A fish keeps its whole school in step by watching just one neighbour at a time

A school of fish turns as one, with no leader calling the moves — so how does each fish keep up? Researchers led by Guy Theraulaz at France's National Center for Scientific Research let a single rummy-nose tetra swim in a bowl while images of four virtual fish were projected onto the walls around it, each programmed with different rules for following the group. Across eight one-hour trials for each setup, the real fish behaved as if it were following just one neighbour at a time — whichever one mattered most in that moment — and that partner kept changing as the group moved. Its sharp turns matched computer simulations only when the simulated fish used the same one-neighbour rule, the team reports in PRX Life. The study used eyesight alone, so how fish combine this with the water movements they feel remains an open question.