68 Memoir Sears Foundation for Marine Research 



elled, has returned to continue feeding on the carcass of a whale, or on offal thrown over- 

 board, or even to take the hook a second time. However, some of their senses are of a much 

 higher order than the foregoing might suggest, particularly their sense of smell. It has 

 been shown by experiment that the Smooth Dogfish {Mustelus) seeks its prey chiefly by 

 smell (p. 248) and it can be only because of their keen scenting ability that sharks gather 

 so quickly around a whale that is being cut up, or around a dead horse or other carcass 

 in some tropical harbor. As evidence of the ability of a large shark to scent a comparatively 

 small object from a considerable distance, we might mention an occasion in the Gulf 

 Stream, off Key West, Florida, when we saw a large carcharhinid tracking our bait (a 

 Spanish mackerel) up-current, its dorsal fin cutting the surface as it tacked back and forth 

 across the trail, and finally dashing forward on a direct line. 



Experiments on the Smooth Dogfish {Mustelus canisY have shown that it has at 

 least fair vision for objects that are close at hand, and this no doubt applies to sharks gen- 

 erally. In experiments, however, they seldom responded to any object until the latter was 

 within one foot of them,' thus bearing out the general concept that sight is of very little 

 importance in the lives of sharks. 



No evidence of any response by sharks to vibrations of high frequency (sound) 

 has been reported, although it seems well established that their auditory (8th) nerves, 

 as well as the nerves of the lateral-line system, are sensitive to water vibrations of low 

 frequency.'" 



Luminescence. A few genera are luminescent, as noted below (p. 509), but the great 

 majority are not. 



Food. Sharks are carnivorous without exception. Seaweeds have often been found in 

 the stomachs of one or another species, but no doubt these were taken with the animals on 

 which they were preying, and the more voracious kinds are so indiscriminate in their feed- 

 ing that they often swallow any kind of inedible rubbish." A few that have crushing teeth 

 (e.g., Mustelus and the heterodonts) feed largely on hard-shelled crustaceans (crabs, 

 lobsters) or on mollusks; but the majority prey chiefly on fishes smaller than themselves, 

 on squid and to some extent on pelagic Crustacea. In general the size of the prey is relative 

 to the size of the shark. However, some of the more fiercely predaceous species regularly 

 attack other fish, including other sharks nearly as large as themselves, if they are in a 

 position to do soj sea turtles and seals are a regular item in the diet of some sharks. On the 

 other hand, the two largest species (Whale Shark and Basking Shark) subsist wholly on 

 minute planktonic forms, chiefly Crustacea, and on small schooling fishes. 



Number of Species. In spite of the antiquity of the group, and in spite of the fact 

 that they appear to be as numerous and as varied now as at any time in the past, there are 

 many less species of sharks than of bony fishesj not more than 225 to 250 are now known. 



Danger to Man. Dependable information on the danger of sharks to man is frag- 

 mentary 5 nevertheless, we think it necessary to discuss the subject briefly, since it is of 



8. Parker, Bull. U.S. Bur. Fish., 29, 191 1 : 46. 9. Parker, Bull. U.S. Bur. Fish., 33, 1914: 64. 



10. Parker, Bull. U.S. Bur. Fish., 24, 1905 : 201. 11. For instances, see p. 69. 



