Anatomy of finfish and shellfish

Unit 11 - Sensory System in fishes

11.6. Lateral line system
The lateral line system, found in many fishes and in some aquatic amphibians, is sensitive to differences in water pressure. These differences may be due to changes in depth or to the currentlike waves caused by approaching objects. The basic sensory unit of the lateral line system is the neuromast, which is a bundle of sensory and supporting cells whose projecting hairs are encased in a gelatinous cap. The nueromasts continuously send out trains of nerve impulses. When pressure waves cause the gelatinous caps of the neuromasts to move, bending the enclosed hairs, the frequency of the nerve impulses is either increased or decreased, depending on the direction of bending.
Neuromasts may occur singly, in small groups called pit organs, or in rows within grooves or canals, when they are referred to as the lateral line system. The lateral line system runs along the sides of the body onto the head, where it divides into three branches, two to the snout and one to the lower jaw.
A swimming fish sets up a pressure wave in the water that is detectable by the lateral line systems of other fishes. It also sets up a bow wave in front of itself, the pressure of which is higher than that of the wave flow along its sides. These near-field differences are registered by its own lateral line system. As the fish approaches an object, such as a rock or the glass wall of an aquarium, the pressure waves around its body are distorted, and these changes are quickly detected by the lateral line system, enabling the fish to swerve or to take other suitable action. Because sound waves are waves of pressure, the lateral line system is also able to detect very low-frequency sounds of 100 Hz or less.
An interesting adaptation of the pressure-sensitive systems is seen in the modified groups of neuromasts called the ampullae of Lorenzini, which are found in sharks and certain bony fishes. The ampullae of Lorenzini act as electroreceptors and are able to detect electrical charges, or fields, in the water. Most animals, including humans, emit a DC field when in seawater. This is presumably caused by electrical potential differences between body fluids and seawater and between different parts of the body. An AC field is also set up by muscular activity (contractions). A wound, even a scratch, can markedly alter these electrical fields. The cat shark, Scyliorhinus, is known to catch prey by using its ampullae of Lorenzini to detect the electrical field generated by flatfish (plaice) buried beneath the sand.
It provides “distinct touch sense” of objects. The lateral line system is an integral part of the acoustico laterials system which includes ear. It involves sensory lines distributed on the head and body, pit organs and Ampullae of Lorenzine.

Ampullae of Lorienzine are present in the head region of sharks and rays. They are small sac like structure, which open outside by small pores. The Ampullae of Lorienzine are sensitive to mechanical and weak electrical stimuli as well as change in salinity. The receptors of lateralline system are called “neuromast” Each consists of individual hair cells with an attached “cupula”.

Fig: Diagrammatic comparison of various sensory epithelia of the acoustico - lateralis system

Most of the lateral line organs of the head region are innervated by sensory fibres of the lateralis anterior root of cranial nerves VII (facialis). The rest of the organs of systems are innervated by the laterials posterior root of the vagus (X).