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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 finding, publishing on February 7 in the open-access journal PLOS Biology, suggests that fish might possess far higher cognitive powers than previously thought, and ignites a high-stakes debate over how we assess the intelligence of animals that are so unlike ourselves. ...

The ability to perceive and recognise 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 behaviour of "cleaning" external parasites from client fish -- by placing a coloured 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.