Hearing thresholds of the horn shark, Heterodontus francisci.

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....