Variability in the role of the gasbladder in fish audition

The teleost gasbladder is believed to aid in fish audition by transferring pressure components of incoming sound to the inner ears. This idea is prima rily based on both anatomical observations of the mechanical connection bet ween the gasbladder and the ear, followed by physiological experiments by V arious researchers. The gasbladder movement has been modeled mathematically as a pulsating bubble. This study is extending the previous work on fish w ith a physical coupling of the gasbladder and ear by investigating hearing in two species (the blue gourami Trichogaster trichopterus. and the oyster toadfish Opsanus tau) without a mechanical linkage. An otophysan specialist (the goldfish Carassius auratus) with mechanical coupling, is used as the control. Audiograms were obtained with acoustically evoked potentials (e.g. , auditory brainstem response) from intact fish and from the same individua ls with their gasbladders deflated. In blue gourami and oyster toadfish, re moval of gas did not significantly change thresholds, and evoked potentials had similar waveforms. In goldfish thresholds increased by 33-55 dB (frequ ency dependent) after deflation, and major changes in evoked potentials wer e observed. These results suggest that the gasbladder may not serve an audi tory enhancement function in teleost fishes that lack mechanical coupling b etween the gasbladder and the inner ear.