One of the mysteries of the English language finally explained.
Any of a class of light-sensitive pigments found in the retinas of freshwater vertebrates, analogous to rhodopsin in humans but differing from it in containing the aldehyde of vitamin A ₂ rather than that of A₁ and in having a maximum absorption of visible light at a slightly longer wavelength.
1930s. From porphyro- + -opsin. Compare later opsin.
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