VISUAL SYSTEM: STATE OF THE ART 31 



increase in the area due to pupillary dilation was about one log unit, or 

 tenfold. 



According to Franz (quoted by Walls 1942), dogfish develop "mydriatic 

 pupillary rigor," i.e., pupils first constrict, then dilate widely and remain 

 dilated in an animal placed in continuous light for several days. Lemon 

 sharks treated in this manner do not develop pupillary rigor (Gruber 1967). 



Kuchnow and Gilbert (1967) measured pupillary responses of Negaprion 

 to light and dark. After 25 min dark adaptation, the eye was illuminated 

 with 1000 fc (1080 lx). The pupil rapidly constricted to a minimum diam- 

 eter, then gradually adjusted to a steady state value some 5% larger. The 

 entire process took about 4 min. They also followed pupillary changes dur- 

 ing sunrise and sunset, noting that the greatest dilation (twofold) occurs as 

 light falls between 10~ 3 and 10~ 5 fc (lx); the greatest pupillary constriction 

 upon light adaptation occurs at 10 2 and 10 4 fc (lx). Differential rate of 

 change at different illumination levels suggested separate mechanisms, which 

 agreed with the morphology and early physiological studies of Young (1933) 

 and von Studnitz (1933). 



Kuchnow (1970) investigated details of pupillary activity in the diurnal 

 Mustelus and nocturnal Scyliorhinus. Scyliorhinus was 2.5 times more sensi- 

 tive (i.e., less light was required for pupillary constriction) than Mustelus. 

 Response to light in both species was graded; the pupil of Scyliorhinus 

 constricted to 18% after 1 min light adaptation and 5% of its dilated size 

 after 5 min. The pupil of the Mustelus reacted differently: a drop to 18% 

 after 2 min was followed by redilation to 20% after 5 min. The rate of 

 pupillary constriction in Scyliorhinus shifted in the mid-range of light in- 

 tensities. Kuchnow suggested that this shift signaled changeover from rod- to 

 cone-control of a retinally mediated pupillary reflex. However, the diurnal 

 Mustelus, known to possess retinal cones (Stell and Witkovsky 1973b), did 

 not shift in sensitivity. 



Pupillary activity of both species under monochromatic lights yielded 

 an action spectrum similar to that of rhodopsin (Figure 7). Thus the melanin 

 in the iris muscles cannot be the receptive pigment responsible for in- 

 dependent pupillary activity as suggested by Franz (1931). A rhodopsin-like 

 pigment must therefore be present in the iris tissues. 



In a final paper, Kuchnow (1971) photographed pupillary activity in 13 

 elasmobranchs. Nocturnal and diurnal species had mobile pupils; the deep- 

 sea sharks Oxynotus and Apristurus had fixed pupils. Dilation in all other 

 species was slow, typically taking 30 min. Constriction in Rajaformes ordi- 

 narily required about 5 min. Pupillary constriction in bright light was com- 

 paratively slower— about 15 min for the five species tested. Most unexpected 

 was the rapid dilation observed in Carcharhinus galapagensis (Plate I A shows 

 a closely related species, C. limbatus). This shark fully dilated in only 1 min, 

 faster even than constriction, which required 2 min. The observation was 

 made on shipboard and may represent an abnormal condition since dilation 

 is apparently much faster in this species than in any known elasmobranch 

 including other carcharhinids. However, Sivak and Gilbert (1977) have re- 

 cently observed rapid dilation in the sandbar shark, C. milberti. None of 



