240 CHEMICAL SENSES 



beats completely. Lemon sharks may also interpolate extra gill beats into 

 the usual rhythm, especially when swimming very strongly, as in the Fig- 

 ure 6 example. These differences in the records from nurse and lemon 

 sharks appear to be adaptations to obvious differences in the overall pat- 

 terns of activity in the two species. The initial lunge of the previously 

 immobile nurse shark may place a higher premium on streamlining, even 

 at the cost of interrupting oxygen intake, the constantly swimming lemon 

 shark briefly prolongs the posture of the gill covers in the closed stream- 

 lined position, but very quickly the need for oxygen to sustain its vigorous 

 swimming takes precedence, even requiring extra gill beats to be inter- 

 polated later in some instances. 



With both species, it is possible to differentiate these complete initial re- 

 actions to chemical stimuli from reactions of "awareness" only. In the latter 

 cases, the sharks may change body posture, rearing up heads toward the 

 stimulus source (nurse sharks) or turning toward the stimulus source (lemon 

 sharks), with or without changes in the rhythm of gill beats, but not giving 

 further reactions. In these cases it is clear that the animals are aware of the 

 chemical stimuli, but do not show sustained orientation or attempts to 

 approach or avoid the stimuli. EEG changes are rarely observed with the 

 simple awareness responses. This undoubtedly is because the sharks can 

 integrate and respond to afferent impulses that are too few, or too subtly 

 different from the usual patterns of neural activity, to be recognized in the 

 EEG traces. It is well established that swimming activity is closely associated 

 with many areas of the CNS, including arousal, escape, feeding, and spinal 

 centers (Demski 1977), and so there is no expectation that a strict separa- 

 tion of awareness responses from at least transient locomotor respones 

 would be usual, although the snout-raising of otherwise immobile nurse 

 sharks is one instance in which such a distinction can be made. 



The problem of designating "negative," aversive, or avoidance responses 

 is difficult when observations are confined to the glass observation tank of 

 the hydrodynamic tunnel. In a few cases (e.g., with holothurin extracts), 

 sharks oriented away from the incoming current and nosed against the 

 restraints on the outflow side of the tank; these are, at least initially, nega- 

 tive responses. As described below, it was necessary to conduct tests in 

 large enclosures, more closely simulating the natural open sea conditions, 

 to determine whether orientation toward or away from these stimuli would 

 be sustained or quickly reversed. 



For those chemical stimuli that evoked strong responses, both EEG and 

 behavioral, within the confines of the observation tank of the hydrodynamic 

 tunnel, additional testing was done in pens measuring 40 by 80 ft (12.3 by 

 24.6 m). These large enclosures were situated in an area of twice daily tidal 

 flow, in the Bimini (Bahamas) lagoon. Stimuli were introduced through 

 Tygon tubing underwater, with release points varied to provide defined 

 olfactory corridors, as shown in Plate I. The precise position of the olfactory 

 corridor, and its rate of movement, depended on the tide conditions at the 

 time of testing. Reactions of sharks were recorded by motion pictures taken 

 from an observation platform overlooking the test pen, and also from under- 



