Temporal shifts in visual pigment absorbance in the retina of Pacific salmon

The visual pigments and photoreceptor types in the retinas of three species of Pacific salmon (coho, chum, and chinook) were examined using microspectrophotometry and histological sections for light microscopy. All three species had four cone visual pigments with maximum absorbance in the UV (_max: 357–382 nm), blue (_max: 431–446 nm), green (_max: 490–553 nm) and red (_max: 548–607 nm) parts of the spectrum, and a rod visual pigment with _max: 504–531 nm. The youngest fish (yolk-sac alevins) did not have blue visual pigment, but only UV pigment in the single cones. Older juveniles (smolts) had predominantly single cones with blue visual pigment. Coho and chinook smolts (>1 year old) switched from a vitamin A₁- to a vitamin A₂-dominated retina during the spring, while the retina of chum smolts and that of the younger alevin-to-parr coho did not. Adult spawners caught during the Fall had vitamin A₂-dominated retinas. The central retina of all species had three types of double cones (large, medium and small). The small double cones were situated toward the ventral retina and had lower red visual pigment _max than that of medium and large double cones, which were found more dorsally. Temperature affected visual pigment _max during smoltification.