VISUAL SYSTEM: STATE OF THE ART 77 



were essentially the same; thus, none of these elasmobranchs displayed a 

 Purkinje shift (i.e., shift in spectral sensitivity during light adaptation). 



A Purkinje shift was found, however, in the rays Holorhinus tobijei, 

 Dasyatis akajei, and Urolophus fuscus. The maximum scotopic sensitivity of 

 these rays occurred at 500 nm except for Dasyatis, which peaked at 525 nm. 

 Light adaptation shifted the maximum sensitivity of Holorhinus and 

 Dasyatis to 575 nm. Urolophus shifted to 525 nm. These results were cor- 

 related with the depth in which the animal was found and its behavior. 



Using S-potentials as an index of sensitivity, Tamura et al. (1966), Tamura 

 and Niwa (1967), and Niwa and Tamura (1975) measured spectral sensitivity 

 in a number of elasmobranchs (Table 5). These results were correlated with 

 the presence of a duplex retina. Of the elasmobranchs examined, only 

 Dasyatis possessed chromatic S-potentials. The authors attributed this to the 

 possession of cones, which were said to be absent in the sharks examined. 



However, Gingly mo stoma, a close relative of Orectolobus, possesses a 

 large number of cones in addition to rods (Hamasaki and Gruber 1965, 

 Wang 1968). It is therefore surprising that cones as well as the associated 

 C-type S-potentials were not found. 



The spectral sensitivity of Mustelus canis could be described by a single 

 curve peaking at 500 nm regardless of state of adaptation (Dowling and 

 Ripps 1971, 1972; Stell et al. 1970, 1971, 1975). Cones have been found in 

 this retina, but the absence of a Purkinje shift probably reflects their low 

 number and the predominance of rods (Stell and Witkovsky 1973a, 1973b). 



Using the lemon shark Negaprion brevirostris, which possesses rods and 

 cones in the ratio of 12:1, Cohen et al. (1977) found a peak scotopic sen- 

 sitivity at 530 nm. This did not correlate with the X max of the visual pig- 

 ment (Bridges 1965b) and was shown not to be due to preretinal absorp- 

 tion or to the influence of the tapetum lucid um. It was suggested that the 

 cones were operating under scotopic conditions and contributing to the 

 scotopic spectral sensitivity. In addition, upon moderate light adaptation a 

 shift in the peak spectral sensitivity to 544 nm was also demonstrated. Thus 

 a Purkinje shift is characteristic of the retinal activity of this animal. 



Adaptation— The eyes of vertebrates can detect stimuli over a great 

 range of light intensities. This ability to change thresholds with different 

 light levels is termed "adaptation." The changes in threshold can be 

 monitored electrophysiologically to determine the rate and range of adapta- 

 tion, which can shed light on the mechanisms that control adaptation. 



Dark-adaptation curves obtained by monitoring the ERG after exposure 

 of the retina to a background-adapting stimulus was first done on the dogfish 

 Mustelus and several rays by Kobayashi (1962). In these experiments 

 changes in threshold amounted to only 2-3 log units and reached a stable 

 level after only 10 min. In addition, only smooth dark-adaptation curves 

 were obtained, with no evidence of any rod-cone breaks in spite of the 

 presence of both rods and cones in the retina of at least one of the rays. 



In contrast, Hamasaki and Bridges (1965) and Hamasaki et al. (1967) 

 reported changes of 6 log units in dark-adaptation experiments on lemon 



