Most marine animals rely on sound for survival and depend on unique adaptations that enable them to communicate, protect themselves, locate food, and navigate underwater. Animals change the rate of sound production and the structure of the sounds to send different messages.

Just like humans, marine animals use sound to communicate with one another. The majority of studies on communication have been done with marine mammals. Marine mammals use sound to communicate over long and short distances. Communication over long distances is usually associated with reproduction, territoriality, and maintenance of group structure. Communication over short distances is used in social interactions involving aggression, individual identification, and to maintain mother-offspring contact. Most marine mammals use sound to regain contact when members of a group are separated.

Dolphins travel in large groups, therefore sound is important for communication to maintain group structure. Photo courtesy of NOAA.

Similar to sonar systems on navy ships, some whales use sound to detect, localize, and characterize objects, including obstacles and other whales. By emitting clicks, or short pulses of sound, these marine mammals can listen for echoes and detect objects underwater. Some whales and dolphins use echolocation to locate food. They send out pulsed sounds of high intensity and frequency that are reflected back when they strike a target. This echo helps the dolphin or whale identify the size and shape of an object, the direction in which the object is moving and enables them to estimate how far away the object is. Echolocation is a very sophisticated way of locating prey and can even be used to find prey that is hidden in the sand. Echolocation also helps these whales and dolphins to actively navigate through the water.

Toothed whales, such as these orcas, use sound to locate prey and to navigate through the water. Photo courtesy of the National Marine Mammal Laboratory (NMML).

Whales and dolphins are celebrated for their sounds, but many species of fish and marine invertebrates also use sound. Fish produce various sounds, including grunts, croaks, clicks, and snaps, that are used to attract mates as well as ward off predators. For the toadfish, sound production is very important in courtship rituals. Sound is produced by the male toadfish to attract the female for mating and is especially important in the murky waters that toadfish inhabit where sight is limited. Fishes also produce sound when feeding. When a fish eats hard food, such as coral, it will produce a sound. Fishes sometimes gnash their teeth without the presence of food, which may be a way to scare away predators.

Garibaldi produce sounds using their pharyngeal teeth while grinding on coral. Photo courtesy of Robert Fenner.

The life history of most coral reef fishes includes a pelagic larval stage that undergoes metamorphosis to the juvenile stage. Late stage larvae and transforming juveniles need to seek out suitable reef habitats. There is evidence that underwater reef sounds can be detected by the coral reef fish (and invertebrate) larvae guiding them to coastal areas and allowing them to identify suitable settlement habitats ^{[1] [2] [3] [4] [5] [6]}. Different coastal habitat types have been found to produce different ambient sounds over short distances (km’s) , and larval and adult fish may be able to discriminate between these sounds. One study suggests that settlement-stage larvae of several coral reef fish species prefer the higher-frequency component (570-2000Hz) over the low-frequency component (<570Hz) of audible reef noise ^[7]. Adults and juveniles of some reef fish species may also use underwater sounds coming from different habitat types to guide their nocturnal movements ^{[8] [2]}. Some fish species forage in pelagic habitats at night, but return to coral reefs for shelter and protection during the day. Movements between the two habitats occur during dawn and dusk to reduce predation risks. Unique, acoustic differences between habitats may therefore be an effective cue for organisms moving among marine habitats during periods of low light.

Some coral reef fish species that migrate between reef and pelagic habitats, like these squirrelfish, may use sound to navigate back to a reef after foraging in open waters at night. Image credit: NOAA.

Invertebrates may use sounds for courtship or to ward off predators, such as the loud sounds produced by spiny lobsters. Little research has been done on marine invertebrates that produce sounds, but for those that do, like the spiny lobster, sound is very important for protection from predators.