In fish, the lateral line is a sense organ used to detect movement in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail.

The receptors in the line, known as neuromasts, each consist of a group of hair cells, whose hairs are surrounded by a protruding jelly-like cupula, typically 1/10 to 1/5 mm long. The neuromasts are usually at the bottom of a pit or groove, which is large enough to be visible. Teleosts and elasmobranchs usually have lateral-line canals, in which the neuromasts are not directly exposed to the environment, but communicate with it via canal pores. Additional neuromasts may appear individually at various locations on the body surface.

The development of the lateral-line system depends on the fish's mode of life. For instance, active swimming types tend to have more neuromasts in canals than on the surface, and the line will be further away from pectoral fins, presumably to reduce the "noise" generated by fin motion.

Uses of the lateral-line system include collision avoidance, orientation relative to water currents, and predation. For instance, blind cavefish have rows of neuromasts on their heads, which could be used to precisely locate food without the use of sight, and killifish can sense ripples caused by insects struggling on the surface of the water. Experiments with pollack have shown that the lateral line is also an key enabler for schooling behavior.

See also: fish hearing

References

• A.N. Popper and C. Platt , "Inner ear and lateral line", in The Physiology of Fishes, 1st ed. (CRC Press, 1993)
• N.A.M. Schellart and R.J. Wubbels , "The auditory and mechanosensory lateral line system", in The Physiology of Fishes, 2nd ed. (CRC Press, 1998)