A reexamination of the teleost swimbladder as an acoustic source

Abstract

Classically swimbladders are considered pulsating resonant bubbles that are omnidirectional monopole sources capable of translating acoustic pressure to the ears. Swimbladder sounds are driven by sonic muscles (the fastest in vertebrates), yet high speed would seem unnecessary to excite a resonant structure. Recent studies in the oyster toadfish Opsanus tau and the weakfish Cynoscion regalis suggest that the classic generalizations may not apply to all fishes. The toadfish bladder is a low-Q inefficient (highly-damped) resonator that moves in a quadrupole fashion producing sound amplitude proportional to bladder velocity. Slow movements do not produce audible sound. When the bladder is stimulated artificially, dominant frequency is determined by the waveform of the excitation stimulus and not fish size. The sound field, measured underwater, is mildly directional, and deflation of the bladder does not change auditory thresholds. The dominant frequency of weakfish sounds (pulses produced by individual muscle contractions) appears to be determined by timing parameters of muscle twitches and not the natural frequency of the bladder. [Work supported by NIH.]