The distribution of vitamins A1 and A2 has been determined in the eye tissues and livers of a number of fishes. The vitamins were differentiated by means of the antimony chloride reaction, which yields with A1 a band at 615–620 mµ and with A2 a band at about 696 mµ. In the retina the presence of vitamin A1 is diagnostic of the operation of a rhodopsin, and vitamin A2 of a porphyropsin cycle.
The eye tissues of all permanently marine fishes examined, except the tautog, contain vitamin A1 alone. Those of all permanently freshwater fishes possess only vitamin A2. Those of all euryhaline (potentially migratory) fishes, except possibly the alewife, contain mixtures of both vitamins A, and always predominantly that one which ordinarily is associated with the environment in which the fish is spawned.
These correlations extend in part to the liver oils, but most livers contain mixtures of both vitamins A, and occasionally in proportions the reverse of those in the eye tissues.
The vitamin A configuration does not depend upon environmental circumstances, but is determined genetically. The transfer from vitamin A1 to A2 metabolism appears associated phylogenetically with migration of marine teleosts into fresh water.