Physiology of Finfish and Shellfish (2+1)

3.3.3.2. Lateral line

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. Like the ear, the lateral line senses vibrations. It functions mainly in detecting low-frequency vibrations and directional water flow, and in distance perception. This consists of a series of fluid-filled canals just below the skin of the head and along the sides of a bony fish's body where it is associated with the scales. The canals are open to the surrounding water through tiny pores. Lateral line canals contain sensory cells (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

The lateral line of fishes provides a distant touch sense. By means of mechanoreceptors similar to those in the auditory and equilibrium systems, water movements around the fish can be detected. The receptors are called neuromasts and each consists of individual hair cells with an attached cupula. (Fig).

Water movements bend the protruding cupula which stimulates the hair cell by bending the attached cilia (sense hairs). Most teleosts and elasmobranchs have also developed lateral line canals, in which the neuromasts lie between canal pores which open to the environment (Fig). The cupulae of the canal neuromasts are sensitive to movements of the watery, endolymph fluid through the canal. Like the auditory hair cells, the neuromasts continuously send neural impulses to the brain.

The lateral line system is developed and used in various ways by fishes exhibiting different modes of life. Parts of the lateral line system are often specialized for detecting prey. Surface oriented fishes such as killifish (Fundulus) sense surface ripples from struggling insects by sing the short canal neuromast system on the flattened dorsal surface of their heads. Blind cave fish have rows of free neuromasts on the head which stand out in ridges, to aid the cavefish in locating the food so precisely that it can be ingested without sight.

Neuromast function can also be useful to fish for stimuli other than localized water disturbances from distant objects. As the fish swim, they “push” some water in front of their heads, as a boat creates a bow wave. The amount of water pushed or displaced depends on the size of the fish and shape of the head. These forward displacements of water rebound from objects in front of the swimming fish. The rebounds or increases in resistance in front of the fish are apparently sensed by the neuromasts on the head, allowing avoidance of the obstacle.