lateral line

This goldfish uses its lateral line to orient upright in the water (Pogrebnoj Alexandroff).

The lateral line is, literally, an observable line down both sides of a fish.  It is also a sensory organ system that helps fish detect motion in the water around them.  When water along the lateral line is displaced by movement or a vibration nearby, hair cells (similar in form and function to hair cells in a human ear) translate the displacement into an electrical impulse that is transmitted to the brain.  The lateral line helps a fish orient itself upright in the water and relative to other fish (e.g., schooling fish or predators and prey).