Fish always orientate their bodies upstream, a behavioural response called rheotaxis. But just how they know which way the current is flowing has been a matter of some debate. New research shows that a series of sensory organs distributed over the head and neck of fishes (the lateral line system) may be the long sought sensor that detects the direction of current flow. This finding is particularly intriguing as research in the 1960s apparently ruled out involvement of the lateral line system in rheotaxis.
