The role of saccular resonance in fish audition

Abstract

Dynamic characteristics and resonance of the saccule play a fundamental role in audition in all teleosts, including those with direct connections between the swim bladder and inner ear. The saccule is an accelerometer with its rigid mass, the otolith (or saggita), coupled to the sensory epithelium through mechanical impedances of the otolithic membrane and hair-cell ciliary bundles. Relative displacement between the saggita and sensory epithelium induced by sound correlates with hearing sensitivity. Dynamic models of the peripheral auditory system in fishes from five different orders (the oscar, broad whitefish, oyster toadfish, dab, and goldfish) were developed for a comparative analysis. Species selected included one without a swim bladder and one with Weberian apparatus that transmits swim bladder motion directly to the saccule. Results for all fishes agreed with audiograms published in the literature. The lowest frequency marking the band of best sensitivity was found to be at the saccular resonance in all species; however, width of the band depended on excitation of the saccule indirectly from motion of the swim bladder and/or Weberian apparatus. Species with swim bladders and larger saggita had best sensitivities at lower frequencies, but with smaller bandwidths because the saccule could not respond to indirect stimulation.