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. Sphyrna lewini responded to a 400 Hz pulsed-tone with an abrupt increase in tailbeat frequency (97 beats per 30 s, versus 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, versus 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. We 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. Our results show that S. lewini is adapted 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. This article is protected by copyright. All rights reserved.