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Under the sea - sotto voce no more

Scientists are beginning to link certain sounds made by fish with certain behaviors

By Lori ValigraSpecial to The Christine Science Monitor. / November 8, 2001

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.

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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.

Are the sounds purposeful?

"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.

Loud is better

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.