250 CHEMICAL SENSES 



tempting to speculate that the relative ineffectiveness of globulin extracts is 

 related to their higher molecular weights and relative insolubility. Much 

 more needs to be known about the fates of these purified fractions in sea- 

 water, and more quantitative comparisons of electrophysiological responses 

 such as the EEGs should be obtained before additional conclusions can be 

 drawn from the studies on blood fractions. 



Chemical stimuli eliciting negative responses— A large amount of 

 effort has been invested in the search for chemicals acting as powerful and 

 consistent repellents as sharks. Rather than reviewing those chemical com- 

 pounds and past studies in detail, it can be said, in summary, that no ideal 

 repellents have been found. Indeed, there are persuasive theoretical reasons 

 for believing that repellents capable of practically deterring attacking sharks 

 do not exist. This does not mean, however, that no chemicals repel sharks 

 in their normal environments. The whole question of shark repellents, as 

 well as new approaches to their study, is considered in more detail below. 



MECHANISMS OF ORIENTATION-INTEGRATION 

 OF STIMULI 



The large, 40-by-80-ft (12.3 by 24.6 m) observation and test pens used in 

 these behavioral studies on sharks provided a good opportunity to analyze 

 further the mechanisms used by nurse and lemon sharks in locating sources 

 of chemical stimuli. These mechanisms proved to differ in the two species. 



Contributing to the analysis of orientation mechanisms was the fact that 

 tidal flow was at different rates on the two sides of the observation pen. 

 This made it possible to introduce a chemical stimulus on the strongest cur- 

 rent flow or in an area of weak flow. Consequently, swimming patterns of 

 stimulated sharks could be studied as a function of the direction and rate 

 of water flow, in addition to the variables of the chemical stimuli. Photo- 

 graphs of locomotor tracks were made with motion pictures in the daylight 

 hours, and with open-shutter photos of towed electronic flash units at night 

 (Hodgson and Mathewson 1971, Mathewson and Hodgson 1972). 



It was found that nurse sharks homed in on a stimulus source via an 

 S-shaped track, using true gradient searching, or klinotaxis (Figure 7). The 

 extent of the side-to-side excursions decreased with the number of the test 

 in a repetitive sequence of trials, suggesting that some learning was involved 

 during repeated testing. In addition to the decreased sideways excursions, 

 under repeated testing the nurse sharks showed increased speed and effi- 

 ciency in approaching a stimulus. 



Although the nurse sharks, unlike the lemon sharks, did not move into 

 the strongest currents following chemical stimulation, there was no part of 

 the observation pen which completely lacked some current flow. In a subse- 

 quent study on mechanisms of localizing chemical stimuli, Kleerekoper and 

 Gruber (1975) found that in stagnant water only generalized localization 

 occurs; hence, some movement of flowing water "provides the direction 

 vector for precise localization" even by nurse sharks (Kleerekoper 1978). 



