28 VISION 



1969a, 1969ft). Most of these studies were done in vivo with the intact eye 

 and thus differ from Nicol's approach. The method of measuring tapetal 

 activity involved curarizing and fixing iridectomized animals in space, then 

 photographing the eye under controlled illumination. Densitometry of the 

 photographic emulsion compared with a standard gray scale was proportional 

 to the amount of light reflected from the tapetum. While absolute values were 

 unobtainable, Kuchnow believed that the photographic technique was simple 

 and gave an accurate picture of the rate and extent of tapetal response. 

 Laboratory experiments were performed on Cephaloscy Ilium, Heterodontus, 

 Mustelus, Negaprion, and Triakis. Field observations were made on Apris- 

 turus, Carcharhinus, Ginglymostoma, and Prionace. As expected, the 

 scyliorhinid shark Apristurus had a nonocclusable tapetum. The tapeta of all 

 other species were occlusable to some degree. Laboratory experiments 

 (mainly on Heterodontus) demonstrated that pigment begins to migrate in 

 the dark-adapted shark when levels exceed 10~ 6 fL (3.4 X 10 -6 cd/m 2 ) with 

 graded responses up to 10" 1 fL (3.4 X 10 _1 cd/m 2 ). Footlamberts are photo- 

 metric values and thus not the most appropriate measure of light for an 

 experiment of this sort. Maximum pigment extension occurred at 10" 1 fL. 

 Conversely, tapetal plates remained occluded until the light fell below 1 fL. 

 Maximum pigment aggregation occurred at levels just below 10~ 3 fL (3.4 X 

 10~ 3 cd/m 2 ). Data from a number of species indicated that light adaptation, 

 i.e., response of the melanin to light, was graded, requiring 60-90 min for 

 complete occlusion; dark adaptation was invariably faster, taking anywhere 

 from 30 to 60 min. At onset of both light and darkness, a lag of several 

 minutes was noted before change in reflectivity began. This lag represents 

 movement of pigment granules through channels on the tapetal plates to an 

 anatomical position were changes in light absorption can first take place. 



Research on the elasmobranch tapetum has been especially fruitful. Of all 

 animals, the elasmobranchs have the most elaborate tapetum (Pirie 1965). 

 While other animals may have occlusable tapeta, none is known to combine 

 sensitivity of occlusion, regularity of multilayer crystals, specific plate 

 orientation, and specular reflection into such an ordered tapetum as do the 

 elasmobranchs. Biochemistry of tapetal guanine and melanin are reasonably 

 well known, but the actual mechanism(s) subserving tapetal occlusion re- 

 mains to be discovered. Everyone seems to agree that tapetal pigment flows 

 through fixed channels, and thus tapetal occlusion is not pseudopodal as had 

 first been thought (Franz 1931). The primary unresolved question is: How is 

 the tapetal pigment response triggered and controlled? 



While we understand the basis of eyeshine in the elasmobranchs, specu- 

 lation on its value to the organism has not been entirely convincing. Cer- 

 tainly, the tapetum is an important optical device, since eyeshine has evolved 

 independently in many phyla and from a number of unrelated structures. 

 The tapetum is clearly useful only in dim light. However, its value as a 

 sensitivity mechanism alone must be seriously questioned on the basis of 

 Nicol's (Denton and Nicol 1965) observation on the relation between den- 

 sity of visual pigment and possession of a reflecting tapetum. Simply stated, 

 certain teleosts have twice the density of visual pigment compared to sharks 



