140 UNDERWATER GUIDE TO MARINE LIFE 



The various shaiks are best told apart by shape and action. Except for a few 

 species, they offer little in the way of pattern or coloration as an aid to 

 identification. 



Sharks are predominantly fish-eaters, but they are neither fast nor agile enough 

 to catch fast healthy fish so a great part of their diet consists of the disabled 

 or slower fishes. They prefer the easy mark, a wounded fish or a straggler. 

 (Exceptions to this are the fast and active members of the family Isuridae, 

 the mackerel sharks.) Sharks, contrary to popular account, do not have to turn 

 on their sides or backs to bite, even though they occasionally do so. 



We shall examine the various sharks' diets under later species discussion, but 

 one item of diet deserves more than fleeting attention. This concerns the ever- 

 discussed question: Do sharks attack man? The answer is an undeniable yes. 

 But we must be careful in this assertion to be attentive to the circumstances 

 under which attack is most likely to occur and which species might be 

 responsible. 



There is a great tendency to overdramatize the possibility of attack. This was 

 particularly damaging to the morale of service men deployed in "shark waters" 

 during the last war. There is an equal tendency toward debunking. Having 

 met sharks and not having been chewed to bits, some people, especially under- 

 water swimmers, have made the extreme assertion that sharks are not dangerous 

 at all. The authors have heard some underwater swimmers claim that sharks 

 are cowards. This seems to be based on the fact that these unfortunate people 

 had once or even several times met a shark which happened to run when 

 aggressively approached. 



Sharks seem to hunt their prey in a manner somewhat as follows. The first 

 stimulus to feed comes from two sources: smell (olfaction) and, to a lesser 

 extent, reception of low frequency water vibration (through the lateral line). 

 These have the effect of increasing the speed of the shark's movements, 

 causing it to make short hunting turns and figure eights which are of an 

 exploratory, food-seeking nature. These responses reinforce the original olfactory 

 stimulus and also notify other sharks of the presence of food through the stimulus 

 of low frequency vibration. Gradually, as the shark or sharks approach the food, 

 the exploratory turns become more and more direct until the food is struck. 

 Sight may play a part in these last stages. Contact with food is the final stimulus 

 which brings on the "feeding mood." At this stage, sharks are extremely 

 aroused, active, dangerous, and not discriminating in taste. Tiger sharks, 

 Galeocerdo, have been known to become so excited when in the "feeding 

 mood" as to bite blindly at any object including sacks of coal, old shoes, and 

 tin cans. 



The sharks that are implicated in attack on man are: 



1. White Shark or Man-Eater, Carcharodon carcharias. This is the sole 

 deliberate attacker and is always dangerous. It does not seek out man but 

 is liable to attack because it habitually eats large-sized prey. It is aggressive, 

 and its triangular, trenchant teeth (to 2 inches long in a 15-foot specimen) 

 are extremely efficient. Large ones can swallow objects as large as a whole 

 man. Fortunately, this shark is not common. 



