Ultraviolet Light from the Nighttime Sky Enhances Retinal Sensitivity of Limulus.

Abstract

Here we report that stimulation of the median ocelli of the horseshoe crab, Limulus polyphemus, by UV light from the nighttime sky enhances the sensitivity of the lateral eyes to visible light. At night a circadian clock in the brain of Limulus generates and transmits neural activity via efferent optic nerve fibers to the lateral eyes and median ocelli (8), significantly increasing their sensitivity (9,lO). Experiments carried out in the laboratory show that illumination of the median ocelli at night with UV light enhances the clock’s efferent output and thus further increases the sensitivity of the lateral eyes and median ocelli (11). We tested whether this occurs under natural conditions; that is, whether ocellar exposure to UV light from the nighttime sky increases visual sensitivity of the lateral eyes. We carried out our experiments in an outdoor shed located in a remote area of Woods Hole, MA, where no artificial light was detectable at night. We mounted animals securely to a platform in an aquarium so that their gills were irrigated with recirculated seawater, but their lateral eyes and median ocelli remained above water. An opaque chamber was placed over the right lateral eye of each animal to occlude the eye from ambient light. A wick electrode and light emitting diode (LED) within the chamber allowed us to record electroretinograms (ERGS) from the eye in its dark-adapted state while the animal was exposed to normal diurnal changes in illumination. A computerbased system recorded ERGS every 30 min in response to a 10 ms light flash from the LED (12). Changes in the amplitude of the ERG of the occluded right eye provided a measure of the clock’s effect on retinal sensitivity and the ocellar influence on the clock’s action. Animals (n = 7) were maintained in this apparatus for approximately one month. During this period, ERGS were monitored while the median ocelli were either fully exposed, covered by a UV blocking filter (blocks y < 420 nm), a UV bandpass filter (ymaX = 360 nm, 54 nm bandwidth at half maximum) or occluded to block all incident light. Figure 1 shows the ERG data recorded from the lateral eye of an animal during five days of a month-long experiment (1990). The amplitude of the ERG increased at night and decreased during the day. The extent of the change depended, in part, on the light incident on the median ocelli. Fully exposing the ocelli to the nighttime sky on 19