VISUAL SYSTEM: STATE OF THE ART 89 



and thus narrowed the spectrum impinging on the photoreceptors. They 

 therefore concluded that the pineal receptors contained rhodopsin. 



Sensitivity determinations indicated that the pineal system responds to 

 changes of illumination on the order of 4 X 10~ 6 lm/m 2 which is far below 

 moonlight. Actually the sensitivity of the pineal approaches that of the rod 

 system of the lateral eye. It is difficult to understand why the epiphyseal 

 photoreceptors are morphologically conelike yet behave like rods and 

 possess rod visual pigment. 



CONCLUDING REMARKS 



The increasing importance of the elasmobranch visual system as an object of 

 study has been made abundantly clear in this article. We were astonished by 

 the number and variety of recent investigations on the eyes of cartilaginous 

 fishes. Objectives for studying the elasmobranch visual system varied from 

 phylogenetic and systematic considerations through use of the shark eye as a 

 simple model of the human eye. 



From comparison with the older literature, it was clear that a quantum 

 increase has been achieved in available information on the elasmobranch 

 visual system. This has radically altered our views on structure, function, and 

 adaptation of that system. For example, Duke-Elder (1958) reviewed the 

 elasmobranch eye and listed six characterizing features. We now know that 

 three of these— (1) sluggishly mobile iris without nerve supply; (2) shallow 

 anterior chamber without annular ligament or Schlemm's canal; (3) retina, 

 with few exceptions, provided only with rods —are no longer correct. 



While the eyes of the approximately 250 species of cartilaginous fishes 

 share many features in common, there is so much variation that it is difficult 

 to describe the "elasmobranch eye." 



Elasmobranchs swim in nearly all major habitats in the marine environ- 

 ment, from the deepest abyssal zones to the littoral surf zone. Pelagic and 

 benthic species are known from Arctic to tropical waters and a few species 

 are even restricted to freshwater. Obviously this ecological diversity will be 

 reflected by adaptations of their organ systems. Compounding this is the wide 

 variation in behavior; some species are apparently highly dependent on vision 

 while others seem to use tactile, olfactory, or other nonvisual cues more 

 frequently. Thus we are presented with an array of elasmobranch visual 

 systems in which the diameter of the eye varies from nearly 10% of the 

 standard length in the big eye thresher, Alopias superciliosus, to less than 1% 

 in a large nurse shark, Ginglymostoma cirratum. Some sharks have fixed 

 eyelids while others possess a completely mobile nictitating membrane. Cer- 

 tain species have rapidly constricting hides; in others, the iris is nearly im- 

 mobile. Variability in pupil shape characterizes the elasmobranchs. Some 

 species can even "stop" the pupil down to one or more stenopaic apertures. 

 Many elasmobranchs have a partially or totally occlusable tapetum but the 

 tapetum of Scyliorhinus, for example, is fixed. And so the list can be con- 

 tinued. Thus it is difficult to meaningfully define the average "elasmobranch 

 eye." 



