Pressure and particle motion detection thresholds in fish: a re-examination of salient auditory cues in teleosts

The auditory evoked potential technique has been used for the past 30 years to evaluate the hearing ability of fish. The resulting audiograms are typically presented in terms of sound pressure (dB re. 1 mu Pa) with the particle motion (dB re. 1 m s(-2)) component largely ignored until recently. When audiograms have been presented in terms of particle acceleration, one of two approaches has been used for stimulus characterisation: measuring the pressure gradient between two hydrophones or using accelerometers. With rare exceptions these values are presented from experiments using a speaker as the stimulus, thus making it impossible to truly separate the contribution of direct particle motion and pressure detection in the response. Here, we compared the particle acceleration and pressure auditory thresholds of three species of fish with differing hearing specialisations, goldfish (Carassius auratus, weberian ossicles), bigeye (Pempheris adspersus, ligamentous hearing specialisation) and a third species with no swim bladder, the common triplefin (Forstergyian lappillum), using three different methods of determining particle acceleration. In terms of particle acceleration, all three fish species have similar hearing thresholds, but when expressed as pressure thresholds goldfish are the most sensitive, followed by bigeye, with triplefin the least sensitive. It is suggested here that all fish have a similar ability to detect the particle motion component of the sound field and it is their ability to transduce the pressure component of the sound field to the inner ear via ancillary hearing structures that provides the differences in hearing ability. Therefore, care is needed in stimuli presentation and measurement when determining hearing ability of fish and when interpreting comparative hearing abilities between species.