Phase relationship between ocular and behavioral circadian rhythms in Bulla gouldiana exposed to different photoperiods.

To determine whether the ocular circadian pacemakers of the marine snail Bulla gouldiana exert phase control over the circadian rhythm in locomotor activity, we measured the phase angles for entrainment of the ocular pacemakers and the activity rhythm on four different photoperiods: L:D 15:9, 12:12, 9:15, and 4:20. We found that the phase angle for ocular entrainment was progressively advanced relative to dawn as photoperiod decreased, although the phase was fixed relative to the middle of the day. In contrast, activity began near dusk on all photoperiods. On subsequent release into constant conditions, the free-running locomotor activity commenced near the time of previous activity onset. Thus, activity phase on light cycles represents the entrainment of a light sensitive pacemaker. The resulting lability in phase between ocular and behavioral rhythms in Bulla exposed to light cycles suggests that the ocular pacemakers are not the only determinants of locomotor activity phase in Bulla.