THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



visual pigments from various fish illustrate the transition from the 

 rhodopsin system (the permanently marine dogfish) to the porphy- 

 ropsin system (the anadromous white perch), wald assumed that 

 the intermediate spectra obtained from euryhahne fish were due to 

 mixtures of rhodopsin and porphyropsin. 



From the results of these and other investigations wald (1949) 

 summarizing the position regarding rod pigments wrote *true land 



ASO SCO SSO 6C 



Fig. 2.5. Density spectra of photopigments from various fish. The 

 curves illustrate the hypothesis that the permanently marine dogfish 

 possesses rhodopsin alone, the catadromous eel and killifish predomi- 

 nantly rhodopsin, the anadromous brook trout predominantly and the 

 anadromous white perch exclusively porphyropsin. All maxima equated 

 to 1-0 to facilitate comparison. 

 {Wald, 1941) 



and sea vertebrates have the rhodopsin system; true fresh- water 

 vertebrates have porphyropsin; while those equivocal forms which 

 can distribute their lives between fresh water and one of the other 

 environments frequently possess both photopigments mixed or in 

 temporal succession.' In 1953 wald reiterated his conclusion that 

 'in the rods of vertebrates there is substantial evidence for the exist- 

 ence of only two visual pigments, rhodopsin and porphyropsin. The 

 distribution of these pigments has been explored to the point at which 

 it begins to seem probable that there are no others.' 



37 



