How Fish Produce and Detect Sound
It is ironic that one of the early heroes of coral reef research -- Jacques Cousteau -- chose to call his most famous book, The Silent World. In fact, it is far from silent; these waters are a constant lively hubbub of noise. Recent research has shown that fish communicate extensively through sound and are capable of both producing and receiving sounds. Water is an excellent medium for conducting sound; it travels much faster (at about one mile [1,500 m] per second) and much farther in water than in air, so fish can exploit this to get their message across.
How do fish produce sounds?
Fish lack a voice box and must find other means of making themselves heard. They use two main methods: stridulation," where they rub their teeth, spines, or other skeletal parts together and "drumming," where muscles around the swim bladder contract rapidly and, as the name suggests, use the swim bladder just like a drum.
The fishes' "songs" can range from the quite simple "click" like a person cracking their knuckles, to buzzing and growling. Butterflyfish use this whole spectrum of sound, during their pair bonding, including high-frequency clicks, low-frequency pulses of sound and even long and very specific series of pulses. The next step is to find out what it is that they are saying to one another!
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 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 this vibration is picked up by sensory cells in the inner ear and transmitted to the brain. In some species, the swim bladder amplifies underwater sounds by picking up the pressure waves of the sound. If you have ever stood near a loudspeaker and been able to "feel" the sound, especially the low frequency bass, this is essentially the same thing. Picking up sounds is especially important to the butterflyfish, which has some remarkable adaptations to help it listen effectively. One is a unique link between the lateral line and the swim bladder; another is a highly specialized swim bladder that features both amplifying extensions and links to the skull.
What are they telling each other?
Just as in humans, some elements of communication are intentional, while in others eavesdroppers listen in. Damselfish use different calls extensively to deter others from invading their territories. Recent research has shown that the fish are extremely good at telling the difference between individual fish, based on their calls.
For instance, bicolor damsels can distinguish between strangers and the fish that hold neighboring territories based solely on the calls that each makes -- in other words, they are capable of voice recognition.The calls that males make are important in courtship as well; female damsels choose between the males at least partly
on the basis of the series of different chirps and grunts they make. Fish also listen out for what their competitors are up to; if they hear other fish feeding on a different part of the reef, they home in on the sound in an attempt to join the feast. Sound is also extremely important in attracting larval fishes to reefs. Each reef has its own specific sound, produced not only by the fish that live there, but also by the currents that swirl around it and the waves that break over it, and the youngsters home in on these sounds before settling.