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Melatonin is a ubiquitous and conservative biogenic amine implicated in a variety of bio-physiological processes. Although the daily variations of melatonin concentrations have been determined in many crustacean species, results have been conflicting. The present study was undertaken to evaluate the effect of different photoperiods on the circadian rhythms of melatonin in Eriocheir sinensis by using high performance liquid chromatography. The circadian rhythms of melatonin in the eyestalk of E. sinensis were influenced by external light stimulation. When under consistent light (24L:0D) and consistent dark (0L:24D), the melatonin rhythm in eyestalks and hemolymph was disturbed with respect to that of the control group (12L:12D). However, disturbance owing to different photoperiod treatments decreased with increasing treatment duration. Our results underscore the exquisite sensitivity of melatonin content to different photoperiods. These findings may have real-world implications in terms of the disruption of circadian rhythms induced by exposure to different photoperiods. [ABSTRACT FROM AUTHOR]

This study is the first to examine the circadian rhythms of melatonin in Eriocheir sinensis and Palaemonetes sinensis, two economically important crustaceans. We collected haemolymph and optic lobes from both species every 4 h over a whole day cycle. Melatonin content was measured with high-performance liquid chromatography. E. sinensis haemolymph exhibited significant (p < 0.05) peaks in melatonin at 16:00 (0.180 ± 0.020 μg·mL−1) and 24:00 (0.244 ± 0.055 μg·mL−1), while eyestalks had significant peaks at 16:00 (72.377 ± 18.100 μg·eyestalk−1) and 24:00 (98.756 ± 30.271 μg·eyestalk−1). In P. sinensis, melatonin peaked significantly only at 16:00 in optic lobes (12.493 ± 1.475 μg·eyestalk−1) (p < 0.05); no significant peaks were present in haemolymph. Thus, both E. sinensis and P. sinensis exhibit species-specific melatonin rhythms. Time of day should therefore be considered when examining the physiological status of both crustaceans, given the potential influence of fluctuating daily melatonin concentrations. [ABSTRACT FROM AUTHOR]

Many physiological functions of crustaceans show a rhythmic change to adapt to daily environmental cycles. However, daily variation in the immune and antioxidant status and its possible correlation with circulatory melatonin levels during the daily cycle have not been reported in the Chinese mitten crab, Eriocheir sinensis. In this study, the specific activities of immune and antioxidant enzymes of E. sinensis during the 24 h cycle and its relationship with injected doses of melatonin were evaluated. The results showed that the immune parameters in the hemolymph, such as total hemolymph count, alkaline phosphatase, lysozyme, acid phosphatase, and phenol oxidase, exhibited bimodal patterns during the 24 h cycle, these parameters were synchronized with the activity of antioxidant enzymes such as malondialdehyde (MDA), superoxide dismutase (SOD), glutathione peroxidase, and catalase. However, there was only one peak in the muscle (during 1200–1600 h) and gills (during 0400–0800 h). The survival rate reached approximately 80% in 5 days when melatonin concentrations were lower than 0.05 g/L, significantly decreasing as melatonin concentrations increased. Four hours after melatonin injection, MDA levels in the muscle and hemolymph were significantly lower than those in the control group. Eight hours after melatonin injection, SOD levels in the hemolymph were significantly higher than those in the control group. These findings highlight the importance of considering circadian regulation of innate immunity when comparing immune responses at fixed times. • Circadian rhythms in Eriocheir sinensis were studied. • Evaluation of specific activities of immune and antioxidant enzymes in E. sinensis. • Results showed immune items in the hemolymph exhibited bimodal patterns during 24 h. • These patterns were synchronized with the activity of antioxidant enzymes. • Melatonin can improve antioxidant capacity of E. sinensis 4–8 h after treatment. [ABSTRACT FROM AUTHOR]

Abstract The eyestalks of crustaceans play an essential role in controlling a variety of physiological functions by converting light into hormonal signals. To obtain a more complete description of eyestalk biology in the commercially important Chinese mitten crab (Eriocheir sinensis), we conducted comparative transcriptome analysis of eyestalks during the day and at night using high-throughput sequencing on an Illumina HiSeq 4000 platform. We obtained 47,092 unigenes—including 4771 differentially expressed genes (DEGs)—from eyestalks during the day and at night. We found that 4269 DEGs were upregulated during the day and 502 DEGs were upregulated at night. We identified five DEGs that may contribute to molting, including molt-inhibiting hormone, cuticle, catalase, aquaporin, and ubiquitin-conjugating enzyme; hence, similar to other crustaceans, Eriocheir sinensis may molt at night. We further identified eight DEGs related to behavior regulation, including three glutamate dehydrogenase genes that were upregulated during the day. Thus, changes in the eyestalks may partially compensate for daily changes in illumination in the Chinese mitten crab's normal environment. Our present study is the first genome-wide transcriptome analysis of the eyestalks of Eriocheir sinensis during the day and at night. Our findings provide a valuable insight into the molecular basis of circadian cycle regulation in crustaceans. Highlights • This is the first transcriptome analysis of crustacean eyestalks by day and night. • We used the commercially important Chinese mitten crab (Eriocheir sinensis). • Of 47,092 unigenes obtained from eyestalks, 4771 were differentially expressed. • These included 5 DEGs associated with molting, and 8 DEGs related to behavior. • The data reveal the molecular basis of circadian cycle regulation in crustaceans. [ABSTRACT FROM AUTHOR]