Findings from new research suggest that fish might possess far higher cognitive powers than previously thought. More specifically, the results suggest that fish appear to recognize themselves in the mirror.

Caution should be used interpreting these results as (1) there will need to be numerous replications studies and (2) the results have further escalated debates within the scientific community about how intelligence is defined in animal cognition studies. Furthermore, these results will no doubt ignite new heated debates within the scientific, political and corporate communities over how we assess the intelligence of animals, how we define self awareness in nonhuman animals, and how things will change moving forward with laws and policies dictating how we treat and protect nonhuman animals. In the meantime, the study results offer one more piece of evidence that nonhuman animals may be far more intelligent than many in the scientific community have previously acknowledged.

Fish Appear to Recognize Themselves in the Mirror

The results of new scientific research have demonstrated that a species of fish, the cleaner wrasse (Labroides dimidiatus), responds to its reflection and attempts to remove marks on its body during the mirror test — a method held as the gold standard for determining if animals are self-aware. The findings suggest that fish might possess far higher cognitive powers than previously thought.

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“The behaviors we observe leave little doubt that this fish behaviorally fulfills all criteria of the mirror test as originally laid out. What is less clear is whether these behaviors should be considered as evidence that fish are self-aware — even though in the past these same behaviors have been interpreted as self-awareness in so many other animals.”  -Dr. Alex Jordan, senior study author

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Study overview

The ability to perceive and recognize a reflected mirror image as self (mirror self-recognition) is considered a hallmark of cognition across species. To test for this phenomenon in fish, the researchers applied the classic ‘mark’ test to the cleaner wrasse (Labroides dimidiatus) — a marine fish best known for its behavior of “cleaning” external parasites from client fish — by placing a colored mark on fish in a location that can only be seen in a mirror reflection. In order to gain a ‘pass’, the test requires that the animal must touch or investigate the mark, demonstrating that it perceives the reflected image as itself. This is clearly a challenge for animals such as fish that lack limbs and hands.

The researchers observed that fish attempted to remove the marks by scraping their bodies on hard surfaces after viewing themselves in the mirror. Fish never attempted to remove transparent marks in the presence of a mirror, or colored marks when no mirror was present — suggesting that marked fish were responding to the visual cue of seeing the mark on themselves in the mirror. Further, unmarked fish did not attempt to remove marks from themselves when interacting with a marked fish across a clear divider, nor did they attempt to remove marks placed on the mirror itself — suggesting that fish were not innately reacting to a mark resembling an ectoparasite anywhere in the environment, for instance due to hard-wired feeding responses.

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Journal Reference: Masanori Kohda, Takashi Hotta, Tomohiro Takeyama, Satoshi Awata, Hirokazu Tanaka, Jun-ya Asai, Alex L. Jordan. If a fish can pass the mark test, what are the implications for consciousness and self-awareness testing in animals? PLOS Biology, 2019; 17 (2): e3000021 DOI: 10.1371/journal.pbio.3000021