How Do Fish Detect Sound?
Until relatively recently, fish were considered to be silent creatures; they clearly do not have external ears and even though they do have some internal hearing apparatus, it lacks some basic structures. But experiments performed early in the 20th century showed that fish could be trained to emerge to feed on hearing a whistle. Water is an excellent medium for conducting sound; it travels much fester (over four times as fast at about 5,000 feet (1,500 meters) per second) and much farther in water than in air, so fish can exploit this to get their message across.
How do fish hear?
Sound travels as a series of waves or vibrations through the water. Because a fish's body is a similar density to the surrounding water, the waves pass through it. However, in the fish's inner ear there are a number of ear bones known as otoliths. Being bones, these are of a different density to much of the rest of the fish's body. The sound waves cause the otoliths to vibrate and it is this vibration that is picked up by sensory cells in the inner ear and transmitted to the brain. In some species, the swimbladder acts to amplify underwater sounds by picking up the pressure waves of the sound -- if you have ever stood near to a loudspeaker and been able to "feel" the sound, especially the low frequency bass, this is essentially the same thing.
Can fish make noise as well as listen to it?
Fish lack a voice box and so must find other means of making themselves heard. To do this, freshwater fish use two main methods: "stridulation" where they rub their teeth, spines or other skeletal parts together, and "drumming" where muscles around the swimbladder contract rapidly and, as the name suggests, use the swimbladder just like a drum. The fishes' "songs" can range from the quite simple "click" made by loaches (using a similar process to people cracking their knuckles) to the buzzing and growling common among cichlids. Croaking gouramis produce their eponymous sounds using specially adapted pectoral fins.
What are they telling each other?
Most fish use their sounds for two main purposes: aggression and courtship. Cichlids, such as the jewel (Hemichromis spp.) and the convict (Archocentrus nigrofasciatus), growl if an intruder enters their territory when they are defending young.
The click noise produced by some loaches can be heard well beyond the aquarium. This again is an aggressive signal, warning other loaches not to approach the territory.
Male croaking gouramis produce sound as they square up to a rival -- the depth of the sound that each fish is able to produce is related to its size and ultimately its strength. The stronger the fish, the deeper the aggressive call.
But not ail fish vocalizations are concerned with aggression; indeed, some evidence suggests that submissive fish produce "appeasement" sounds to try to dissuade an aggressor from launching an attack. Fish may also make sounds during courtship to impress a mate. Talking catfish, which make a surprising noise when taken out of water, may do so to frighten a predator. It is thought that the shock of the found may just occasionally save the fish's life by causing the predator to drop its prey.