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.
Distribution - Anatomy - Circulation - Respiration - Air Breathing
Body Temperature - Water
Balance - Swimming - Gas Bladder
Lateral Line System - Evolution
- Reproduction