The directional hearing abilities of two species of bamboo sharks

Auditory evoked potentials (AEPs) were used to measure the directional hearing thresholds of the white-spotted bamboo shark Chiloscyllium plagiosum and the brown-banded bamboo shark Chiloscyllium punctatum at four frequencies and seven directions, using a shaker table designed to mimic the particle motion component of sound. Over most directions and frequencies there were no significant differences in acceleration thresholds, suggesting that the sharks have omni-directional hearing abilities. Goldfish Carassius auratus were used as a baseline to compare a species with specialized hearing adaptations versus sharks with no known adaptations, and were found to have more sensitive directional responses than the sharks. Composite audiograms of the sharks were created from the average of all of the directions at each frequency and were compared with an audiogram obtained for C. plagiosum using a dipole stimulus. The dipole stimulus audiograms were significantly lower at 50 and 200 Hz compared to the shaker audiograms in terms of particle acceleration. This difference is hypothesized to be a result of the dipole stimulating the macula neglecta, which would not be stimulated by the shaker table.