Evaluation of Circadian Rhythms in the Limulus Eye.

The visual system of the horseshoe crab, Limulus polyphemus, shows a remarkable circadian rhythm in sensitivity. About the time of sunset, a circadian oscillator in the brain of the animal generates efferent optic nerve signals to the lateral eye modulating both its structure and function. The overall increase in sensitivity nearly compensates for the reduction in ambient light at night (1, 2). High nighttime visual sensitivity appears to be important for mating, a visually guided behavior that the animals can accomplish as well at night as in the day (3). The mechanisms underlying the elegant adaptation of the Limulus eye to dim nighttime illumination include anatomical changes in the retina that increase photon catch at the expense of spatial resolution, and physiological changes that enhance the summation of photon events at the expense of temporal resolution. Here we evaluate further the effect of the circadian clock on two