What is a lateral line in fish?

The lateral line is a sense organ that consists of a row of scales that most fish have along their sides, extending from their head to tail. Under these scales are a system of fluid-filled canals and specialized cells which transmit vibrations to the brain of the fish. The lateral line helps fish to detect movement and vibration in the surrounding water including predators and prey.

The lateral line or similar organs in fish such as blind cavefish (which has rows of neuromasts on their heads) are used precisely to locate food without the use of sight. Killifish can sense ripples caused by insects struggling on the surface of the water. Scientific experiments with Pollack Fish have shown that the lateral line is also used for schooling behavior.

Identification:

Lateral lines are usually visible as faint lines running lengthwise down each side, from the area around the gill covers to the area near the base of the tail. Sometimes parts of the lateral line are modified into electro-receptors (biological ability to receive and make use of electrical impulses), which are organs used to detect electrical impulses.

How the Lateral Line Works:

There are receptors in the line, called neuromasts, each consist of a group of hair cells, these cell hairs are surrounded by a protruding cupula (an organ that gives an animal a sense of balance). 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. These neuromasts are usually at the bottom of a pit or groove in the fish, which is large enough to be visible.

Skates, rays and sharks usually have lateral-line canals, in which the neuromasts are not directly exposed to the environment, but communicate with it via canal pores. The hair cells in the lateral line are similar to the hair cells inside the others vertebrates inner ear (such as the cupula in humans where hair cells within the cupula sense rotational acceleration), indicating that the lateral line and the inner ear share a common origin. Some active fish that are constantly swimming tend to have more neuromasts in canals than on the surface, and the lateral line will be further away from pectoral fins, to reduce the noise generated by fin motion.

The lateral line system, found in many fishes, is sensitive to differences in water pressure. These differences are thought to be due to changes in depth or to the current like waves caused by approaching objects. 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. 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 turn or to take other actions. Because sound waves are waves of pressure, the lateral line system is also able to detect very low-frequency sounds of about 100 Hz or less.

An adaptation of the pressure-sensitive systems is seen in the modified groups of neuromasts called the Ampullae of Lorenzini (special sensing organs, forming a network of jelly-filled canals), which are found in sharks, rays and a few bony fishes. The Ampullae of Lorenzini are able to detect electrical charges, or fields, in the water. Most animals, including humans, emit a DC (Direct Current) field when in seawater. This is thought to be 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 contractions. A wound, even a scratch, can alter these electrical fields.

Diseases of the Lateral Line:

Often the lateral line in fish (marine in particular) will get infections or degenerate from water conditions.

Vitamin deficiencies such as Vitamin C and possibly Vitamin B complex are one cause of degeneration. Proper feeding of foods high in these vitamins such as found in Spirulina Algae will help in this case.

Proper lighting is another (or lack thereof) is another parameter for a healthy lateral line in fish. Full spectrum lighting such as a combination 10,000 K or 20,000 K bulb and Actinic (UVA) bulb will help with this. The thought is that full spectrum lighting aids in the assimilation of certain vitamins, much as in humans and Vitamin D.

An un-grounded aquarium can cause electrical fields that both interfere and degenerate the lateral line in fish. You can test this by using a pocket multi-meter on AC volts with a probe in the wall ground and a probe in the water. To remedy this, you can run a wire from your tank or electrical aquarium equipment to a ground in the wall, or simply by running a wire into a nail or spike driven into the ground outside your home. If connecting directly to the tank, I recommend using a titanium probe in the aquarium.

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What is a Lateral Line in fish? The functions and diseases of the lateral line.