76 VISION 



from rod to cone vision. It is thought that rods follow only slow temporal 

 changes while cones resolve higher flash rates (Graham 1965). This has 

 recently been confirmed by intracellular recordings from turtles, which 

 showed that rods had a slower time course than cones (Baylor and Fuortes 

 1970). 



It is not surprising, then, to find that elasmobranchs with very few cones 

 can resolve intermittent stimuli only at very slow rates. Kobayashi (1962), 

 monitoring the ERG in Mustelus, reported a cff of 10 Hz and 6 Hz in Narke 

 and Urolophus, respectively, while Dowling and Ripps (1970) found a cff of 

 5 Hz for the skate Raja. In contrast to these is the ability of the lemon shark 

 Negaprion brevirostris to follow intermittent stimuli at a higher rate, up to 

 45 Hz (O'Gower and Mathewson 1967; Gruber 1969, 1975; Gruber and 

 Hamasaki, in preparation), which is reasonable in light of their duplex retina 

 (Gruber et al. 1963). 



Although a discontinuity in the curve relating cff to intensity and the 

 ability to resolve intermittent stimuli at high rates is indicative of a duplex 

 retina, recent work on the skate retina has shown that this might not neces- 

 sarily be so. Green and Seigel (1973, 1975) demonstrated that under certain 

 conditions responses from the all-rod retina of the skate produced a double- 

 branched cff log I curve. Stimuli up to 30 Hz were resolved. This was much 

 higher than the value given by Dowling and Ripps (1970). Green and Siegel 

 came to the unorthodox conclusion that both parts of the curve rose as a 

 result of a single photoreceptor having only one photopigment. These were 

 presumably rods, since the spectral senstivity of both high- and low-intensity 

 segments of the cff log I curve followed the action spectrum of rhodopsin. 

 The high cff was obtained by measuring not only the b-wave of the ERG but 

 also the receptor potential and S-potential. The exact mechanism was not 

 described, but by recording the S-potential it was shown that to follow 

 high-frequency stimuli requires that certain conditions be met: (1) the 

 stimuli must be intense enough to saturate the S-potential and (2) they must 

 be prolonged. 



Spectral Sensitivity— The spectral sensitivity of the eye can be investi- 

 gated to identify photopic and scotopic activity; to provide indirect evidence 

 regarding color vision; to help identify the photopigment underlying vision; 

 or as an important component of phylogenetic, comparative, or ecological 

 studies (Armington 1974). 



Since early investigators (i.e., Franz 1931) labeled elasmobranchs as 

 nocturnal predators possessing all-rod retinas, the goal of most investigations 

 on spectral sensitivity of elasmobranchs has been to compare the sensitivity 

 of the animal with its type of photoreceptor and habitat. Kobayashi (1962), 

 using the b-wave of the electroretinogram, found the peak sensitivity of 

 Mustelus manazo to be at 505 nm. He correlated this with its life in oceanic 

 waters where light of 480-500 nm penetrates most effectively. Like Mustelus, 

 the scotopic (dark -adapted) spectral sensitivity of the torpedo Narke peaked 

 at 500 nm and did not shift upon light adaptation. The data for Raja porosa 



