412 



Comparative Animal Physiology 



or are found there in greater concentration than elsewhere. In those cases in 

 which the action spectrum of a photoreceptive process and the spectral ab- 

 sorption data of carotenoid or conjugated protein pigments are available, the 

 precise correlation between them predicates the conclusion that these pig- 

 ments are active in absorbing the radiant energy which subsequently init- 

 iates other events in the optic pathway. 



The Chemical Evolution of Photosensitive Pigments. One of the most 

 interesting stories to emerge from the realm of retinal physiology is that of 

 the phylogenetic origin of two photosensitive pigments, rhodopsin and por- 

 phyropsin. The following discussion is adapted largely from Wald,^*'*' and 

 earlier publications. 



In the years 1879-1880 Kiihne,^^^ one of the classic investigators of photo- 

 chemical systems, observed that the retinae of frogs and higher vertebrates 

 were rose-colored, whereas those of fresh-water fishes were distinctly purple 

 in color. Subsequently, Kottgen and Abelsdorff^^^ demonstrated that solu- 

 tions of photopigments from eight species of fresh-water fish possessed spect- 



1.0 



o 



^ 0.6 



YitaminaA Retinenes 

 I 



Rhodopsin Fbrphyropsin 

 I I 



500 

 Wavelenqth - m/i 



Fig. 120. Absorption spectra of the rhodopsin and porphyropsin systems obtained from 

 crude extracts of retinae of the marine scup (broken lines), and the fresh-water calico 

 bass (solid lines). All maxima have been brought to the same height to facilitate compari- 

 son. FromWald.'~ 



ral absorption properties different from the spectral properties of other verte- 

 brate pigments. Many vears later Wald^*'*' found that the spectral properties 

 of photopigments extracted from retinae of marine fishes resembled those of 

 rhodopsin. The spectral properties of pigments from the eyes of fresh-water 

 and marine fishes were dissimilar. Those of the latter were identical with 

 the spectral properties of rhodopsin; those of the former were not. The pig- 

 ment from the eyes of fresh-water fishes was called porphyropsin. Figure 120 

 indicates that the rhodopsin and porphyropsin systems are parallel systems: 

 rhodopsin decomposes into two carotenoids, retinenci, vitamin Ai, and a 

 protein; porphyropsin decomposes into retinene^., vitamin A2, and a protein. 

 This clear-cut distinction between photopigments of fresh-water and ma- 

 rine fishes (stenohaline) naturally led to the investigation of photopigments 

 in fishes that are transitional between fresh-water and marine environments, 

 or euryhaline fishes. Examination of the salmonids, which spawn in fresh 



