54 VISION 



They also presented clear evidence of cones in the retina of Narcine brasi- 

 liensis and Paratrygon motoro. From a photgraph given in their paper, we 

 estimate the receptor ratio in P. motoro at about 8 rods to 1 cone. 



To study visual adaptation and its relation to photochemical and neural 

 mechanisms, Dowling and Ripps (1970) searched for an animal that would 

 provide a convenient, long-lasting electrophysiological preparation with a 

 single type of receptor. After careful examination under the light microscope 

 and preliminary study under electron optics, they concluded that retinas of 

 the skates Raja erinacea and R. oscellata failed to reveal any receptoral detail 

 that could be considered characteristic of cones. This interpretation has been 

 confirmed by A. I. Cohen (personal communication). 



Gruber et al. (1975) reported the presence of cones in the retinas of 

 three lamnid and four carcharhinid sharks. Studies of 5-jum-thick paraffin 

 sections under the light microscope revealed that all seven species possess 

 duplex retinas with rods predominating by about 10:1 in the requiem sharks 

 and 6:1 in the more active mackerel sharks. A tally of the recently published 

 photoreceptor literature makes it clear that the prevailing view that the 

 retina of sharks contains but a single type of receptor is in error. 



Detailed study of the fine structure of elasmobranch photoreceptors has 

 been reported by Dunn (1973), Stell (1972a), and Stell and Witkovsky 

 (1973a). The most complete description of receptor morphology, fine struc- 

 ture, and synaptic connection appears to be the work of Stell (1972b) on 

 Squalus. The receptor layer of Squalus forms about 50% of the retina com- 

 pared to only about 30% in most other vertebrates. Cones make up only 2% 

 of the total receptor population but were easily distinguished from the rods. 

 Figure 10 shows the main ultrastructural difference between the rods and 

 cones at the level of the outer segment. 



In Squalus, visual pigment containing membranous disks (saccules) of the 

 cone outer segments are open to the ventricular space but are completely 

 enclosed by plasma membrane and probably free floating in rods. In horizon- 

 tal section the disks are circular in cones but lobulated in rods. These differ- 

 ences imply two very different cellular renewal processes in the rods and 

 cones, as suggested by Dunn (1973). The contiguity of cone disks to the 

 extracellular space appears to be universal in the vertebrates (Cohen 1972), 

 and we have confirmed that the cone outer segments of Carcharodon differ 

 from the rods in this unambiguous way. 



At the level of the inner segment, the cones bear radiating finlike proc- 

 esses while the rod inner segments appear to be scalloped in outline. This is 

 not a universal feature, however, since Dunn (1973) reported weakly devel- 

 oped fins on the rod inner segments of several vertebrates, including the 

 guitarfish, Rhinobatos. Other gross differences characterize the cone inner 

 segments. In Squalus they taper from 6 jiim at the external "limiting mem- 

 brane" to 3 {Jim at the base of the outer segment. Rods are untapering. The 

 mitochondrion-rich ellipsoids of the cones occupy a level below their rod 

 counterpart, which was clearly shown by Stell. As in Negaprion, receptor 

 nuclei and cell bodies of Squalus can lie at any level in the outer nuclear 

 layer, thus differing, for example, from those of Ginglymostoma, in which 



