The sea worm’s internal circalunar clock can be demonstrated by the lunar equivalent of free-running circadian experiments. The circalunar clock must first be entrained, not by sunlight, but by moonlight (or in the laboratory by exposure to dim light for a few hours at night). If, after a period of entrainment, worms are kept in the laboratory under a constant day-night cycle, they nevertheless exhibit a 30-day reproductive rhythm. How do these worms keep track of this 30-day cycle? Do they use their circadian clock as a pendulum with a period of 1 day and count up to 30?
