Researchers at the University of the Ryukyus placed a plain-body octopus (Callistoctopus aspilosomatis) into a tank filled with polymer beads. Whenever the cephalopod sat on the beads, the team covered one of the limbs with a fake arm affixed to an opaque sheet and then gently stroked both arms at the same time. When the researchers pinched the fake arm with tweezers, the octopus reacted defensively by changing its body color, retracting its arm, or escaping. But in trials where only the fake arm was stroked, both arms were stroked out of sync, or the real arm was in a different position than the fake one, the mollusk showed little or no response.

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