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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Based on previous work in which we proposed midgut as a putative peripheral oscillator responsible for circadian reduced glutathione (GSH) crayfish status, herein we investigated the retina and optic lobe-brain (OL-B) circadian GSH system and its ability to deal with reactive oxygen species (ROS) produced as a consequence of metabolic rhythms and light variations. We characterized daily and antioxidant circadian variations of the different parameters of the glutathione system, including GSH, oxidized glutathione (GSSG), glutathione reductase (GR) and glutathione peroxidase (GPx), as well as metabolic and lipoperoxidative circadian oscillations in retina and OL-B, determining internal and external GSH-system synchrony. The results demonstrate statistically significant bi- and unimodal daily and circadian rhythms in all GSH-cycle parameters, substrates and enzymes in OL-B and retina, as well as an apparent direct effect of light on these rhythms, especially in the retina. The luminous condition appears to stimulate the GSH system to antagonize ROS and lipid peroxidation (LPO) daily and circadian rhythms occurring in both structures, oscillating with higher LPO under dark conditions. We suggest that the difference in the effect of light on GSH rhythmic mechanisms of both structures for antagonizing ROS could be due to differences in glutathione-system coupling strength with the circadian clock.

This work was carried out to study the antioxidant circadian system of two species of crayfish of different latitude origin. We investigated (1) whether both species possess glutathione circadian rhythms and (2) whether both species' rhythms differ in their ability to synchronize to 24 h cycles. Two batches of Procambarus clarkii and P. digueti were kept in (1) light–dark (LD) 12:12 low irradiance (LI) cycles and then exposed to (2) 72 h of complete darkness, (3) LD 12:12 high irradiance (HI), (4) LD 20:4 LI and (5) LD 20:4 HI for 2 weeks. The midgut and hemolymph were sampled and reduced and oxidized glutathione as well as glutathione reductase and glutathione peroxidase were assayed. Cosinor and analysis of variance revealed differences between both species. Procambarus clarkii robust antioxidant circadian rhythms are able to entrain to all conditions resetting to lights on or off. However, the P. digueti weak circadian glutathione system did not entrain to the LD cycles, showing a random distribution of phases. In this species, LD 12:12 and 20:4 HI evidenced significant daily rhythms indicating a damped circadian antioxidative system that is enhanced by the effect of light. This suggests that each species' photoperiodic history determines the adaptive abilities of the circadian antioxidative mechanisms.

The present study investigated the rhythmic changes in glutathione status in midgut gland and hemolymph as well as in glutathione reductase (GR) activity in the crayfish Procambarus clarkii. In order to determine the circadian nature of these rhythms different groups of crayfish were submitted to constant‐darkness conditions for 24 or 72 h after they had spent 15 days under light–dark 12:12 cycles. The animals of the different batches were killed at 6 h intervals during a 24 h cycle. Reduced glutathione (GSH) and oxidized glutathione (GSSG) in hemolymph and midgut as well as midgut GR activity were determined in midgut gland and hemolymph by fluorometric and spectrophotometric methods. Data analysis by chronogram and single Cosinor revealed circadian rhythmicity for GSH and GSSG concentration in both tissues as well as midgut GR activity. The rhythm parameters revealed oxidative stress induced by light. The possible correlation between the glutathione rhythm and other metabolic and behavioral rhythms of crayfish as well as the importance of the glutathione circadian temporal order in the adaptation of crayfish are discussed.