Whence the Shadoivs? 229 



from laboratory experiments such as this, the belief grew that while 

 sharks could see nearby objects they had limited distance vision. 



But tests conducted within recent years by Dr. Perry W. Gilbert, 

 chairman of the newly formed Shark Research Panel, have indicated 

 that sharks depend considerably on their eyes in hunting prey. Gilbert 

 reported that a temporarily blinded shark blundered into a wall and 

 knocked itself out. 



Working not with dogfish but with such big sharks as Tiger sharks 

 (Galeocerdo ciivieri) and Lemon sharks (Negaprion brevirostris), Gil- 

 bert anesthetized these fish, put opaque plastic caps over their eyes, 



The nictitating membrane, found in certain sharks, is 

 a kind of eyelid which moves upward from the bottom 

 of the eye. The eye shown is that of a Hammerhead 

 {Sphyrna diplana). The membrane, found also in 

 birds and reptiles, acts as a windshield wiper which 

 keeps the eye clean. 



Courtesy, The Sears Foundation for Marine Research from 



Fishes of the Western North Atlantic by Henry B. Bigelow 



and WiUiam C. Schroeder, 1948 



and turned them loose in 80- by 40-foot outdoor pens. The pens, at 

 the Lemer Marine Laboratory in Bimini, in the Bahamas, provide an 

 environment more natural than that of an indoor laboratory tank. And, 

 in this environment, Gilbert reports that a blinded shark is often helpless. 



Gilbert believes that the shark's eyes become very important as it 

 nears the food that its olfactory senses have detected. He reports that 

 recent tests have indicated that sharks depend more and more on vision 

 as they near their prey. At about 100 feet from the prey, Gilbert says, 

 the sense of vision seems to take over. The distance depends on how clear 

 the water is. 



The tests from which this theory stems were conducted by tempo- 

 rarily blocking vision or smell, and observing the results. When Gilbert 

 and his associates obliterated both senses simultaneously, by putting 

 shields over the eyes and plugging the nostrils, the sharks swam about 

 helplessly, usually injured themselves by crashing into the pen barriers, 

 and died in 3 to 5 days. 



Though there seems to be some evidence that sharks can— and do- 

 distinguish between light and dark objects, they are generally thought 



