Fishy sounds in slow motion

Many fish make sounds to communicate with each other. Fish commonly generate sound with an organ called the swim bladder, which also controls their buoyancy. Biological acousticians treat the swim bladder as a damped, pulsating underwater bubble that can radiate sound initiated by some of the fastest known muscles in vertebrates. For example, the oyster toadfish (Opsanus tau) routinely contracts its swim-bladder muscles at 200 Hz, generating a sound of the same frequency. Now Eric Parmentier (University of Liège, Belgium), Michael Fine (Virginia Commonwealth University, Richmond), and their colleagues have found a surprising new mechanism in at least one group of fish. Pearlfish of the genus Carapus take up housing inside sea cucumbers (see photo) and produce sounds that resemble taps on a drum or cymbal—the specific frequency varies among species. At the November 2006 joint meeting in Honolulu of the Acoustical Society of America and the Acoustical Society of Japan, Fine presented evidence that carapid fish do not use their swim-bladder muscles at high speed; rather, the muscles slowly stretch a “window” near the front of the balloonlike bladder, much like drawing a bowstring or a rubber band. At a critical point, the window is released and snaps back, causing a wedge-shaped bony plate to vibrate and resonate with the bladder. The process is akin to sound production in cicadas, where muscle power drives a small mechanical resonator that in turn drives a larger acoustic resonator. (E. Parmentier et al., J. Exp. Biol. 209 , 2952, 2006 .)