The Acoustical Biology of Elasmobranchs

A report on the auditory capabilities and their associated functions of elasmobranch fishes along with a brief review of the physics of underwater sound as it relates to hearing by fishes is provided. The inner ears of elasmobranchs possess structures that are no different from other fishes, except for an enlarged macula neglecta, which is unique among fishes. Hearing abilities among sharks (the only elasmobranchs examined) have demonstrated highest sensitivity to low frequency sound (40Hz to approximately 800Hz), which is sensed solely through the particle-motion component of an acoustical field. Free-ranging sharks are attracted to sounds possessing specific characteristics: irregularly pulsed, broad-band (most attractive frequencies: below 80Hz), and transmitted without a sudden increase in intensity. Such sounds are reminiscent of those produced by struggling prey. A sound, even an attractive one, can also result in immediate withdrawal by sharks from a source, if its intensity suddenly increases 20dB [10 times] or more above a previous transmission. Present evidence also shows that the lateral line system does not respond to normal acoustical stimuli. Morphological and physiological evidence suggest that the maculae neglecta possess acoustical relevance and may explain directionality of response despite physical principles that provide still other hypotheses for acoustical directionality. Brain centers controlling acoustical response, particularly among sharks, must be explored in the near future with careful consideration of the habits of subjects based on indications of species-differences regarding the importance of acoustical stimuli to their activities. Numerous facts and ideas about the acoustical biology of elasmobranch fishes make certain that future research will be most rewarding.