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

Abstract

The metamorphic molt of the American lobster (Homarus americanus) occurs in the form of a population-based, recurring daily rhythm that is photoperiodically entrained. Molting occurs predominantly in the scotophase (dark period) at temperatures of 11, 15, and 20°C and in both normal and reversed photoperiod cycles. Molting is arrhythmic in larvae reared in continuous illumination (LL), but when larvae reared in LL are transferred to cyclic photoperiod 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 rhythm continues for 3 days before dampening. The rhythm appears to be phase-set by the onset of darkness: similar proportions of larvae (79-89%) reared in scotophase lengths of 6, 10, and 12 h molted in the 12-h period following lights-off. The results demonstrate that the synchrony of the metamorphic 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 pacemaker.