The lateral line is a sensitive organ in fish that detects movement and vibrations in the surrounding water. It is used for both orientation and hunting. Externally, it looks like a thin line on either side of the body, extending from the gill slits to the base of the tail. In some species, some of the lateral line receptors have been converted into electroreceptors and can pick up electrical vibrations in the environment. Some crustaceans and cephalopods have similar organs.
The lateral line receptors are called neuromasts, each consisting of a group of hair cells. The hair cells are housed in a convex, jelly-like cupule about 0.1-0.2 mm in size. The hair cells and cupules of the neuromasts are usually located at the bottom of the furrows and fossae that make up the lateral line organs. The hair cells of the lateral line are similar to the hair cells of the inner ear, suggesting that these organs have a common origin.
The lateral line organs of bony fishes and platypods are usually in the form of canals in which the neuromasts are not directly connected to the external environment, but through canal pores. In the lateral lines of some fishes, and in various parts of the fishes' body surface, there may also be free-floating neuromasts that are not connected to the canals.
The development of the lateral line organs depends on the animal's lifestyle. For example, in actively swimming fish, the neuromasts are usually located in the canals. The lateral line itself is as far away from the pectoral fins as possible, which probably reduces the distortion that occurs when the fish moves.
Lateral line organs help fish navigate, sense the direction and speed of currents, and detect prey or predators. The blind cave fish Astyanax mexicanus, for example, has rows of neuromasts on its head that are used to locate food. Some carps are able to sense ripples when an insect moves on the surface of the water. Experiments with coalfish have shown that the lateral line plays a key role in the social movement of fish.