Literature References: Photoreceptor protein found in the retinal cone cells of fresh water and migratory fish, lampreys, and certain amphibians; corresponds to the rod pigment, porphyropsin, q.v. Absorption maximum ~620 nm. Composed of the chromophore 11-cis-3-dehydroretinal, q.v., bound to a photopsin, the specific protein component of cone cells (see Opsins). Proposed existence as a visual pigment and in vitro synthesis from 3-dehydroretinal and chicken photopsin: G. Wald, Science 118, 505 (1953). Demonstration of natural occurrence by microspectrophotometry: P. Liebman, G. Entine, Nature 216, 501 (1967). Methods for prepn and assay: R. Hubbard et al., Methods Enzymol. 18, 615-653 (1971). Photochemistry and biological activity: G. Wald, Science 162, 230 (1968). A trichromatic cone system utilizing 3-dehydroretinal as chromophore has been noted to occur in certain amphibians and fish. Three photochemically distinct pigments, corresponding to those of retinal-based color vision, have been identified: P. A. Liebman in Handbook of Sensory Physiology Vol. VII(1), H. J. A. Dartnall, Ed. (Springer-Verlag, New York, 1972) pp 481-528. A visual system is generally based on one type of chromophore combined with various opsins. However, pigments utilizing both types of chromophore have been found to co-exist in the retina of some of these species. Demonstration of paired-pigment cone systems: E. R. Loew, H. J. A. Dartnall, Vision Res. 11, 551 (1976); A. T. C. Tsin et al., ibid. 21, 943 (1981). Absorption spectrum: J. E. M. Mooij, T. J. T. P. van den Berg, ibid. 23 (1983).