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October 26, 2001

Distant Touch

What purpose does a fish's lateral line serve? and how does it work?

Q: When I tag fish, I know that I'm supposed to avoid the lateral line. But what does the lateral line do? I've heard that fish use it to hear, or to sense temperature or pressure. Which is it and how does
it work? - Matthew Stickney, Jacksonville, Florida

A: Fish use their lateral lines to feel low-frequency vibrations or pulsing movements in the water. The sense is somewhere between our senses of hearing and touch. Some researchers have called it the sense of "distant touch."
The lateral line is composed of many tiny organs called neuromasts, set in a line of pits along each side of the fish. From each neuromast, a capsule containing sensory hairs sticks out of the fish's skin. When a pulse of moving water - created, for example, when another fish swims by - passes over a fish's lateral line, each capsule bends slightly, triggering the hairs inside to send a nerve signal to the fish's brain.
The lateral line helps fish avoid bumping into things they can't see because it detects pulses of water bouncing off solid objects. It is this sense that allows fish to school, swim together and change directions simultaneously. It also gives fish the ability to sense a predator approaching, and predators the ability to detect the abnormal swimming patterns of an injured fish.
Fish can use this sense because they live in water, which is much denser than air, and low-frequency vibrations can trigger a physical response in the lateral-line organs. Fish have a separate sense called the labyrinthine system (comparable to our inner ears) for detecting the high-frequency vibrations that we call sound.