VISUAL SYSTEM: STATE OF THE ART 85 



curve agrees in shape and position of maximum sensitivity with the rho- 

 dopsin absorption curve, the light-adapted shark had somewhat different 

 sensitivity characteristics. The effect of adaptation with white light caused 

 the peak of this curve to shift some 20 nm toward the red and broadened the 

 curve. This shift has been confirmed electrophysiologically (Gruber 1973, 

 Cohen et al. in press). Thus the Purkinje phenomenon is a feature of the 

 lemon shark's visual system and is further evidence of cone function in this 

 species. 



Gruber (1969, 1975) described a direct test for color vision in which 

 lemon sharks were trained to respond when a chromatic adapting field was 

 silently switched to another color. Controls on stimulus brightness were 

 included in the test. Statistical analyses of the data indicated that the subject 

 was using chromatic cues in responding to the stimulus changes. However, 

 results were presented with reservation since data from only one subject 

 were available. 



Visual Orientations— Harden- Jones (1963) studied the reactions of 

 various fishes, including the dogfish Scyliorhinus, to a rotating striped drum. 

 Fishes typically responded by swimming around the periphery of the tank 

 following the movement of the background stripes. Although Hardin-Jones 

 was interested in the relation of visual cues to orientation in a current, he 

 was actually measuring what is known as the optomotor response. A number 

 of species including Scyliorhinus failed to respond to the moving background 

 and thus did not exhibit an optomotor response. 



Wallace (1972) reported on the reaction of sharks to gill nets in con- 

 nection with a meshing program along South African beaches. Although nets 

 are successful at protecting beaches, it was not known exactly how the nets 

 act as barriers or how sharks react to them. In one test, Wallace presented 

 bull sharks, Carcharhinus lucas, with nets of various reflectances and colors. 

 Results indicated that sharks avoided the brighter colored (yellow) nets, thus 

 Wallace suggested that avoidance of nets in the experimental situation was 

 based on visual cues. 



Other workers have suggested that sharks, in the field, are attracted to or 

 avoid brightly colored objects. For example, Hess (1962) reported that 

 oceanic sharks are naturally attracted to fluorescent orange objects. Some- 

 what more meaningful was the report by McFadden and Johnson (1971) 

 that survival gear painted yellow was attractive to free-ranging sharks while 

 the same gear painted black was apparently ignored by the same animals. 

 This report is reminiscent of a preliminary field study (Gruber, unpublished 

 results) in which silky sharks, C. falciformis, were acoustically attracted to 

 an array of three polyethylene globes 40 cm in diameter. The globes were 

 separated by 6 m. In this study, sharks clearly avoided the fluorescent orange 

 globe but readily removed bait from a black globe and less frequently from a 

 white globe. Again, this is similar to a study by Limbaugh (1963) in which 

 baits were placed in fluorescent orange or white bags. In this case, sharks 

 preferred the white over the orange target, striking the white bag three times 



