Cleaner fish with parasite tattoos will try to scrape them off if they catch sight of themselves in a mirror, indicating that these fish have mirror self-recognition (something Deb from Finding Nemo lacked). An exciting revelation (suggested by a new study) when you consider that being able to spot and identify your own reflection is considered evidence for self-awareness, a trait that’s so far only been conclusively shown in chimpanzees.

The new experiment, published in the journal PLOS Biology, builds on previous research into fish mirror self-recognition (MSR) which was later criticized in its conclusions. The earlier work found that injecting a visible brown mark into the cleaner fish Labroides dimidiatus triggered them to scrape their throats when they spotted it in their reflection.

This exciting find, however, was only present in three out of four fish studied so other researchers questioned the findings’ generalizability and reliability. To tackle these concerns, the team returned to their cleaner fish this time using 18 animals, of which 17 (94 percent) exhibited the same MSR-induced cleaning behavior seen in the earlier experiment.

The parasite tattoos were crucial to the research as they created the optical illusion that there was something on the cleaner fish’s necks for them to remove and eat (they love a good parasite, hence the name). This reasoning crafted the methodology from the first study, but some argued that the physical sensation of tattooing a fish could be triggering the throat scratching rather than mirror recognition.

To tackle this limitation the researchers added a new step to their methodology by testing if the fish would respond to a physical stimulus that wasn’t visible. They gave fish the same parasite tattoo only this time it went three times deeper into the skin so that it wasn’t visible.

Under these conditions, the fish would scratch their throats the same amount irrespective of whether or not there was a mirror present. So, to further solidify the ecological context of a parasite being crucial to the MSR response, the researchers gave fish visible tattoos that were green and blue and therefore didn’t resemble food.

This condition revealed that none of the fish tried to scrape off these marks, indicating that the visual (rather than physical) perception of parasite-like marks was a likely explanation for the fish’s MSR throat scratching.

The lifestyle of a cleaner fish goes some way towards explaining why they appear to tackle the MSR experiments effectively.

“Cleaner fish will pay attention to small parasites on other fish bodies and try to pick up and remove them,” lead author on the study Professor Masanori Kohda of Osaka City University told IFLScience.

“So, if we use a color mark resembling the parasite, the fish will show a good response to the mark. During the long 50-year history of mirror tests of animals, this study is the first test that uses the mark to which the subject animals pay attention. Hence, this fish shows the highest passing rate for mark-test, exceeding that of chimpanzees, dolphins, and elephants.”

A fascinating find, then, and while it’s gone some way to addressing the criticisms of the past Prof. Kohda says there is still a way to go.

“We still have much work to be done, especially quantitatively, to show that fish, as well as other animals, have the capacity for MSR,” Kohda concluded in a statement. “However, as a result of this study, we reiterate the conclusion of our previous study that either self-awareness in animals or the validity of the mirror test needs to be revised.”