Photoreception 



413 



water but spend the greatest part of their adult Hfe in the ocean (i.e., anad- 

 romous fishes), revealed that thev possess both rhodopsin and porphvropsin, 

 the latter predominating. This predominance of porphvropsin is further 

 exemplified by the brook trout, which can enter the ocean but rarely does 

 so, and the anadromous white perch, which has only the porphvropsin system 

 (Figure 121). Examination of the fresh-water eel, which spawns in the 

 ocean but spends the greatest part of its adult life in fresh water (i.e., a ca- 

 tadromous fish), revealed that, although it possesses both pigments, rhodop- 

 sin dominates (Fig. 121). The killifish, which spends its entire life in 

 brackish water, is intermediate between the eel and the brook trout. The 

 alewife, an anadromous fish, spends almost all of its life in the sea and vet 

 has only porphyropsin in its retina. In general, it may be said that eury- 

 haline fishes possess predominately or exclusively that photosensitive pigment 

 associated with their spawning environment, i.e., rhodopsin for marine and 

 porphyropsin for fresh-water environment. 



Since the fresh-water fishes are believed to have a major place in verte- 

 brate evolution, it was of interest to examine the photopigments of other ver- 

 tebrates in the evolutionary line. The cyclostomes are the only living primi- 



T 



WaY9fengt/t 



Fig. 121. Absorption spectra of retinal extracts from various fishes, showing the tran- 

 sition from the rhodopsin to the porphyropsin system. The dogfish possesses rhodopsin 

 alone; the anadromous white perch possesses porphyropsin alone, and the other species 

 are intermediate. From Wald.^*" 



tive vertebrates which have eyes sufficiently well developed to permit analysis 

 of photopigments. The sea lamprey, Petromyzon niarinus, an anadromous 

 lamprey, possesses a great preponderance of porphyropsin. Retinas of the 

 lungfish, Protoptenis, have been found to contain vitamin Ao, indicating a 

 porphyropsin system. Examination of the amphibia led to an interesting 

 discovery. The urodele amphibian, Triturus viridescens, utilizes porphyrop- 

 sin as its visual pigment, which is in accord with its fresh-water spawning 

 habits and its probable ancestry. However, the anuran amphibian, the com- 

 mon frog, uses rhodopsin exclusively as its rod pigment. Yet the frog also 



