Comparison of acoustic particle acceleration detection capabilities in three shark species

Behavioural studies have shown that sharks are capable of directional orientation to sound. However, only one previous experiment addresses the physiological mechanisms of directional hearing in sharks. Here, we used a directional shaker table in combination with the auditory evoked potential (AEP) technique to understand the broadscale directional hearing capabilities in the New Zealand carpet shark (Cephaloscyllium isabellum), rig shark (Mustelus lenticulatus) and school shark (Galeorhinus galeus). The aim of this experiment was to test if sharks are more sensitive to vertical (z-axis) or head-to-tail (x-axis) accelerations, and whether there are any differences between species. Our results support previous findings, suggesting that shark ears can receive sounds from all directions. Acceleration detection bandwidth was narrowest for the carpet shark (40-200 Hz), and broader for rig and school sharks (40-800 Hz). Greatest sensitivity bands were 40-80 Hz for the carpet shark, 100-200 Hz for the rig and 80-100 Hz for the school shark. Our results indicate that there may be differences in directional hearing abilities among sharks. The bottom-dwelling carpet shark was equally sensitive to vertical and head-to-tail particle accelerations. In contrast, both benthopelagic rig and school sharks appeared to be more sensitive to vertical accelerations at frequencies up to 200 Hz. This is the first study to provide physiological evidence that sharks may differ in their directional hearing and sound localisation abilities. Further comparative physiological and behavioural studies in more species with different lifestyles, habitats and feeding strategies are needed to further explore the drivers for increased sensitivity to vertical accelerations among elasmobranchs.