I found a few studies in fish that showed constant light exposure changing rod density. First one in goldfish:
Raymond PA, Bassi CJ, Powers MK.
Lighting conditions and retinal development in goldfish: photoreceptor number and structure.
Invest Ophthalmol Vis Sci. 1988 Jan;29(1):27-36.
The retinas of 63 goldfish were examined after varying durations of exposure to one of three environmental lighting conditions beginning before hatching: constant light (340 lux), cyclic light (12 hr 320 lux, 12 hr dark) and constant dark. Up to 8 months, no effects of constant light or dark on photoreceptor numbers or structure were apparent. Densities of rod and cone nuclei were normal and all retinal layers appeared normal by light microscopy. Exposure to constant light for 12 months or longer resulted in a reduction in rod density by 37%. Cone numbers were unaffected by constant light, even with exposures of 3 yr, and rod and cone outer segments were normal in length at 11-20 months under all environmental conditions. Due to poor survival, only one animal was available for quantitative examination from the group reared in constant dark 12 months or longer. Photoreceptor size and number in this retina were similar to those in the constant light condition. The results suggest that the formation and maturation of rods and cones in goldfish retina is unaffected by rearing in constant light. However, long-term exposure (greater than or equal to 12 months) may disrupt maintenance of differentiated rods.
See also http://www.vision-in-cichlids.com/fenotypic_plasticity.htm where they say rod density changes in early retina development of Tilapia leucosticta.