Comparison of auditory evoked potential thresholds in three shark species

Auditory sensitivity measurements have been published for only 12 of the more than 1150 extant species of elasmobranchs (sharks, skates and rays). Thus, there is a need to further understand sound perception in more species from different ecological niches. In this study, the auditory evoked potential (AEP) technique was used to compare hearing abilities of the bottom-dwelling New Zealand carpet shark (Cephaloscyllium isabellum) and two benthopelagic houndsharks (Triakidae), the rig (Mustelus lenticulatus) and the school shark (Galeorhinus galeus). AEPs were measured in response to tone bursts (frequencies: 80, 100, 150, 200, 300, 450, 600, 800 and 1200 Hz) from an underwater speaker positioned 55 cm in front of the shark in an experimental tank. AEP detection thresholds were derived visually and statistically, with statistical measures slightly more sensitive (∼4 dB) than visual methodology. Hearing abilities differed between species, mainly with respect to bandwidth rather than sensitivity. Hearing was least developed in the benthic C. isabellum [upper limit: 300 Hz, highest sensitivity: 100 Hz (82.3±1.5 dB re. 1 µm s-2)] and had a wider range in the benthopelagic rig and school sharks [upper limit: 800 Hz; highest sensitivity: 100 Hz (79.2±1.6 dB re. 1 µm s-2) for G. galeus and 150 Hz (74.8±1.8 dB re. 1 µm s-2) for M. lenticulatus]. The data are consistent with those known for 'hearing non-specialist' teleost fishes that detect only particle motion, not pressure. Furthermore, our results provide evidence that benthopelagic sharks exploit higher frequencies (max. 800 Hz) than some of the bottom-dwelling sharks (max. 300 Hz). Further behavioural and morphological studies are needed to identify what ecological factors drive differences in upper frequency limits of hearing in elasmobranchs.