Operant conditioning as a tool to assess hearing abilities in sharks

Sharks (elasmobranchs) are an ancient, diverse group of fishes, representing a basal stage in the evolution of vertebrate hearing. Yet, our understanding of behavioural measures of hearing abilities in sharks is limited. To address this, an operant conditioning paradigm was designed, and scalloped hammerhead Sphyrna lewini and rig (spotted estuary smooth hound) Mustelus lenticulatus were successfully trained to respond to pure-tone acoustic stimuli from an underwater speaker. After 2-3 weeks of training, both species showed distinctive responses to these acoustic stimuli and retained this behaviour when reinforced. S. lewini responded to a 400 Hz pulsed tone with an abrupt increase in tailbeat frequency (97 beats per 30 s vs. 69 beats for a 2 kHz control and 70 beats for no signal) and sustained vigorous swimming (arousal response) for at least 30 s. In response to a 200 Hz pulsed tone, M. lenticulatus visited a target area under the speaker significantly more frequently (13.4 ± 4.3 times per minute vs. 1.4 ± 1.5 times for a 1.2 kHz control and 0.9 ± 0.01 times for no signal) and swam circles under the speaker to search for food. The authors used S. lewini arousal responses to pure-tone stimuli of 40, 80, 200, 400, 600 and 800 Hz to generate a provisional hearing-threshold curve. The results show that S. lewini adapts to low-frequency hearing (greatest sensitivity at 200 Hz, upper limit 800 Hz), which is like other coastal pelagic sharks that have been investigated so far. Despite challenges operant acoustic conditioning studies are a viable method for revealing auditory capabilities of sharks.