64 VISION 



visual elements are the GGC's, which presumably feed information to the 

 primary visual areas of the brain. The function of the small cells was "less 

 clear." 



Finally, Walls (1942) mentioned that displaced retinal neurons such as 

 displaced GGC's are indicative of a crude retina. However, Dunn (1973) 

 reported that displaced ganglion cells are known from all vertebrate classes 

 with the possible exception of teleosts, and therefore the significance of 

 these displaced neurons is not known. 



Visual Pigments— Visual pigments are organic dyes composed of a 

 protein, opsin (molecular weight about 27,000), complexed onto a short 

 chain prosthetic or chromophore molecule related to Vitamin A. These pig- 

 ments are located in the photoreceptors; specifically, rhodopsin constitutes 

 up to 35% dry weight of the rod outer segments (Kropf 1972). While most 

 retinal components are relatively transparent, visual pigments absorb light 

 and give the retina its characteristic purple or pink color. The early retinolo- 

 gists were almost certainly aware of the coloration associated with the 

 retina (Crescitelli 1972), but it was Boll (1876) who first reported that the 

 color quickly fades in light. He correctly associated this bleaching with rod 

 photoreceptors and related it to the mechanism of vision. He also observed 

 sehrot in the retina of sharks, and Krause (1889) reported sehpurpur from 

 the rod outer segments of a ray. However, the most important pioneer work 

 in visual pigments was that of Kuhne (Crescitelli 1972), who coined the term 

 "rhodopsin," extracted and characterized this material, produced a crude 

 absorption spectrum, and observed regeneration of rhodopsin from its prod- 

 ucts of bleaching. 



It was not until 1936 that Bayliss et al. first investigated elasmobranch 

 visual pigments in any detail. Since then retinas from about a dozen species 

 have been extracted, primarily in digitonin, and their visual pigments 

 characterized (Table 4). In addition, Denton and Nicol (1964) and Dowling 

 and Ripps (1970) have investigated elasmobranch visual pigments in situ by 

 the methods of differential density and fundus reflectometry . 



The absorbance maximum of the ordinary elasmobranch visual pigment 

 lies at about 500 nm, which is typical of rhodopsin. The chromophore has 

 been characterized as Vitamin Ai -based retinene (i.e., Beatty 1969). 



Bayliss et al. (1936) and Clarke (1936) independently suggested that 

 fishes inhabiting the deeper ocean might be visually adapted to the spectral 

 quality of light at depth. This was confirmed by Denton and Warren (1956), 

 who reported that the retinas of three teleosts living below 500 m contained a 

 golden colored pigment (chrysopsin) in high density. These fishes had visual 

 pigments shifted some 20 nm toward the blue and were thus well adapted to 

 make use of the fraction of daylight that reaches that depth. In a remarkable 

 example of parallel evolution, Denton and Shaw (1963) reported that retinas 

 of three elasmobranch species caught at 1150 m contained a golden pigment 

 similar to the chrysopsin of deep-sea teleosts. While the chromophore of 

 elasmobranch chrysopsin was not chemically identified and the visual pig- 

 ments were not checked for homogeneity, the authors clearly demonstrated 

 that these pigments were environmentally tuned. Of the three shark retinas 



