Hearing is the universal alerting sense in all vertebrates. Sound is so important because animals are able to hear events all around them, no matter where their attention is focused. Many species of blind amphibians, reptiles, fishes and mammals are known, but no naturally profoundly deaf vertebrate species have been discovered. Although hearing is important to all animals, the special qualities of the undersea world emphasize the use of sound.

Sound travels far greater distances than light under water. Light travels only a few hundred meters in the ocean before it is absorbed or scattered. Even where light is available, it is more difficult to see as far under water as in air, limiting vision in the marine environment It is similar to looking through fog on land. So, the best opportunity for long-range vision underwater -- especially in murky water -- is to swim beneath objects and see their silhouettes.

In addition to vision, marine animals use other mechanisms, such as sound, to gather information and communicate. Photo of harbor seal courtesy of Sean A. Hayes, Salmon Ecology Group, NMFS

The undersea world presents very different conditions for hearing as well as seeing. Sound travels much farther underwater than in air. The sounds produced by many marine mammals can project for miles. Strong echoes are always present underwater, because sound travels without much loss and there are many underwater surfaces that reflect sound. So, it can be tricky to communicate using sound underwater, because a listener may have to sort through many different sounds and confusing echoes to hear the message. Marine mammal sounds are probably structured so that they can be recognized in spite of all the echoes.