PHYSIOLOGY OF CHEMORECEPTION 261 



products of rotting shark flesh and to clouds of dye. Since the significant 

 breakdown products of shark meat were shown to contain large quantities 

 of acetic acid, and copper ions appeared to have at least sporadic repellent 

 effects, the combination of copper acetate was chosen as the best approxi- 

 mation of a repellant known at the time. The dye cloud, which at least 

 temporarily inhibited approaches by some large sharks under some condi- 

 tions, was even more important as a psychological boost to the users. The 

 limitations of Shark Chaser are demonstrated in Plate I, which illustrates 

 a positive response to the "chaser" when the dye was being used to map 

 olfactory corridors in a large observation pen. 



It is important to note that a repellent may have some useful functions, 

 even though it does not work against sharks in a feeding frenzy or produce 

 long-lasting effects against all species. Despite a research crash program, and 

 tacit admission by the producers that Shark Chaser was not "the ultimate 

 repellent," experiments during the 1960s produced no strikingly better 

 alternative. In fact, studies on the kinetics of responses by sharks to water- 

 borne drugs strongly indicated that the most powerful repellents, capable 

 of incapacitating attacking sharks, are unlikely to be found. This conclusion 

 was based, in large part, on experiments by Baldridge (1969a) concerning 

 the actions of incapacitating drugs on lemon sharks. 



Baldridge determined the minimum drug concentrations in seawater 

 necessary for producing states of excitation, depression, and narcosis or 

 incapacitation in sharks. The effects produced by drugs such as quinaldine 

 were found to involve mass-action effects on a number of systems in which 

 drug— receptor combinations are relatively weak and highly reversible. In 

 practice, very high concentrations of such drugs would have to be built up 

 and sustained in a shark's body to produce significant behavioral alterations. 

 Strychnine and hydrocyanic acid are more selective and specific in their 

 actions, but still would be unlikely to incapacitate sharks in open water 

 without reaching concentrations that would be impractical to deliver and 

 would be dangerous for human beings in the area (Baldridge 19695). 



The general impact of the studies in the 1960s was to turn investigations 

 away from unrealistic hopes of finding "ultimate" repellents, capable of 

 incapacitating sharks. Attention has been shifted to chemicals that might 

 render an environment uninviting, decrease the palatability of potential 

 prey, or (if specific chemoreceptor sites could be characterized) compete 

 with normal stimuli at the transduction sites to block the effectiveness of 

 sensory cues. The experiments with tracer-labeled stimuli suggest that the 

 latter approaches will not be easy, at least with amino acids, since any 

 binding of stimulating molecules appears to be weak and quickly reversed, 

 and the normal rinsing actions of flow through the nasal sac are extremely 

 effective. Recent evidence suggests, however, that further attention might 

 profitably be given so-called "natural repellents." 



Naturally Occurring Repellents 



The experiments of von Uexkull (1895) strongly indicated the existence 

 of at least one naturally occurring shark repellent (in Aplysia), even before 



