396 



MECHANICAL AND ACOUSTICAL SENSES 



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60 - 



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20 - 



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□ = TOTAL SIGHTINGS DURING 



TEST PERIOD 



 = MAXIMUM SIMULTANEOUS SIGHTINGS 

 DURING TEST PERIOD 



□ = TOTAL SIGHTINGS DURING CONTROL 



PERIOD 



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Figure 1 Attraction of sharpnose sharks Rhizoprionodon, sp., by 

 acoustic signals, showing upper effective frequency limit. Signals 

 consisted of irregularly pulsed, overdriven sine waves, having funda- 

 mental frequencies from 200 to 1500 Hz. Peak sound pressure level 

 at 18.5 m from sound source was approximately 20 dB above broad- 

 band ambient noise. Each test and control period— 3 min. (Myrberg 

 etal. 1969) 



branch, the skate, Raja clauata. Also, the most effective frequency range for 

 purposes of attraction, between 10 and 100 Hz, approximates the range of 

 frequencies associated with spike discharges from vibration-sensitive areas of 

 the labyrinth in this skate (Lowenstein and Roberts 1951). Although the 

 authors attached no physiological significance to their data, that their 

 findings correlate with those of others at a different level of integration 

 indicates a functional significance between these neural and behavioral 

 events. Additional examination of hearing ability and its neural correlates in 

 other elasmobranch fishes will certainly provide further insight into such a 

 suggestion. 



Pure tones, regardless of frequency, do not attract free-ranging sharks 

 (Myrberg et al. 1969, Richard 1968).- The scarcity of biologically produced 

 pure tones in the aquatic environment may explain this failure to react, thus 

 reinforcing the belief that the attraction response to low-frequency, broad- 

 band sounds has a biologically adaptive basis. 



Repetitive Pulsing— The second quality of an attractive sound is its 

 repetitive pulsing. Continuous sound, regardless of frequency, does not elicit 



