412 MECHANICAL AND ACOUSTICAL SENSES 



critical point; rather, it is the manner whereby a given intensity is reached, 

 i.e., rapidly or slowly, relative to some unknown reference. This area of re- 

 search seems highly promising for reasons of biological interpretation and 

 because of its general implications with regard to present theories of with- 

 drawal processes in other animal groups. 



The numerous theoretical and practical difficulties that have faced scien- 

 tists during recent years regarding the problem of directional hearing in 

 fishes seem now to have been alleviated, at least in part. There is presently 

 good evidence that at least some teleosts and sharks do possess directional 

 hearing, and as research proceeds the number of species demonstrating this 

 ability will surely increase. Interest appears now to be shifting to the mor- 

 phological and physiological mechanisms that impart directionality to hear- 

 ing in these animals. There are a number of hypotheses and very few facts. 

 Testing these and other ideas will be an important area of research for the 

 next few years. Such research, it is hoped, will include sharks and other 

 elasmobranchs among it subjects. 



ACKNOWLEDGMENTS 



This manuscript is an expanded version of the lecture presented in honor of 

 Al Tester on the occasion of the first Albert L. Tester Memorial Symposium, 

 held before the Department of Zoology, University of Hawaii at Manoa, in 

 April 1976. 



Additional aid from NSF grants GB 5894 and GB 31000X is gratefully 

 acknowledged. Success of the program has been due to many individuals, but 

 special appreciation must go to Charles R. Gordon, Arnold Banner, A. Peter 

 Klimley, Samuel H. Gruber, Joseph D. Richard, and Juanita Spires. 



Sincere appreciation is also due Donald P. de Sylva, who critically 

 reviewed the manuscript, and to Joseph Richard, who aided greatly in discus- 

 sions of the acoustical portion of the work. I am extremely grateful to the 

 officers and staff of the Atlantic Undersea Test and Evaluation Center 

 (AUTEC), West Palm Beach, Fla., and Fresh Creek, Andros Island, Bahamas, 

 for their cooperation and kindness during our work in the Tongue of the 

 Ocean. Finally, I wish to thank the Government of the Bahamas for allow- 

 ing us to carry out research in their territorial waters. 



REFERENCES 



Allee, W. C, and J. C. Dickinson. 1954. Dominance and subordination in the 

 smooth dogfish Mustelus canis (Mitchill). Physiol. Zool. 27:356-364. 



Aronson, L. R., F. R. Aronson, and E. Clark. 1967. Instrumental condition- 

 ing and light-dark discrimination in young nurse sharks. Bull. Mar. Sci. 

 17(2):249-256. 



Banner, A. 1967. Evidence of sensitivity to acoustic displacements in the 

 lemon shark, Negaprion brevirostris (Poey). Pages 265-273 in P. Cahn, 

 ed. Lateral line detectors. Indiana University Press, Bloomington, Ind. 



