Under the sea - sotto voce no more

It's an underwater nightclub scene that could drown out the overtures of even the most virtuosic terrestrial Romeo. The relentless rat-a-tat-tat of the male cusk eel and the hours-long humming of the midshipman fish may sound like downtown street noise to the human ear. But to potential mates, these underwater troubadours are the piscine versions of Tony Bennett and Frank Sinatra, crooning love songs in the moonlight.

Although whale song has long been documented in ships' logs, it wasn't until World War II that scientists and the military first noticed the sounds of fish and the snapping of crustaceans. So far, more than 700 species of saltwater and freshwater fish throughout the world are known to vocalize, but scientists say the total number is likely much higher. Most of the time, the sounds come from male fish during mating.

But fish also are known to screech, croak, and growl when they defend their turf, become startled, or get caught or hurt by predators. Some female fish also can make sounds, but they tend to be softer. Male and female fish can also communicate through their mating songs to synchronize the release of eggs and sperm.

Much remains unknown about the vocabulary and meaning of fish utterances, but scientists are beginning to match certain sounds with behaviors such as mating. Sounds also can reveal the overall health of the fish, as sick animals generally don't mate, and thus are quieter.

Such monitoring of fish sounds can tell researchers about the negative impacts of pollution, ship travel, sonar, and other environmental changes.

Listening in on the underwater concerts also gives scientists an idea of the diversity of species in an area. In addition, researchers are applying their observations of fish to humans - for example, studying how fish process sounds into actions - in order to better understand how the human brain comprehends sound.

"Hundreds of fish are known to make sounds, and undoubtedly thousands actually make sounds," says Phillip Lobel, associate professor of biology at the Boston University Marine Program in Woods Hole, Mass. "The question is whether they are making purposeful sounds exclusively dedicated to one behavior, such as specific sounds for courtship, reproduction, and spawning."

Dr. Lobel has rigged up a special underwater microphone, or hydrophone, coupled with a video camera to match fish sounds and actions. One thing he and his colleagues already have discovered is that the most vigorous-sounding male damselfish - the one that makes the most sounds per hour - appears to be more attractive to females, and ends up with more eggs in the nest.

He credits such discoveries with the advent of new technology. "A lot of what we know now is because of new technology, such as being able to film in low-light conditions with handycams," Lobel says.

But matching sound to action is a challenging endeavor, because fish species vary widely in their spawning and other habits.

Some fish have elaborate mating rituals that involve various sounds, touch, and intertwined swimming. Some fish mate for life, while others take a more hit-and-run approach. Damselfish and midshipman males protect the nest until the eggs hatch and baby fish can live on their own. Other males and females simply produce eggs and then leave them to their own destiny.

To a female fish, the loudness, depth, and duration of the serenade reveal quite a bit about the male lurking in murky water or under a rock. Perhaps most important, a fish can tell whether the suitor is of the same species. And a deep, strong song usually indicates that the male is large and fit, a big plus in the undersea world of unseen predators and territory battles.

"The larger the fish, the lower the frequency, so the female can tell how big the male is, his physical fitness, and whether he is dominant," says Rodney Rountree, an adjunct assistant professor at the University of Massachusetts-Amherst. "Sound is an integral part of the courtship behavior." But it also is dangerous for the fish to sing, because predators may be attracted to the sound. A large male fish may be able to ward off would-be hunters.

Most of the time, humans can't hear all the drumming, rumbling, jackhammering, and pecking that takes place beneath the water's surface, because the sounds are reflected back under when they hit the surface, Dr. Rountree says.

Scientists use hydrophones to hear the sounds. But sometimes, when many fish are spawning in one location at the same time, it is possible to hear them with the unaided ear, says Rountree, who heard a troop of cusk eels hammering for a date when he was in a small boat on a salt marsh.

And about a decade ago, houseboat owners in Sausalito, Calif., became alarmed at what sounded like an electric power plant buzzing and vibrating underneath their boats, says Andrew Bass, a professor of neurobiology and behavior at Cornell University in Ithaca, N.Y.

The noise turned out to be hundreds of midshipmen fish humming to beckon potential mates to the nests they had fashioned under the rocks in the intertidal zone. The fish can hold a single humming note for as long as an hour. It can make other sounds as well, such as growling to scare away other suitors, and grunting to frighten potential predators away from eggs in the nest.

The midshipman, also known as California singing fish, is found on the west coast of the United States from Alaska to southern California. It can grow to 28 centimeters long as an adult. It is part of the toadfish family, other members of which make a clatter up and down the Eastern Seaboard of the country.

But Dr. Bass is studying the homely midshipman mainly to understand the human brain better. "We are looking at how the midshipman brain works to encode sound signals," he explained. "We've learned that the fish's brain can separate the sounds of two males when they are singing at the same time."

Most fish make sound by moving special muscles near their swim bladder, an air sac used to control buoyancy. The muscles can contract extremely rapidly against the swim bladder to make a fast drumming sound, or more slowly and at different intervals to make a variety of sounds. The air bladder can act as an amplifier for sounds as well.

Prof. Robert Baker of New York University Medical School discovered that part of the midshipman's brain contains a kind of vocalization pacemaker that tells the muscles around the swim bladder to contract in a certain rhythm that produces a hum at a frequency of about 100 hertz.

Fish also can make noise by grinding their teeth, quivering their fins, or snapping air bubbles. Catfish, for example, can stridulate (make noise) by quivering their fins when pulled out of the water. And croakers make quite a clatter when fishermen pull them onto the deck of a boat.

Fish hear with their ears, but sound can be conducted through their jawbone to the ears. Sound travels about five times as fast under water as through the air, and much farther. That means mating sounds can attract predators at quite a distance, and distress noises of the prey can warn off other fish.

"I've seen predators such as a wrass eat a damselfish that made noise when it was caught," says Lobel. "We don't know what the fish is thinking when it makes the sounds, but it is not happy that it is caught."

Lobel, Rountree, and other marine scientists believe fish sounds can be used for fisheries management, and to monitor the health of fish.

"Scientists are interested in sounds of the sea, as well as how we impact fish. We can ask fishermen not to catch certain fish when they are spawning in order to try to preserve species," says Rountree. "This area of science will grow rapidly in the next decade," he says.