Two mechanisms of retinal photoreceptor plasticity underlie rapid adaptation to novel light environments.

Fishes experience different light environments over short time periods that may require quick modulation of photoreceptor properties to optimize visual function. Previous research has shown that the relative expression of different visual pigment protein (opsin) transcripts can change within several days following exposure to new light environments, but whether such changes are mirrored by analogous modulation in opsin protein expression is unknown. Here, Atlantic halibut larvae and juveniles raised under white light were exposed to blue light for 1 week and their retina compared to that of controls, which remained under white light. Blue light-treated larvae showed increased expression of all cone opsin transcripts, except rh2, over controls. They also had longer outer segments, and higher density of long wavelength sensitive (L) cones in the dorsal retina. In contrast, only the lws transcript was upregulated in juveniles exposed to blue light compared to controls but their L cone density was greater throughout the retina. These results demonstrate two mechanisms of rapid photoreceptor plasticity as a function of developmental stage associated with improved perception of achromatic or chromatic contrasts in line with the animal's ecological needs.