The lateral line is an important determinant of detecting prey, current speed and direction, providing useful information to fishes that live in streams or where tidal flows dominate. The lateral line has been one of the most mysterious of all vertebrate sensory systems. The sensory organ is a hollow tube with pores in connection to the surrounding in the skin, which is extending mid-laterally along the length of the body and into the tail and it is found in all fish and some amphibia and responds to hydrodynamic stimuli such as water movement and vibration. In other words, the lateral line detects the presence of nearby animals and objects that cause or disrupt water flow. For example, a fish is swimming diagonally behind a shark and disrupts the water movement; this disruption reaches one of the many pores first and flows through the tube, allowing the shark to detect the location of the fish.

Drawing of the lateral line – uploaded by Joachim Mogdans

The receptors in the lateral line consist of the same sensory hair cells as found in the ear. However, the hair cells are organized into groups called neuromasts. Each neuromasts probably consists of up to 100 cells.

The lateral line is involved with schooling behaviour, where fish swim in a cohesive formation with many other fish. The lateral line tells the fish where the other fish are in the school, and helps the fish maintain a constant distance from its nearest neighbor. The lateral line also is used to detect the presence of nearby moving objects, such as food, and to avoid obstacles, especially in fishes that cannot rely on light, such as the cave fishes that live underground. The fact that many species of fish form schools suggests that the behavior offers a clear evolutionary advantage. Yet, we are only just beginning to understand how fish form and maintain schools, and the roles of the lateral line system and vision in this behaviour.

In the past, researchers studied the role of the lateral line system in a behaviour by removing it surgically and observing changes in the behaviour. Heavy metal ions, such as cobalt chloride, and aminoglycoside antibiotics, such as gentamycin, are toxic to hair cells in the neuromasts. After these chemical treatments, the hair cells regrow over a few days to a few weeks. The posterior lateral line (on the side of the body) nerve can also be surgically severed to disable the trunk lateral line, e.g. bite wound from a predator fish, which will take 1-2 weeks to heal and 4-8 weeks for the fish to again school normally. Disabling the lateral line system, either chemically or surgically, does not prevent fish from schooling, but it does change the position they maintain within a school. The fish can still school, but they stayed closer to their neighbors. Some even caused collisions among individuals, with strong enough forces to stun the fish.

So we can conclude that fish use the lateral line system to maintain their positions within the school, but that disabling it does not prevent the schooling behaviour.