Sounding out the way fish hear.

To submarine commanders, fish have an enviable ability to detect the sounds of predators and prey without necessarily making their own presence felt. In contrast, a submarine's sonar system, which sends out acoustic pulses and then detects reflections from nearby objects, signals the submarine's location as clearly as it finds its targets. Perhaps the U.S. Navy can learn a lesson from the way fish hear.

As part of a Navy effort to study fish hearing, researchers at the GEorgia Institute of Technology in Atlanta have invented an underwater, ultrasonic technique for measuring how a fish's sound-sensitive organs respond to low-frequency sound waves. This technique is better than previous methods, says Georgia Tech's Mardi Cox, a mechanical engineer, because the fish's organs do not have to be removed or exposed for study. Experiments can be done on live fish.

Almost all fish have a swim bladder--a skinlike sack filled with gas, which allows a fish to adjust its density and control its distance from the water surface. In goldfish, a row of tiny bones called otoliths connects the swim bladder with the fish's inner ear. The swim bladder seems to function as a sound amplifier and transmitter.

"There's little information about how these organs respond to acoustic waves," says Cox, who with her colleague Peter H. Rogers described their research at last week's Acoustical Society of America meeting.

In their experiments, the researchers scan the immobilized body of a goldfish with a 10-megahertz sound source while subjecting the fish to a low-frequency sound wave of about 200 hertz. The fish's swim bladder oscillates in response to the low-frequency sound. A detector picks up the resulting echo. This signal reflects the swim bladder's motion.

So far, the researchers have demonstrated that the system works. Swim bladder motions are clearly visible, although otolith movements have not been detected yet. Eventually, Cox and Rogers hope to detect displacements as small as 25 angstroms when the sound waves are focused to a spot on the fish only 0.2 millimeter across.

Rogers is particularly interested in testing a new hypothesis suggesting that a bony fish actually processes some data in the ear itself rather than in its central nervous system, as most theories had assumed. Rogers proposes that a goldfish's otolithic organs act somewhat like accelerometers, which provide information about a body's velocity in various directions. This would help fish "calculate" where sound sources may be located.