Effects of Complete and Skeleton Photoperiods on the Circadian Rhythm of the Electroretinogram of the Crayfish

Abstract

Applying complete photoperiods (PPc), with photophases that varied from 4 to 20 h of the 24-h cycle, we studied entrainment of the circadian rhythm of the electroretinogram (ERG) of crayfish by external light signals. All the PPc regimens tested synchronized the rhythm, which adjusted its circadian period to exactly 24 h; the duration of the photophase affected only other circadian parameters, such as the activity level and the ratio of the duration of active to resting phases. Entrainment of the ERG by skeleton photoperiods (PPs) was also studied. All the PPs tested (each PPs corresponded to a specific PPc regimen) also entrained the ERG rhythm. The use of PPs revealed that the bistability zone of the rhythm lay between light-dark (LD) 11:13 and LD 13:11. When the interval of darkness between the periods of illumination during PPs became less than 11 h, the circadian rhythm of the ERG exhibited a “jump” phase in which the maximal activity period jumped to the period of longer darkness; this result implies that the crayfish requires a minimum tolerable night.