Use of the Clock for Adjustment to Tidal and Lunar Cycles

Eggs of Dictyota dichotoma were liberated in a rhythm with periods of about 16 days under laboratory conditions (light-dark cycles of 14 : 10 hr). An “artificial moon”, i. e. replacement of the 10 hr dark period by light, controls the phases of the rhythm, even if this “moonlight” is of low intensity (3 lux) and lasts for one night only. The next maximum appears 9 days after this “moon”. Lowering the temperature from 20° to 14 ° or 16° has no strong effect on the normal length of these cycles. Exposure to “wrong” days (light+dark period 23,5 hr instead of 24 hr) causes a shortening of the periods to 13-14 days. The results can be explained by assuming the cooperation of two rhythms, giving beats with distances of a little more than 2 weeks. One of these two rhythms should correspond approximately to the periods of the tidal cycles (about 12,4 hr), the other one to the diurnal periodicity. Entrainment of the diurnal component to 23,5 instead of 24 hr explains the decreased distance of the beats. For Carcinus maenas it can be shown that in the periodicity of the running activity 2 rhythms with the postulated properties are participating.