Structure and Function of the Elasmobranch Auditory System

Abstract

Behavioral evidence indicates that sharks detect underwater sound at frequencies up to 1000 Hz, and that certain low frequency signals attract sharks from large distances. It appears that the adequate stimulus for “sound detecting” systems of the shark is panicle motion, as opposed to fluctuations in sound pressure. The elasmobranch ear consists of the three semi-circular canals for detecting angular accelerations, and otolith organs for detecting linear motion and accelerations due to gravity. Two of these organs, the sacculus and macula neglecta, have been shown to be responsive to vibratory motion, with the macula neglecta having best sensitivity to vertical movements. A direct vibrational pathway exists to the macula neglecta from the parietal fossa of the dorsal chondrocranium. It is not clear at present, however, whether it is the inner ear or the lateral line system which is responsible for hearing. Both detection systems are theoretically capable of providing information to the brain about sound source location using non-parallel arrays of directionally sensitive hair-cell receptors. Recent theories of underwater sound localization by fishes and sharks suggest that the ability to detect a vertical displacement component of an acoustic signal (e. g., via the macula neglecta) is necessary for instantaneous location decisions. It is not known, however, whether the sharks localize by processing information about various aspects of the sound field simultaneously (in parallel), or whether the sound field is sampled successively at different points in space. Clearly, more experimental work on the physiology of elasmobranch acoustic behavior is called for.