66 VISION 



examined, the visual pigment of Centroscymnus was most blue shifted (472 

 nm) while Centrophorus was intermediate at 482 and Deania was least 

 shifted (484 nm). Using an entirely separate technique, Denton and Nicol 

 (1964) confirmed the A max position at 472 and 484 nm for Centroscymnus 

 and Deania, respectively. Parallel visual pigment adaptations have recently 

 been reported from invertebrates and marine mammals (Lythgoe 1972). 



In a noteworthy study Pepperberg et al. (1976) investigated the effects of 

 placing various isomers of retinal (the chromophore of rhodopsin) on a 

 physiologically active but strongly bleached retina. Using the all-rod retina of 

 Raja, they first intensively light-adapted the retina with green light, which 

 bleached about 90% of the rhodopsin. Since the retina was isolated from the 

 pigment epithelium, further visual pigment regeneration could not take 

 place. Thus, the amplitude of the receptor potential reached a stable plateau 

 determined mainly by the amount of rhodopsin available in the rod outer 

 segments. Next, aliquots of the all-trans isomer of retinal were dropped on 

 the retina. As expected, no change in sensitivity was recorded since the 

 normal effect of light on unbleached visual pigment is to isomerize the 

 chromophore to the all-trans form. When the 11-cis isomer of retinal was 

 sprayed on the retina a dramatic increase in sensitivity was recorded. Most 

 unexpected was the increase in sensitivity when 9-cis retinal was placed on 

 the retina. Densitometric evidence presented led to the conclusion that 

 external application of 11-cis retinal rapidly promotes the formation of 

 rhodopsin; application of 9-cis retinal similarly forms isorhodopsin, a visual 

 pigment not naturally found in skate photoreceptors. Thus, the receptor 

 mechanism that subserves those sensitivity changes dependent upon concen- 

 tration of visual pigment is able to use both rhodopsin and isorhodopsin. 

 This was apparently the first demonstration of changes in receptor sensitivity 

 directly dependent upon 11-cis retinal. 



It is assumed that rhodopsin occurs only in the outer segments of rod 

 visual cells. Cone pigments of mixed retinas are not ordinarily obtained 

 through the extraction procedures. Recently, however, analysis of the light- 

 absorbing properties of individual cone outer segments of the goldfish has 

 been achieved by microspectrophotometry (Marks 1965). Harosi and Gruber 

 attempted to characterize the cone outer segments of Negaprion by this 

 method but were unsuccessful. Thus, nothing is known of the cone pigments 

 of elasmobranchs. 



Retinal Electrophysiology— Electrical recording from the fish retina 

 was first attempted by Dewar and McKendrick (1873). However, it was not 

 until the early 1960 's that researchers turned toward elasmobranchs, to use 

 their retinas for electrical recordings to answer questions of ecology, 

 behavior, physiology, and pharmacology as they pertain to the animal in 

 particular and visual science in general. 



Electroretinogram (ERG)— The early electrophysiological studies on 

 elasmobranchs concentrated on recording a massed response of several 

 retinal cell types known as the ERG to determine waveform and other 



