Circadian Rhythms in Limulus Visual Sensitivity Compensate for Day-Night Changes in Light Intensity

Abstract

At night a circadian clock transmits efferent optic nerve activity to the lateral eyes of Limulus polyphemus, increasing their sensitivity to light. The animal’s use of vision to find mates at night suggests that the clock’s effect on retinal sensitivity compensates for the nighttime decrease in ambient illumination. To test this possibility we recorded activity from optic nerve fibers in situ while the animal was exposed to day-night changes in illumination in its natural environment. Using a waterproof recording chamber we monitored the maintained activity of single or small clusters of optic nerve fibers. Placing an animal at a meter depth near the shoreline of Woods Hole, Massachusetts, enabled us to monitor optic nerve activity with a remote amplifier and recorder connected to the animal via a tether. A single optic nerve fiber typically responds to ambient daylight illumination at rates ranging from 4 to 14 impulses/s. In one experiment the steady-state firing rate was 4.7 ips in the early evening when the ambient intensity was 85 cd/m 2 . At 8 p.m. the rate increased to 9.2 ips as the light intensity decreased to 0.054 cd/m 2 . The rate was 11.8 ips under bright illumination the following day (22,000 cd/m 2 ) and dropped to 5.3 ips at 0.040 cd/m 2 that night. Similar results were recorded from animals that were placed in an aquarium next to a south-facing window in Woods Hole. The nearly 1,000,000fold decrease in ambient illumination at night appears to be associated with only about a two-fold decrease in optic nerve response. We conclude that the circadian clock’s input to the lateral eye nearly compensates for the diurnal changes in light intensity. Supported by NSF, NEI, NIMH, RPB and the Lions of Central New York.