por·phy·rop·sin 
(pôr′fə-rŏpsĭn)
(pôr′fə-rŏpsĭn)n.
Any of a class of purple, light-sensitive pigments similar to rhodopsin, found in the retinal rods of the eyes of freshwater fishes and certain frogs.
[Greek porphurā, purple dye, purple garment; see PURPLE + OPSIN.]