A visible population of sand tiger 

 sharks frequents the wreckage of the 

 USS Tarpon, which rests 137 feet below 

 the surface of the Atlantic, 26 miles 

 southeast of Cape Hatteras. The sharks, 

 which apparently have become accus- 

 tomed to the increasing amount of 

 human activity at the site, attract many 

 expert divers, scientists and biologists. 

 Here people can watch sand tigers 

 cruise the derelict submarine for 

 sluggish schools of Atlantic spadefish, 

 an easy meal. 



The Tarpon wreck is illuminated 

 in a program Lovin filmed for the 1991 



N.C. Public Television series "Return 

 to the Sea." "Secrets of the Shark" also 

 featured footage Lovin shot at Cocos 

 Island, off the Pacific Coast of Costa 

 Rica. The program includes awesome 

 scenes of reef whitetips, swirling 

 schools of giant hammerheads and a 

 graceful and harmless shark relative, 

 the enormous manta ray. 



"Secrets of the Shark" and many 

 other recent nature programs are 

 making a difference in the way many 

 humans perceive sharks. And sharks 

 have a considerable prejudice to 

 overcome: Sharks are mindless killing 



machines. Sharks are all the same. 

 Sharks are dumber than goldfish. 



"I do believe the wheel of igno- 

 rance has slowly turned," says Samuel 

 Gruber, a shark biologist at the Univer- 

 sity of Miami. Gruber has been studying 

 shark behavior, particularly lemon 

 sharks, for more than 30 years, not to 

 mention diving, photographing and 

 training them too. 



"It so happens that sharks are pretty 

 smart animals," he says. "Sharks can 

 learn a simple, conditioned reflex faster 

 than a rabbit or a cat using the same 



Continued 



Shark Senses 



Sharks were once thought 

 of as dull, shortsighted crea- 

 tures that lurked the depths, 

 eating anything they happened 

 to stumble upon. We now 

 know that this highly evolved 

 animal has acute senses that 

 have enabled it to hunt suc- 

 cessfully and survive for at 

 least 350 million years. 



With more than half the 

 shark's brain dedicated to 

 smell, its most discriminating 

 sense may be olfactory. Some 

 sharks can detect one part of 

 blood in 1 00 million parts of 

 water, which may explain why 

 they are so sensitive to chum 

 and bleeding animals. Sharks 

 smell with their nostrils, but 

 don't breathe through them. 

 Instead they breathe by passing 

 water over their gills, which lie 

 just behind the head. 



Sharks also have fine-tuned 

 senses of touch and hearing, 



which are intimately linked; they 

 both involve sensitivity to pressure 

 waves. Tiny hairlike receptors 

 extend throughout the shark's 

 lateral line, which runs from the 

 tail down the shark's side and to 

 the head, including its ears. With 

 this sophisticated system, sharks 

 can feel and hear surrounding 

 vibrations, including the splashing 

 of a wounded fish. On the under- 

 side of the head or snout are 

 modified receptors, fluid-filled 

 pores called ampullae of Lorenzini. 

 These electroreceptors enable 

 sharks to detect the minute electric 

 fields that are given off by all living 

 beings. With this keen sense, 

 sharks can locate prey buried 

 beneath sand on the seafloor or 

 estuary bottom. 



The ampullae of Lorenzini 

 may also explain the shark's 

 "compass" ability. So sharp are 

 its senses that a shark can detect 

 the magnetic fields emanating 



from the earth's core. This 

 tool may help them find their 

 way and return to breeding 

 and pupping grounds. 



As for taste, sharks know 

 what they like. They have taste 

 preferences, which they can 

 sense with a profusion of taste 

 buds lining the mouth and 

 throat. Some species, such 

 as nurse sharks, have barbels 

 protruding from their lips, 

 similar to those on a catfish. 

 These barbels are also lined 

 with tastebuds. 



And when all else fails, 

 sharks have their sight. Sharks 

 have relatively good vision, and 

 their eyes are very sensitive to 

 light. One feature of their sight 

 is similar to that found in cats: 

 Reflecting plates in back of the 

 eye function like a mirror, 

 allowing sharks to enhance 

 light to improve their vision at 

 night and at great depths. □ 



COASTWATCH 7 



