Timing of the metamorphic molt of the American lobster (Homarus americanus) is governed by a population-based, photoperiodically entrained daily rhythm

The metamorphic molt of the American lobster (Homarus americanus) occurs in the form of a population-based, recurring daily rhythm that is photoperiod ically entrained. Molting occurs predominantly in the scotophase (dark peri od) at temperatures of 11, 15, and 20.C and in both normal and reversed pho toperiod cycles. Molting is arrhythmic in larvae reared in continuous illum ination (LL), but when larvae reared in LL are transferred to cyclic photop eriod conditions (LD 12:12), the rhythm is reinstated in a stepwise manner over 3 days. When larvae are transferred from LD 12:12 to LL, the molting r hythm continues for 3 days before dampening. The rhythm appears to be phase -set by the onset of darkness: similar proportions of larvae (79-89%) reare d in scotophase lengths of 6, 10, and 12 h molted in the 12-h period follow ing lights-off. The results demonstrate that the synchrony of the metamorph ic molt with the scotophase is not fortuitous and suggest that the molting rhythm may result from a gated event under the influence of an endogenous p acemaker.