Hearing thresholds of the horn shark, H e t e r o d o n t u s f r a n c i s c i

Horn sharks were conditioned to pure-tone audio signals in two different laboratory test situations. In the first experiment, conducted in a small, air-speaker-driven enclosure, four sharks were conditioned to cross a barrier to avoid electrical shock. The sharks responded from 25 to 160 Hz, with the lowest pressure threshold at 40 Hz. The second experiment, conducted in a large tank with an underwater speaker, used four different sharks and employed a heart-rate technique in which shock and sound were paired to elicit conditioned bradycardia. These sharks responded over a similar frequency range of 20 to 160 Hz. When tested at two subject-to-transducer distances, their acoustic pressure thresholds increased with distance while their acoustic particle-motion thresholds remained the same. The lowest pressure threshold was at 40 Hz (12 dBμb), and the lowest particle-motion threshold was at 80 Hz, with a displacement of 1.4×10−6 cm and a velocity of 7×10−4 cm/sec (104 μvar). Subject Classification: 80.50, 80.60; 65.50.