For years, researchers led by Masanori Kohda, a biologist at Graduate School of Science in Osaka City University, have been on a quest to prove that the fish species Labroides dimidiatus, known as the cleaner wrasse, can recognise itself in a mirror, opening up the possibility that fish can be self-aware. 

The team first reported their findings about these wrasses in a 2019 study in PLOS Biology, which caused controversy in the field, and now they are back with a follow-up study in the same journal aimed at proving their point. 

Kohda and his colleagues came to the conclusion that wrasse can achieve mirror self-recognition by subjecting them to a famous experiment known as the mark test, in which an artificial mark is placed on the body of an animal while it is anaesthetised. If the animal inspects the mark on its body in a mirror, or tries to touch it or remove it, it suggests that it has identified itself in the reflection, rather than mistaking the mirror image for some other individual.

While some mammals and birds have passed the mark test, Kohda and his colleagues were the first to claim that a fish had achieved this cognitive milestone. The researchers selected the cleaner wrasse specifically because the fish eats parasites off the skin of other animals and so is already attuned to recognising strange marks in its environment. Kohda said in an email:- 

“The cleaner fish (Labroides dimidiatus) is the only fish that passes the mark test, because other fish have no motivation to touch the mark as the mark has no meaning for other fish…This is because this fish is a cleaner fish that will pay attention to small parasites on other fish bodies and try to pick up and remove them.”

Some of the main criticisms levelled at the 2019 study involved its small sample size of four individual fish and its conclusions that the fish were genuinely self-inspecting in the mirrors. De Waal also pointed out that the marking technique might have been physically irritating to the fish, which could muddy their reaction to their reflections by introducing a competing stimulus.

To counter those concerns in the new study, Kohda’s team increased the sample size to 18 individuals and experimented with different injection techniques to clarify whether the fish were scraping their throats due to irritation or self-recognition, or a combination of the two sensations. Instead of only injecting the brown pigment at a depth of one millimetre under the skin, they also tried placing it at a more invasive depth of three millimetres. 

In this version of the test, the mark was barely visible and the fish scraped their throats regardless of whether or not a mirror was present, suggesting that they were reacting to the potentially irritating sensation of the deeper injection. However, fish with the shallower one-millimetre mark only scraped their throats when a mirror was present, hinting that the visual cue of the brown spot provoked the behaviour rather than any sensations. 

In addition, fish injected with green and blue spots, which don’t resemble parasites, did not exhibit the scraping behaviour in front of the mirror. This finding further strengthens the evidence that the wrasse really do recognise themselves in the mirror, because they only took action to remove the spots that resembled parasites. Furthermore, Kohda and his team report that their experiment “shows the highest rate of passing with this large sample size,” with only one “failing fish” out of the 18 that took the test, according to the study. 

In an email, de Waal said that many of his concerns with the initial 2019 study were sufficiently addressed by the new research, which he called “quite convincing.” de Waal noted: -

“I believe this experiment takes care of some of the questions I had last time…I suggested then that *feeling* the mark might contribute to the self-recognition in the mirror as it involves two senses (sight and physical pain) instead of just one.”

“This new series of experiments takes care of this issue, showing that the reaction is color-dependent (which you wouldn't expect if feeling the mark helps) and that a more painful (deeper) mark triggers a response w/o mirror, whereas the usual mark doesn't…With these new experiments the evidence for [mirror self-recognition] is about as strong as it can get, and moreover shows that the marks need to be ecologically relevant in that they need to be of the color that cleaner fish generally respond to.”

Indeed, both Kohda and de Waal emphasized that these tests challenge the notion that humans are the only species capable of higher cognitive abilities such as self-consciousness. Kohda said: -

“We do not know how many animals are intelligent exactly, and regard humans as the best intelligent organism in the world, that God made..But this is a large misunderstanding.”

“We do not properly understand animals!” he added.

To that end, Kohda and his colleagues plan to follow up on the study by trying to establish how the cleaner wrasse recognize themselves in the mirror, which he said is a “much more interesting question.” The researchers suspect that the fish are able to identify themselves by their faces, similar to humans, though it will take more experimentation to verify that hypothesis.

In the meantime, the new study reveals that these cleaner fish may have evolved a sophisticated sense of themselves that is potentially on par with animals that are much more familiar to humans, such as chimpanzees and orangutans.

“There will still be scientists who don't want to take this seriously, but clearly, if a primate had shown all of these responses no one would be skeptical,” de Waal said. “These fish are remarkable!”