Evidence of active sound production by a shark

Elasmobranchs are an evolutionarily ancient group of cartilaginous fishes that can hear underwater sounds but are not historically viewed as active sound producers. Three recent reports of several species of rays producing clicks in response to approaching divers have cast doubt on this long prevailing view and resulted in calls for more research into sound production in elasmobranchs. This study shows that the rig, Mustelus lenticulatus, produces clicks (mean SPL_rms = 156.3 dB re. 1 μPa ± 0.9 s.e.m. at approx. 30 cm) when handled underwater, representing the first documented case of deliberate sound production by a shark. Clicks were broadband (mean bandwidth = 23 kHz ± 0.1 s.e.m.), with peak energies between 2.4 and 18.5 kHz (mean peak frequency = 9.6 kHz ± 0.3 s.e.m.), and mean duration of 48.42 ms ± 2.9 s.e.m. Clicks contained considerably less energy in frequencies below 1 kHz, which overlap with the hearing range of the rig. We propose that forceful snapping of flattened teeth may be the sound producing mechanism based on the plated tooth morphology and the acoustic characteristics of these clicks. Further behavioural studies are needed to test whether clicks are incidental to the handling or a natural acoustic response of behavioural significance.