Adult crayfish exhibit a variety of overt circadian rhythms. However, the physiological mechanisms underlying the overt rhythms are controversial. Research has centered on two overt rhythms: the motor activity and the retinal sensitivity rhythms of the genus Procambarus. The present work reviews various studies undertaken to localize pacemakers and mechanisms of entrainment responsible for these two rhythms in adult organisms of this crustacean decapod. It also describes an ontogenetic approach to the problem by means of behavioral, electrophysiological, and neurochemical experiments. The results of this approach confirm previous models proposed for adult crayfish, based on a number of circadian pacemakers distributed in the central nervous system. However, the coupling of rhythmicity between these independent oscillators might be complex and dependent on the interaction between serotonin (5-HT), light, and the crustacean hyperglycemic hormone (CHH). The latter compound has, up until now, not been considered as an agent in the genesis and synchronization of the retinal sensitivity rhythm.
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