A team of researchers led by Professor Masanori Kohda from the Graduate School of Science at Osaka Metropolitan University has made a groundbreaking discovery – fish have an internal sense of self, as demonstrated by their recognition of their own face in a picture. This marks the first time such a discovery has been made in animals. The findings have been published in the Proceedings of the National Academy of Sciences.

The researchers conducted experiments with Labroides dimidiatus, also known as cleaner fish, which are known for their ability to recognize themselves in mirrors and for attacking other cleaner fish who intrude on their territory. The fish were presented with four photographs – a photo of themselves, a photo of an unfamiliar cleaner, a photo of their own face on an unfamiliar cleaner’s body, and a photo of an unfamiliar cleaner’s face on their own body. The results showed that the fish did not attack photos with their own faces but did attack those with the faces of unfamiliar cleaners, indicating that the fish determined who was in the photo based on the face, similar to how humans do.

To rule out the possibility that the fish considered photographs of themselves as close companions, a mark-test was conducted using photographs. Fish were presented with a photo of themselves with a parasite-like mark on their throat, and six of the eight individuals that saw the photo rubbed their throats to clean it off. However, showing the same fish photos of themselves without parasite marks or of a familiar cleaner fish with parasite marks did not result in them rubbing their throats.

According to Professor Kohda,

“This study is the first to demonstrate that fish have an internal sense of self. Since the target animal is a fish, this finding suggests that nearly all social vertebrates also have this higher sense of self.”