Light intensity appears to be more important than an endogenous seasonal clock for regulating structural rhythms in the lateral eye of the horseshoe crab

Abstract

The lateral compound eye of the American horseshoe crab, Limulus polyphemus, is an established model system for studying visual and circadian processes. Previous studies have shown there is an endogenous rhythm in retinal structure and this rhythm is enhanced by light. The purpose of this study was to determine whether changes in natural lighting influence these rhythms independent of season. To test this concept, horseshoe crabs were maintained in an aquarium tank and exposed to natural lighting only. The angle of the sun and foliage outside the room where the horseshoe crabs were maintained were such that more natural lighting reached the aquarium tank during winter days than summer days despite the fact that sunlight is more intense in the summer. Horseshoe crabs were sacrificed 2 hours, 5 hours, and 8 hours following local sunrise on a winter day (January 11th) and a summer day (June 19th), respectively, and the dimensions of several retinal parameters were quantified. Preliminary data indicate that certain structural rhythms were significantly more pronounced on the winter day when natural lighting was more intense. For example, 8 hours following sunrise, apertures were 4.2 times as long and 3.2 times more narrow on the winter day than on the summer day. These data suggest that the intensity of natural lighting is more important for regulating structural rhythms in the lateral eye than an endogenous seasonal clock.