. When changing water pressure or something similar to sound approaches and compresses these hairs, the message is sent to the sensory cells, along the nerve line to the spinal cord and to the brain, where it is picked up and tells the fish what it needs to know. This can be useful for a variety of things, such as letting the fish know how deep it is in the water, allowing the fish some idea as to where it is when it is too dark to see, whether or not other fish are around, and such