The porphyropsin-vitamin A2 cycle has been found heretofore only in the retinas of bony fishes capable of existence in fresh water. Cyclostomes, due to their primitive and isolated phylogenetic position, might be expected to possess the rhodopsin-vitamin A1 cycle common to marine elasmobranchs, almost all marine teleosts, and all terrestrial vertebrates so far examined. Yet the anadromous sea lamprey, Petromyzon marinus, possesses primarily the porphyropsin system, like an anadromous teleost. This observation greatly extends the phylogenetic association of vitamin A2 with the capacity for freshwater existence.
Compared with freshwater and anadromous teleosts, the lamprey retina contains the porphyropsin system in extremely low concentration.
The remaining eye tissues, like the retina, contain both vitamins A1 and A2, the latter greatly predominant. The livers of larval and adult lampreys, however, appear to contain vitamin A1 alone. This situation also is not without teleost precedent, since the carp and certain anadromous salmonids display similar reversals of vitamin A pattern in the liver and eye tissues.