Whence the Shadows? 235 



female Lemon sharks were successfully conditioned to pressing the 

 empty target and returning for food." 



There were also three Nurse sharks {Gmglymostofym cirratinn) in 

 the pen. They didn't do so well at associating the target with food. But 

 both the Nurses and the Lemons did learn to steal. And, in the summary' 

 of how many times sharks successfully obtained food, a "stealing" factor 

 had to be built in. A "steal" was recorded when one shark rang the bell 

 and another dashed to the spot where the food was dropped. The Lemon 

 sharks "earned" their food by ringing the bell 731 times and stole it 

 from each other 108 times. The Nurse sharks became more adept at 

 stealing than finding food. They earned food 106 times and stole it 118 

 times— either by sneaking it off the target without ringing the bell, 

 or by snatching it from the others. 



When colder weather chilled the water, the sharks lost interest in 

 their schooling. When the w^ater turned warm again, and the target was 

 once more presented to them, the sharks went through their paces, even 

 though they had not seen a target for 10 weeks. 



There was no fighting among the sharks for food. In fact, one 

 curious touch of what a non-scientist would call reverse chivalry was 

 observed. Being a scientist. Dr. Clark put it this way: 



"We have no evidence yet in explanation of the fact that the female 

 refrains from pressing the target until the initial hunger of the male 

 apparently is satisfied." 



As any fish swims, water enters its mouth, beginning a breathing 

 process that is similar to man's. Respiration in fish is essentially the same 

 as respiration in any higher vertebrate: oxygen is absorbed into the blood 

 and carbon dioxide is given off. We extract our oxygen from the atmos- 

 phere; the fish extracts it from air dissolved in water. We use lungs; 

 the fish uses gills, and the Selachian uses gill sHts. In each of the sHts, 

 or clefts, are gill-filaments richly supplied with blood vessels. 



When the shark opens its mouth to inhale water, the clefts close. The 

 water passes over the gill-filaments, carbon dioxide is released from the 

 blood, and oxygen dissolved in the water is absorbed. In addition to their 

 5 to 7 sets of gill clefts, sharks almost invariably also have a less impor- 

 tant respiratory organ, the spiracle. The spiracle, usually located just 

 behind each eye on the shark, is believed to aid in aerating the blood 

 destined for the eyes and the brain. 



When a shark acts sluggishly in an aquarium, apparently because of 

 a lack of oxygen, attendants take it into a shallow tank and "walk" the 

 shark around it. The stroll causes water to flow into its mouth and 

 through the gills, much as swimming would. As soon as the attendants 

 feel the shark beginning to come out of its daze, they prudently leave 

 the pool. 



