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The lateral line is a faintly visible line of dots that runs down the sides of most fish, from the area around the gills to near the tail. It is often clearly visible, but looks like nothing more than a cosmetic feature that serves no purpose. It is, in fact, a sensory organ with microscopic structures underneath the surface of the fish scales, and serves as a primary method for detecting changes in water pressure.
Sensory organs on aquatic species can often be difficult to identify, but, since the lateral line is present in so many species of fish, as well as that of juvenile amphibians, it has been studied rather extensively. Underneath each dot on the line exists a gel cap, and the structure itself is called a neuromast. A neuromast is a nerve-centered sensory cell. The gel detects changes in the surrounding water and passes this sense onto small hairs which activate the nerve tissue in a manner similar to how the human inner ear functions.
The fish nervous system can be extremely precise when considering the lateral line, to the point that a fish can sense minute changes in water pressure as it changes direction in the water or as other fish or objects approach. This allows a fish to swim in the dark as well as in schools of other fish and maintain an established distance from these objects. Sensory organs such as the lateral line also give fish the ability to sense movement near the back of their bodies while their eyes are fixed in a generally forward and sidelong direction. This aids in avoiding predators as well as in finding food sources such as insects on the water surface.
Sharks also have the lateral line, and they use it to detect both vibrations and odor plumes in the water in a process known as eddy chemotaxis. This serves predator species well, as most prey animals like seals leave both turbulence and body oils in their wake as they swim through the ocean. The lateral line is relied on so much by sharks and fish that is is considered almost as important as the sense of sight. Behaviors common to sharks often involve bumping or rubbing up against unknown animals in the sea such as human divers. This is theorized to be a method for the shark of tasting the object by bringing it into contact with the lateral line directly to see if it is safe to eat.