SHARKS AND UNDERWATER SOUND 411 



were recorded when the parietal fossa was stimulated by slight vibrations, 

 the response level falling rapidly as the stimulus was moved away from that 

 location. They also found that the levels of response depended on the loca- 

 tion of stimulation within the surface confines of the fossa. These findings, 

 plus the remarkable similarity of various structures of the region (i.e., 

 parietal fossa, the taut membranes of the fenestrum ovalis, the macula 

 neglecta, and the latter's proximity to the sacculus and its associated en- 

 dolymphatic duct) to the tympanic membrane and its associated structures 

 in higher vertebrates cause one to question if this is merely coincidence. 

 The obvious question is: why would a shark possess a tympanic-like membrane 

 in its auditory system? Could the structure act like the swimbladder of 

 teleosts, which transforms slight pressure fluctuations into appropriate dis- 

 placements so as to increase sensitivity to far-field sound? The main dif- 

 ficulty is the absence of an apparent impedance discontinuity in the system. 

 Could it be the seat of a nondirectional reference for the timing analysis, as 

 required by Schuijf's model of directional hearing? The data strongly suggest 

 that the maculae neglecta, lying directly below the membranes of the fenes- 

 tra ovalis, are sensitive to particle motion (i.e., a velocity detector— Fay et al. 

 1974). Could that structural organization, centering on the macula neglecta, 

 somehow impart directional information about a distant sound source? 

 Further speculation is unjustified until more data confirm the system as part 

 of the auditory system of sharks. 



CONCLUSIONS 



There can no longer be any doubt that sound plays an important role in the 

 lives of sharks. It is used by them to locate food sources and possibly even 

 other objects, such as competitors and predators. Results from studies using 

 acoustical playback techniques have shown that repetitively pulsed, syn- 

 thesized or naturally produced sounds, possessing frequency bands below 

 800 to 1000 Hz, are attractive to many species from a variety of habitats. 

 Probably such sounds will eventually be found to be attractive to most, if 

 not all, sharks. As one lowers the spectral content, attractiveness increases 

 until an optimum is reached at very low frequencies, i.e., 40 Hz or below. 

 Attractiveness increases also as repetitive pulsing increases (at least to 20 

 pulses/s), with irregular pulses being more effective than regular pulse 

 trains. Pure tones and continuous sounds are not attractive to sharks. 



Sharks perform a wide variety of behavioral activities in the vicinity of 

 sound projectors. The technique of acoustic attraction therefore provides an 

 opportunity to observe repeatedly various patterns of movements at times 

 other than those of fortuitous encounters or when the animals have become 

 highly excited by food used to attract them. 



Under specific circumstances sounds can also elicit rapid withdrawal by 

 sharks. Although much work remains to be done before this relationship 

 can be described fully, it appears that some factor of intensity is central. 

 There is evidence indicating that absolute intensity of sound is not the 



