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The Lateral Line, in this case shown on a shark

In fish, the lateral line is a sense organ used to detect movement and vibration 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. Sometimes parts of the lateral organ are modified into electroreceptors, which are organs used to detect electrical impulses. It is possible that vertebrates such as sharks use the lateral organs to detect magnetic fields as well. Most amphibian larvae and some adult amphibians also have a lateral organ.

The Lateral line is related to the Ampullae of Lorenzini.

The receptors in the lateral line are neuromasts, each of which is composed of a group of hair cells. The hairs are surrounded by a protruding jelly-like cupula, typically 1/10 to 1/5 mm long. The hair cells and cupolas of the neuromasts are usually at the bottom of a visible pit or groove in the fish. The hair cells in the lateral line are similar to the hair cells inside the vertebrate inner ear, indicating that the lateral line and the inner ear share a common origin.

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, fish that are active swimming types tend to have more neuromasts in canals than they have on their surface, and the line will be farther away from the pectoral fins, which probably reduces the amount of "noise" that is generated by fin motion.

The lateral-line system helps the fish to avoid collisions, to orient itself in relation to water currents, and to locate prey. For instance, blind cavefish have rows of neuromasts on their heads, which appear to be used to precisely locate food without the use of sight; killifish are able to use their lateral line organ to sense the ripples made by insects struggling on the water's surface. Experiments with pollock have shown that the lateral line is also a key enabler for schooling behavior.

Some crustaceans and cephalopods have similar organs.

See also: fish hearing

some scales of the lateral line of the roach (Rutilus rutilus)

The small holes on the head of this pike contain neuromasts of the lateral line system. Also the eye lens and the nose of this predator are visible.

• 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)