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.