The eels respond by swimming off with the groupers. They can slink through crevices and flush out hidden prey, while the groupers are lethal in open water. When they hunt together, little fish have nowhere to flee.

Working with Bshary, Alexander Vail from the University of Cambridge found that the groupers also use a different signal—a  headstand—to tell the morays where hidden fish can be found. It’s the equivalent of a human pointing a finger—a gesture that says, “The prey’s in here.” These sorts of referential gestures had only been seen in intelligent animals like humans, apes, ravens, dolphins, and dogs. Their use was often taken as a sign of intelligence. The fact that fish—a group hardly known for