The Lateral Line

The lateral line is often thought to be a mysterious, almost magical feature of fish that gives them some sort of “sixth sense”.  The reality isn’t quite so fortean, but is nonetheless interesting.

The lateral line runs down each side of most fish (and amphibian larvae), from the gills to the tail.  It is usually visible as a faint line of dots about halfway down the sides of the fish.  The exact appearance of the lateral line varies from species to species, and these differences are sometimes used to help distinguish closely related fish species.

On a smaller scale, each dot on the line is called a neuromast.  A neuromast consists of a bunch of hairs surrounded by a gel cap.  This whole structure sits in a little hole in the side of the fish, and is connected to a nerve.

The general purpose of the lateral line is to detect changes in water pressure.  The gel cap transfers the pressure changes to the hairs, and on to the nerve, working much like the hairs of the human inner ear.

For example, when a fish is swimming normally with clear water ahead, the pressure is consistent and as expected, a bit like the wave that forms in front of a boat, only in 3 dimensions.  So when the fish is heading for the glass of an aquarium, the pressure of the water flowing over the head and body will change as the shape of the wave changes.  This enables the fish to avoid such invisible objects.

Changes in water pressure will also be caused by other fish swimming nearby, and the lateral line has also been shown to play a part in schooling behaviour in some fish species.

Other species can detect and accurately locate insects on the surface of the water, without having to rely on sight.  This is a good thing in an environment where light does not behave as predictably as in air.  It also means the fish does not need specially adapted eyes that point upwards, and can use them as normal i.e. seeing morsels in close view in front of the fish, and looking out for danger to the sides.

Low frequency sounds are also detected with the lateral line, simply because they cause changes in the water pressure.

Finally, in sharks the lateral line is used to detect changes in electromagnetic fields in water.  A very useful tool for hunting prey.

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