Shedding of rod outer segments is light-driven in goldfish.

Rod outer segment (ROS) shedding in goldfish was quantified by measuring the number of phagosomes in the retinas of goldfish that were maintained under natural or artificial cyclic light, constant light, or constant dark conditions. Fish maintained in cyclic light, whether natural or artificial, had robust daily rhythms of ROS shedding. The ROS tips were shed primarily during the light phase of the cycle, and maximum shedding occurred 2-4 hours into the light period. Fish maintained for 1, 3, or 7 days in constant light or constant dark had no daily rhythms in ROS shedding. In these fish, ROSs lengthened, on average, 2.3 microns/day in constant light and 1.5 microns/day in constant dark. After 3 days in constant light, shedding was induced by placing fish in darkness for 2 hours, then returning them to light. Placing fish in darkness for 0.5 hours did not induce shedding, nor did placing them in darkness for 3 hours without returning them to light. ROS shedding thus appears to be goldfish is completely dependent on changes in ambient illumination; no circadian or endogenous components were found. Previous observations of circadian changes in behavioral visual sensitivity therefore cannot be due to endogenous changes in ROS length.