Use of the electroretinogram to study how changes in photoperiod affect visual sensitivity in the lateral eye of Limulus polyphemus

In 1986, Penn and Williams showed that albino rats homeostatically regulate quantum catch such that approximately the same number of photons get absorbed each day, a phenomenon which they termed photostasis. Since their seminal study, other researchers have corroborated their findings, and there has even been evidence to suggest that photostasis may be a ubiquitous process that occurs in a variety of species. This study investigates how changes in photoperiod affect the response of the lateral eye of the American horseshoe crab, Limulus polyphemus . Horseshoe crabs were entrained to a series of lighting cycles in which day length, light intensity or daily photon flux was systematically varied. In these experiments, the electroretinogram (ERG) was regularly monitored, and its amplitude provided a relative measure of quantum catch. Intensity-response functions were recorded in each diurnal cycle, and relative shifts of these functions signified changes in retinal sensitivity. Reducing light intensity or shortening day length consistently caused the response of the lateral eye to increase. Anatomical studies suggest that the changes in quantum catch were mediated by adjustments in retinal structure. In contrast, varying daily photon flux produced no difference in the lateral eye\u27s response so long as implied day length and light intensity remained constant. These findings indicate that photostasis does not occur in horseshoe crabs. Rather, it is proposed that the changes in the lateral eye\u27s sensitivity were the result of modifications of the dawn and dusk zeitgeber signals. The lateral eye exhibits a robust circadian rhythm in retinal structure, and this rhythm is amplified by light. This study also advances our understanding of the control mechanisms involved in this process by demonstrating that the enhanced movements of the aperture and rhabdom are light-triggered and not light-driven. Fifteen minutes of morning light is sufficient for amplifying the endogenous rhythm in retinal structure; however, longer lengths of light exposure may be necessary to cause the aperture and rhabdom to elongate to the same extent as normally occurs in continuous daylight