THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



Since marine fish possessed rhodopsin, and freshwater fish, por- 

 phyropsin, it became of interest to study those intermediate types 

 which spend part of their Hves in fresh water and part in sea water. 

 These, the euryhahne fishes are divisible into two groups, the fresh- 

 water spawners (anadromous) and the marine spawners (catadro- 

 mous). 



WALD (1939a) investigated the distribution of vitamins A^ and Ag 

 in the retinae, other eye tissues and Hvers of representatives of the 

 various groups. The results he obtained are assembled in the 

 table on p. 36. 



The table contains several points of interest. Thus, considering 

 the results from retinae alone, all but one of the permanently marine 

 fish possess only vitamin A^. The exception is the tautog (tautoga 

 onitis) of the family Labridae, the wrasse fishes. Conversely, the 

 retinae of the permanently freshwater fishes contain only vitamin Ag. 

 With the euryhahne fishes, on the other hand, there is generally a 

 mixture of the two vitamins ; predominantly vitamin Ag if the fish is 

 anadromous, i.e. a freshwater spawner, and predominantly vitamin 

 Aj, in the eel, the single example of a marine spawner. 



A similar distribution pattern is evident in eye tissues other than 

 the retina. This pattern does not extend to the hver, however, which 

 often contains both vitamins when the retina contains only one. 

 When both vitamins are present in the retina, then both are present 

 also in the hver, but not necessarily in the same proportions ; in fact, 

 in the Chinook salmon there is a reversal of the preponderating 

 vitamin in the two locations. 



The presence of both vitamins in the retinae of euryhahne fishes, 

 which spawn either in the sea or in rivers, and the predominance of 

 vitamin A2 in the tautog — a marine fish — shows that the vitamin A 

 pattern is not absolutely determined by the spawning environments. 

 Likewise, it is not governed by the salinity of the adult environment, 

 for salmonids which have never left fresh water still possess both 

 vitamins ; while the eel contains a preponderance of vitamin Aj after 

 years of hving in fresh water, and the alewife a preponderance of Ag 

 after years in the sea. Again, the pattern is not determined by nutri- 

 tion, for the vegetarian carp has the same pattern as the carnivorous 

 pickerel and calico bass and, conversely, eels which had been caught 

 in the same pond as pickerel and calico bass had reverse patterns. 



WALD (1939a) concluded, therefore, that 'the vitamin A configura- 

 tions of both eye and hver are determined by genetic factors. 



35 



