PHYSIOLOGY OF CHEMORECEPTION 251 



With lemon sharks, the influence of water currents was found to be 

 greater. Once stimulated by a chemical, lemon sharks swam upstream in 

 the strongest water current passing through the observation pen at the 

 time. They did this regardless of whether the behavior brought them closer 

 to the source of chemical stimulation. Figures 8 and 9 illustrate one such 

 response, in which a lemon shark circles repeatedly in a corner of the pen 

 where the strongest current enters; the chemical stimulus, which prompted 

 the reaction, remains all the while in a slower current about 10 m away in 

 the pen. The reactions of the stimulated lemon sharks, therefore, are domi- 

 nated by a reaction to water currents, or rheotaxis. Normally, this would 

 bring the lemon shark near to food material or prey, at which time other 

 cues (e.g., visual or electrical) might come into play. Only in the experi- 

 mental situation is the true nature of the lemon shark's rheotaxis revealed. 



It has been postulated (Mathewson and Hodgson 1972) that the use of 

 chemical stimuli primarily to trigger a rheotaxis, as in Negaprion, makes 

 fewer demands on the chemosensory receptor system than does a purely 

 klinotaxic response. Stimulation by a minimum threshold number of mole- 

 cules could suffice to trigger the rheotaxis, without the necessity for 

 "comparing" or "balancing" the number of stimulating molecules hitting 

 chemoreceptors on the two sides of the head. Such a rheotaxis-release 

 mechanism had been postulated by Kleerekoper, and it is known to be 

 operative in many other marine animals (Kleerekoper 1978). 



It remained to be determined whether the observed orientation mecha- 

 nisms were related to the final stages of feeding, when visual stimuli become 

 very important in guiding biting responses and other components of attacks 

 upon prey. Convincing evidence that these patterns of orientation can lead 

 to typical biting, even "frenzied" feeding behavior, came from six cases in 

 which combinations of amines and amino acids elicited strong feeding be- 

 haviors, even though the chemical solutions provided no visual cues for 

 close-range attack. In all these cases, the sharks began biting (lemon sharks) 

 or sucking (nurse sharks) when within 2 m of the stimulus source. This feed- 

 ing behavior appeared to be directed toward whatever small objects the 

 shark encountered in the water when near the highest concentrations of 

 chemical stimuli— bubbles on the water surface, small twigs or grass blades 

 being carried past in the water current, etc. The role of vision in these re- 

 actions was especially clear in Negaprion, as observed and photographed 

 from the underwater observation cage near the stimulus source. As a lemon 

 shark neared the outlet tube for chemical stimuli, its eyes could be seen to 

 move whenever it swam past small objects in the water; the correlation of 

 eye movements and slight shifts in swimming directions, related to what- 

 ever floating objects were visible, were quite different from the responses 

 of the same sharks in other areas of the test pens. The frontispiece of this 

 book provides an illustration of such a response. It shows a lemon shark 

 "attacking" some air bubbles that were originally limited to the water sur- 

 face, about 2 m from the source of a stimulus stream consisting of 0.1 M 

 TMAO mixed with 0.1 M glycine. The shark's biting and side-to-side head 

 movements during the initial attack produced more bubbles, and the shark 



