30 December 1997

If you thought that oceans, rivers, lakes, and streams were worlds of silence, think again! The waters of the world can be noisy places, resonating with a variety of sounds. Although fish, crustaceans, or other invertebrates do not have vocal cords to enable them to produce sounds, they use a number of other methods to do so.   Fish use a repertoire of sounds, including grunts, moans, croaks, and whistles, to convey messages to each other.

The toadfish, one of the most vocal fish, can make noises almost as loud as an underground train! In fact, their sounds are so powerful that, during wartime, they set off acoustic mines meant to be triggered by enemy ships.

The loudest of the sonic, or sound-producing, fish "speak" by vibrating their swim bladders, an air-filled sac in their abdomen that helps them maintain their balance at varying depths. Special drumming muscles vibrate the swim bladder to make loud, low-pitched grunts. The croaker and drum fish, among the best known of the sound producing fish, earned their names by using their swim bladders to emit noises of varying pitch -- from deep, drum-like thumps to high-pitched sounds.

Some fish stridulate, rubbing together the hard parts of their body such as fins, bones, and spines, to produce grating or creaking sounds. Fish of the grunt family get their name from the grunt-like sounds they produce when grinding their upper and lower pharyngeal teeth. Their swim bladders act as sounding boxes, resonating and amplifying these sounds. Even the tiny sea horse makes noises -- by rubbing together certain bony plates on its head, it makes a sound like the snapping of fingers.

In addition to the variety of sounds that fish produce, the sounds themselves may vary in length and pitch and may be emitted at different intervals. All these variables give fish several messages it can send. Evidence suggests that fish, like land animals, use specific sounds to convey different messages -- to indicate aggressive intent, to signal recognition, to attract a mate, or to assist in navigation.

Besides a "vocabulary" consisting of grunts, croaks, clicks, and snaps, fish also use other means of communication such as chemical odors, or pheromones, and electrical signals. Some species of fish recognize one another by odor and associate these odors with position in the local social hierarchy. Others use pheromones to attract mates or to signal danger. Electrical signals, small charges emitted when a muscle contracts, are also used to convey a message of aggression, courtship, or recognition to other fish.