (450 million years ago) and is likely widespread across vertebrates," they write.

The study was published in Scientific Reports.