52 VISION 



Gruber et al. (1963) presented histological evidence at the light micro- 

 scope level that the retina of the lemon shark, Negaprion, is provided with 

 cones. Cones were easily differentiated from rods on the basis of size, shape, 

 and staining properties. For example, rods, which predominated in the ratio 

 of 12:1, had a combined outer and inner segment length of 32 fim compared 

 to 19 jum for cones. The inner segment diameters were 2.3 and 4.7 {Jim, 

 respectively. The staining properties of the paraboloid conferred a character- 

 istic appearance on the inner segments of the cones, making them easily 

 distinguishable from the rods. The cones of Negaprion differed from those of 

 other vertebrates in at least one respect: their nuclei were indistinguishable 

 from those of rods. Mention was made of cone receptors in two other 

 carcharhinids, a hammerhead (Sphyrna mokarran) and a stingray (Dasyatis 

 americana). The cones of Dasyatis were described as typically conelike in 

 appearance in contrast to cones of the shark species. 



Hamasaki and Gruber (1965) presented further histological evidence of 

 duplex retinas in the nurse shark Gingly mo stoma cirratum and the stingray 

 Dasyatis sayi. Again, morphological differences between the rods and cones 

 of both species were obvious and conformed to the criteria set up by Pedler 

 and Tilly (1964). In receptor ratio and nuclear position the cones of Gingly- 

 mostoma were intermediate between those of Negaprion and Dasyatis. That 

 is, some of the cone nuclei of the nurse shark straddled the outer limiting 

 "membrane," while all of the cone nuclei did so in Dasyatis. The rod-cone 

 ratio at the posterior pole of the eye was 7:1 in the nurse shark and 5:1 in 

 the stingray. Counts from different parts of the nurse shark retina estab- 

 lished that there were more cones (ratio 3:2) in the central retina than in the 

 periphery. No evidence of double cones, twin cones, or oil droplets has been 

 reported in any elasmobranch species except for a brief mention by Wolken 

 (1975) of the possible occurrence of oil droplets in 20% of the receptors of 

 Mustelus. Yamada and Ishikawa (1965) casually mentioned the existence of 

 a few cones in the retinas of Dasyatis and Mustelus. 



Wang (1968) extended the work on receptor types in the retinas of 

 Negaprion and Ginglymostoma. He reported a homogeneous distribution of 

 cones in the ratio of 12 rods to 1 cone in Negaprion, while in the dorso- and 

 ventromedial retinal fields of Ginglymostoma, rods predominated by 7 to 1. 

 The receptor ratio increased to 13:1 in the dorso- and ventrolateral fields. 

 His receptor measurements agreed with those of Hamasaki and Gruber 

 (1965). 



In observations involving light- and dark-adapted eyes, Gruber et al. 

 (1963) and Hamasaki and Gruber (1965) did not observe any morphological 

 differences in the photoreceptors dependent upon state of adaptation and 

 thus claimed that photomechanical movements were absent in the elasmo- 

 branchs. Photomechanical movements, present for example in teleosts, are 

 reciprocal contractions and elongations of the rods and cones, often accom- 

 panied by movement of melanin pigment (Ali 1975). 



Wang (1968) agreed that the receptors of Negaprion and the rods of 

 Ginglymostoma were stationary. However, he noted two unusual changes in 

 the cones of the nurse shark with regard to state of adaptation : during light 



