CIRCADIAN-RHYTHMS IN TELEOST RETINOMOTOR MOVEMENTS - A COMPARISON OF THE EFFECTS OF CIRCADIAN-RHYTHM AND LIGHT CONDITION ON CONE LENGTH

Abstract

The long, slender cones of the teleost retina elongate at night and contract during the day. In the Midas cichlid, Cichlasoma citrinellum, this cone excursion is elicited by changes in light conditions and by strong endogenous circadian rhythms. In a normal day/night cycle C. citrinellum cones change length by 69 .mu.m. In this species an endogenous circadian rhythm induces substantial cone excursion in constant light and in constant darkness. Total excursion in constant light is 34% of that seen in a normal cycle. Total excursion in constant darkness is 58% of that seen in a normal cycle. Similar excursions occur on the 2nd and 3rd days of constant darkness. A change from light to darkness at a time in the cycle other than dusk induces elongation averaging 49% of the total excursion observed in the normal cycle. A change from darkness to light at a time other than dawn induces cone contraction averaging 30% of the total excursion observed in the normal cycle. The response of retinal cones to either of the above changes in light conditions is of relatively constant magnitude at all sampling times over 24 h. The endogenous circadian rhythm and the response to changes in light conditions of the cones are required to produce the full excursion observed in the normal day/night cycle. Full elongation of cones occurs only at night in darkness and full contraction of the cones occurs only in the day in light. Changing light conditions at inappropriate times produces intermediate cone lengths.