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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.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


(Biochemistry) a purple pigment occurring in the retina of the eye of certain freshwater fishes
[C20: from Greek porphura purple + -opsis + -in, on the model of rhodopsin]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
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The alevin and fry stages take place in freshwater, and the salmon then migrate to the sea and there is ontogenetic remodeling known as smoltification, which entails shifts in the visual pigments from porphyropsin dominant to rhodopsin dominant.