Daily variation of melatonin content in the optic lobes of the crab Neohelice granulata

A brief and updated introduction to the neuroendocrine system of crustaceans

The neuroendocrine system of crustaceans is complex and regulates many processes, such as development, growth, reproduction, osmoregulation, behavior, and metabolism. Once stimulated, crustaceans' neuroendocrine tissues modulate the release of monoamines, ecdysteroids, and neuropeptides that can act as hormones or neurotransmitters. Over a few decades, research has unraveled some mechanisms governing these processes, substantially contributing to understanding crustacean physiology. More aspects of crustacean neuroendocrinology are being comprehended with molecular biology, transcriptome, and genomics analyses. Hence, these studies will also significantly enhance the ability to cultivate decapods, such as crabs and shrimps, used as human food sources. In this review, current knowledge on crustacean endocrinology is updated with new findings about crustacean hormones, focusing mainly on the main neuroendocrine organs and their hormones and the effects of these molecules regulating metabolism, growth, reproduction, and color adaptation. New evidence about vertebrate-type hormones found in crustaceans is included and discussed. Finally, this review may assist in understanding how the emerging chemicals of environmental concern can potentially impair and disrupt crustacean's endocrine functions and their physiology.

Effects of Photoperiod on Survival, Growth, Physiological, and Biochemical Indices of Redclaw Crayfish (Cherax quadricarinatus) Juveniles

Through a 30-day experiment, this study investigated the effects of five photoperiods (0L:24D, 6L:18D, 12L:12D, 18L:6D, and 24L:0D) on the survival, enzyme activity, body color, and growth-related gene expression of redclaw crayfish (Cherax quadricarinatus) juveniles. The results showed that C. quadricarinatus juveniles under 18L:6D and 24L:0D photoperiods exhibited the highest survival rate, which was significantly higher than the survival rates of juveniles under the other three photoperiods (p < 0.05). However, the 0L:24D group had the highest final body weight and weight gain rate, significantly surpassing those of the 12L:12D, 18L:6D, and 24L:0D groups (p < 0.05). Regarding enzyme activity and hormone levels, juveniles under the 18L:6D photoperiod exhibited relatively higher activity of superoxide dismutase (SOD), acid phosphatase (ACP), and lysozyme (LZM) enzymes than those under other photoperiods, but their levels of melatonin and cortisol were relatively low. In addition, the 24L:0D group showed the highest malondialdehyde (MDA) content. Analysis of gene expression levels revealed that retinoid X receptor (RXR) and α-amylase (α-AMY) genes in C. quadricarinatus juveniles exhibited significantly higher expression levels under the 18L:6D photoperiod than those under the other four photoperiods (p < 0.05). With increasing daylight exposure, the body color of C. quadricarinatus changed from pale blue to yellow-brown. In summary, C. quadricarinatus juveniles achieved high survival rates, good growth performance, strong antioxidant stress response, and immune defense capabilities under an 18 h photoperiod. Therefore, in the industrial seedling cultivation of redclaw crayfish, it is recommended to provide 18 h of daily light. Further, the study demonstrated the ability to manipulate the body color of C. quadricarinatus through controlled artificial photoperiods. These findings provide essential technical parameters needed for the industrial cultivation of C. quadricarinatus juveniles.

Effects of dietary melatonin on antioxidant and immune function of the Pacific white shrimp (Litopenaeus vannamei), as determined by transcriptomic analysis

Melatonin (MT) is regarded as an antioxidant and immunostimulant that can efficiently scavenge free radicals and activate antioxidant enzymes. The aim of this study was to investigate the effects of dietary MT on the growth performance and immune function of the Pacific white shrimp (Litopenaeus vannamei). Six groups of L. vannamei were supplemented with dietary MT at 0, 22.5, 41.2, 82.7, 165.1, and 329.2 mg/kg levels for 2 months. RNA-Seq analysis was performed to obtain transcriptome data of the control group and the group supplemented with dietary MT at 82.7 mg/kg BW. In total, 1220 DEGs (799 up-regulated and 421 down-regulated) were identified. Pathways and genes related to growth performance and immune function were verified by real-time quantitative polymerase chain reaction. The total hemocyte count, phagocytosis rate, and respiratory burst were significantly increased in the MT (82.7 mg/kg BW) group as compared to the control group. Analysis of antioxidant-related enzymes in the hepatopancreas showed that dietary MT (82.7 mg/kg BW) significantly increased activities of superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase, while dietary MT at 41.2 mg/kg BW significantly increased activities of glutathione S-transferase, lysozyme (LZM), and phenoloxidase (PO). At the transcriptional level, dietary MT up-regulated expression levels of genes associated with antioxidant immunity and growth, which included PO, SOD, LZM, GPx, chitin synthase, ecdysone receptor, calcium-calmodulin dependent protein kinase I, and retinoid X receptor. In conclusion, dietary MT may improve the growth performance and immune function of L. vannamei to some extent.

CasEcR and CasMIH Genes in the Blue Crab, Callinectes sapidus: A Temporal Evaluation and Melatonin Effects

Environmental cues synchronize endogenous rhythms of many physiological processes such as hormone synthesis and secretion. Little is known about the diurnal pattern of hormones and gene expression of the Callinectes sapidus molt cycle. We aimed to investigate in the eyestalk and hepatopancreas of premolt and intermolt C. sapidus the following parameters: 1) the diurnal expression of the ecdysteroid receptor CasEcR isoforms, and the molt inhibiting hormone CasMIH; 2) the diurnal hemolymph ecdysteroid and melatonin levels; and 3) melatonin effects on the transcripts of the above-mentioned genes in intermolt C. sapidus. Ecdysteroid levels were higher in the premolt than the intermolt animals at all time points evaluated (ZTs). Premolt crabs displayed a variation of ecdysteroid concentration between time points, with a reduction at ZT17. No difference in the melatonin level was seen in either molt stage or between stages. In the eyestalk of intermolt animals, CasEcR expression oscillated, with a peak at ZT9, and premolt crabs have a reduction at ZT9; CasMIH transcripts did not vary along 24 h in either molt stage. Moreover, the evaluated eyestalk genes were more expressed at ZT9 in the intermolt than the premolt crabs. In the hepatopancreas, CasEcR expression showed a peak at ZT9 in premolt crabs. Exogenous melatonin (10⁻⁷ mol/animal) reduced the expression of both genes in the eyestalk at ZT17. In the hepatopancreas, melatonin markedly increased the expression of the CasEcR gene at ZT9. Taken altogether, our results are pioneer in demonstrating the daily oscillation of gene expression associated to molt cycle stages, as well as the daily ecdysteroid and melatonin levels and the remarkable influence of melatonin on the molt cycle of C. sapidus.

Melatonin: A mitochondrial resident with a diverse skill set

Melatonin is an ancient molecule that originated in bacteria. When these prokaryotes were phagocytized by early eukaryotes, they eventually developed into mitochondria and chloroplasts. These new organelles retained the melatonin synthetic capacity of their forerunners such that all present-day animal and plant cells may produce melatonin in their mitochondria and chloroplasts. Melatonin concentrations are higher in mitochondria than in other subcellular compartments. Isolated mouse oocyte mitochondria form melatonin when they are incubated with serotonin, a necessary precursor. Oocyte mitochondria subsequently give rise to these organelles in all adult vertebrate cells where they continue to synthesize melatonin. The enzymes that convert serotonin to melatonin, i.e., arylalkylamine-N-acetyltransferase (AANAT) and acetylserotonin-O-methyltransferase, have been identified in brain mitochondria which, when incubated with serotonin, also form melatonin. Melatonin is a potent antioxidant and anti-cancer agent and is optimally positioned in mitochondria to aid in the maintenance of oxidative homeostasis and to reduce cancer cell transformation. Melatonin stimulates the transfer of mitochondria from healthy cells to damaged cells via tunneling nanotubes. Melatonin also regulates the major NAD⁺-dependent deacetylase, sirtuin 3, in the mitochondria. Disruptions of mitochondrial melatonin synthesis may contribute to a number of mitochondria-related diseases, as discussed in this review.

Litopenaeus vannamei BMAL1 Is a Critical Mediator Regulating the Expression of Glucose Transporters and Can Be Suppressed by Constant Darkness

Simple Summary

Growing evidence has indicated that glucose absorption exhibits profound circadian rhythmicity, mediated entirely by glucose transporters. We observed that the daily profile of BMAL1, GLUT1 and SGLT1 expression was also synchronized in the intestine and the hepatopancreas of Litopenaeus vannamei. Our result identified for the first time that BMAL1 is a critical mediator regulating the expression of glucose transporters, which could be suppressed by constant darkness in L. vannamei.

Abstract

Aryl hydrocarbon receptor nuclear translocator-like protein 1 (BMAL1) is a core circadian transcription factor that controls the 24-h cycle of physiological processes. In shrimp, the role of BMAL1 in the regulating glucose metabolism remains unclear. Firstly, we observed that the daily profile of BMAL1, GLUT1 and SGLT1 expression were synchronized in the intestine and the hepatopancreas of Litopenaeus vannamei. Then we examined the effects of BMAL1 on the gene expression of glucose transporter type 1 (SGLT1) and sodium-glucose cotransporter 1 (GLUT1) in vivo and in vitro. BMAL1 in L. vannamei shares 70.91–96.35% of sequence identities with other shrimp species and possesses the conserved helix-loop-helix domain and polyadenylation site domain. The in vitro dual-luciferase reporter assay and in vivo RNA interference experiment demonstrated that BMAL1 exerted a positive regulation effect on the expression of glucose transporters in L. vannamei. Moreover, we conducted an eight-week treatment to investigate whether light/dark cycle change would influence growth performance, and gene expression of BMAL1, GLUT1 and SGLT1 in L. vannamei. Our result showed that compared with natural light treatment, constant darkness (24-h darkness) significantly decreased (p < 0.05) serum glucose concentration, and suppressed (p < 0.05) the gene expression of BMAL1, GLUT1 and SGLT1 in the hepatopancreas and the intestine. Growth performance and survival rate were also decreased (p < 0.05) by constant darkness treatment. Our result identified BMAL1 as a critical mediator regulating the expression of glucose transporters, which could be suppressed by constant darkness in L. vannamei. It would be quite interesting to explore the mechanism of dark/light cycles on glucose transport and metabolism in L. vannamei, which might provide a feeding strategy for improving carbohydrate utilization in the future.

Artificial light at night may increase the predation pressure in a salt marsh keystone species

Artificial light at night (ALAN) has the potential to alter ecological processes such as the natural dynamics of predator-prey interactions. Although understanding of ALAN effect on faunal groups has increased in recent years, few studies have explicitly tested for direct consequences of ALAN on predator-prey systems. Here, we evaluated the effect of ALAN on juvenile mortality due to cannibalism and general predation of the South American intertidal burrowing crab Neohelice granulata, a key ecosystem engineer of salt marshes. For this, we conducted tethering and crab enclosure experiments for both night and day periods during successive tidal floods in a semidiurnal tidal regime. Both experimental approaches were deployed simultaneously in the field and they lasted four consecutive days during new moon nights. ALAN was simulated by a white LED lamp (30W) with a solar panel as a source of power in five separated areas selected as replicates. For general predation, juvenile survival under ALAN was 44% lower than during the daytime and 61% lower than under natural dark conditions. For cannibalism, juvenile survival under ALAN and during the daytime was similar and about 30% lower than under natural dark conditions. We also found that the abundance of adult male crabs (cannibals) under ALAN was nearly five times higher than at natural dark conditions. Our field experiments provide evidence that ALAN can increase the mortality of juvenile crabs and is at least partially driven by cannibalistic interactions.

The magnitude of behavioural responses to artificial light at night depends on the ecological context in a coastal marine ecosystem engineer

Artificial light at night (ALAN) is one of the most extensive human geographic disturbances to wildlife. ALAN can have ecological and evolutionary effects on individual organisms, which in turn can affect populations, communities and ecosystems. Although understanding of the effects of ALAN on the ecology and biology of organisms has increased in recent years, most of these advances are in terrestrial environments, but scarce in marine habitats, especially in ecologically important transition areas such as saltmarshes. Here, we study the effects of ALAN on the behavioural budget (i.e. the proportion of time spent performing feeding, burrow maintenance and concealment) of the South American intertidal crab Neohelice granulata, which is an ecosystem engineer of coastal salt marshes. Moreover, we compared the impact of a gradient of ALAN between two different saltmarshes with contrasting environmental characteristics. Our results showed a relationship between ALAN and the behavioural budget. In particular, we showed that an increase in ALAN drove an increase in time spent maintaining burrows at the expense of time spent concealed in the burrow or feeding outside it. Such effects showed slightly different patterns in the two saltmarshes, possibly related to the reproductive value of burrows for mating and to predation risk. Considering the ecosystem role of N. granulata, we argue that the different effect of ALAN on its behavioural budget could have ecosystem effects that differ between the two saltmarshes studied here.

The Hyperglycemic Effect of Melatonin in the Chinese Mitten Crab, Eriocheir sinensis

Melatonin has been identified in a variety of invertebrate species, but its function is not as well understood as in crustaceans. The effects of melatonin on hemolymph glucose levels and tissue carbohydrate metabolism in the Chinese mitten crab, Eriocheir sinensis, were fully investigated in this study. Moreover, whether the eyestalk (an important endocrine center in invertebrate species) involves in this process or not, also were clarified. Analysis revealed that eyestalk ablation, especially bilateral, caused a significant decrease in the hemolymph glucose level. Moreover, injection of melatonin induced hyperglycemia in a dose-dependent manner both in intact and ablated crabs. Based on the expression of CHH mRNA in the 10 different tissues, eyestalk, thoracic ganglion, intestinal tract and hemolymph were selected to estimate the effect of melatonin on the expression of CHH mRNA. Bilateral eyestalk ablation caused a significant increase in the expression of CHH mRNA in the thoracic ganglion, intestinal tract and hemolymph compared with the controls. In addition, injection of melatonin into intact or ablated crabs elevated the CHH mRNA level in the eyestalk, thoracic ganglion and intestinal tract tissues compared with controls. The hemolymph CHH mRNA after melatonin injection was elevated only in ablated crabs. Administration of melatonin resulted in a significant decrease in total carbohydrates and glycogen levels with an increase in phosphorylase activity levels in the hepatopancreas and muscle in intact and ablated crabs. Our findings demonstrated that melatonin can induce hyperglycemic effects in both intact and ablated crabs, suggesting that this effect is probably not mediated solely via eyestalk.

Circadian rhythm in melatonin release as a mechanism to reinforce the temporal organization of the circadian system in crayfish

Melatonin (MEL) is a conserved molecule with respect to its synthesis pathway and functions. In crayfish, MEL content in eyestalks (Ey) increases at night under the photoperiod, and this indoleamine synchronizes the circadian rhythm of electroretinogram amplitude, which is expressed by retinas and controlled by the cerebroid ganglion (CG). The aim of this study was to determine whether MEL content in eyestalks and CG or circulating MEL in hemolymph (He) follows a circadian rhythm under a free-running condition; in addition, it was tested whether MEL might directly influence the spontaneous electrical activity of the CG. Crayfish were maintained under constant darkness and temperature, a condition suitable for studying the intrinsic properties of circadian systems. MEL was quantified in samples obtained from He, Ey, and CG by means of an enzyme-linked immunosorbent assay, and the effect of exogenous MEL on CG spontaneous activity was evaluated by electrophysiological recording. Variation of MEL content in He, Ey, and CG followed a circadian rhythm that peaked at the same circadian time (CT). In addition, a single dose of MEL injected into the crayfish at different CTs reduced the level of spontaneous electrical activity in the CG. Results suggest that the circadian increase in MEL content directly affects the CG, reducing its spontaneous electrical activity, and that MEL might act as a periodical signal to reinforce the organization of the circadian system in crayfish.

Effects of hypoxia and reoxygenation on the antioxidant defense system of the locomotor muscle of the crab Neohelice granulata (Decapoda, Varunidae)

Crustaceans often occur in areas with variations in oxygen and experience situations known as hypoxia and reoxygenation. Consequences of such situations are increased levels of reactive oxygen species. To avoid oxidative damage intertidal crabs appear to possess an efficient antioxidant defense system (ADS). However, to date, studies have not addressed the strategies that are adopted by the crabs when exposed to hypoxia/reoxygenation cycles. Towards this end we evaluated the ADS and the role of melatonin as an antioxidant in the locomotor muscle of the crab Neohelice granulata under conditions of severe hypoxia and reoxygenation. Total antioxidant capacity against peroxyl radicals and the enzymes superoxide dismutase, catalase, glutathione peroxidase (GPx), and glutathione-S-transferase as well as the key enzyme of glutathione synthesis, glutamate cysteine ligase (GCL), were evaluated. Furthermore, GSH, GSH/GSSG index as well as hemolymph and cellular melatonin levels were evaluated. During hypoxia, increased GPx and GCL activity and decreased GSH and mitochondrial melatonin levels were observed, but during reoxygenation catalase activity increased and cytosolic melatonin levels decreased. It appears that the ADS in the locomotor muscle of N. granulata exert a modulating effect when being confronted with hypoxia and reoxygenation to avoid oxidative stress. During hypoxia, the ADS appear to target GPX activity as well as GSH and mitochondrial melatonin. During reoxygenation, however, evidence suggests that catalase and cytosolic melatonin are involved in the recovery of the locomotor muscle from oxidative damage and the suppression of further damage.

Induction of ecdysteroidogenesis, methyl farnesoate synthesis and expression of ecdysteroid receptor and retinoid X receptor in the hepatopancreas and ovary of the giant mud crab, Scylla serrata by melatonin

Melatonin, a chronobiotic molecule, is known to modulate several physiological functions in crustaceans including reproduction, molting and glucose homeostasis. In our earlier studies (Sainath and Reddy, 2010a), we observed hyperglycemia in crabs after melatonin administration and concluded that melatonin is another crustacean hyperglycemic hormone. In the current study, we have further examined the role of melatonin in regulating the levels of methyl farnesoate and ecdysteroid in the giant mud crab Scylla serrata and determined that melatonin indeed is a reproductive hormone. Further, we have determined partial nucleotide sequences of retinoid X receptor (RXR) and ecdysone receptor (EcR) in S. serrata and also studied the effect of melatonin on expression of these genes. Cloned RXR and EcR possess high sequence similarity with other Brachyuran genes. Administration of melatonin elevated circulatory methyl farnesoate (MF) and ecdysteroid levels in crabs. Since MF and ecdysteroid act through RXR and EcR respectively and these receptors are involved in the regulation of reproduction in crustaceans, we measured the expression levels of RXR and EcR in hepatopancreas and ovary after melatonin administration. The expression levels of both RXR and EcR increased significantly in the hepatopancreas and ovary of melatonin injected crabs when compared to the controls. In vitro culture of mandibular organ (MO) and Y-organ (YO) in the presence of melatonin resulted in a significant increase in the secretion of methyl farnesoate and ecdysteroid respectively. From the above studies it is clear that melatonin stimulates YO and MO, resulting in increased synthesis of ecdysteroids and methyl farnesoate, and thereby inducing reproduction in S. serrata.

Structures and functions of insect arylalkylamine N-acetyltransferase (iaaNAT); a key enzyme for physiological and behavioral switch in arthropods

The evolution of N-acetyltransfeases (NATs) seems complex. Vertebrate arylalkylamine N-acetyltransferase (aaNAT) has been extensively studied since it leads to the synthesis of melatonin, a multifunctional neurohormone prevalent in photoreceptor cells, and is known as a chemical token of the night. Melatonin also serves as a scavenger for reactive oxygen species. This is also true with invertebrates. NAT therefore has distinct functional implications in circadian function, as timezymes (aaNAT), and also xenobiotic reactions (arylamine NAT or simply NAT). NATs belong to a broader enzyme group, the GCN5-related N-acetyltransferase superfamily. Due to low sequence homology and a seemingly fast rate of structural differentiation, the nomenclature for NATs can be confusing. The advent of bioinformatics, however, has helped to classify this group of enzymes; vertebrates have two distinct subgroups, the timezyme type and the xenobiotic type, which has a wider substrate range including imidazolamine, pharmacological drugs, environmental toxicants and even histone. Insect aaNAT (iaaNAT) form their own clade in the phylogeny, distinct from vertebrate aaNATs. Arthropods are unique, since the phylum has exoskeleton in which quinones derived from N-acetylated monoamines function in coupling chitin and arthropodins. Monoamine oxidase (MAO) activity is limited in insects, but NAT-mediated degradation prevails. However, unexpectedly iaaNAT occurs not only among arthropods but also among basal deuterostomia, and is therefore more apomorphic. Our analyses illustrate that iaaNATs has unique physiological roles but at the same time it plays a role in a timezyme function, at least in photoperiodism. Photoperiodism has been considered as a function of circadian system but the detailed molecular mechanism is not well understood. We propose a molecular hypothesis for photoperiodism in Antheraea pernyi based on the transcription regulation of NAT interlocked by the circadian system. Therefore, the enzyme plays both unique and universal roles in insects. The unique role of iaaNATs in physiological regulation urges the targeting of this system for integrated pest management (IPM). We indeed showed a successful example of chemical compound screening with reconstituted enzyme and further attempts seem promising.

Daily variation in melatonin synthesis and arylalkylamine N-acetyltransferase activity in the nematode Caenorhabditis elegans

Melatonin influences circadian rhythms and seasonal behavioral changes in vertebrates; it is synthesized from serotonin by N-acetylation by arylalkylamine N-acetyltransferase (AA-NAT) and O-methylation by N-acetylserotonin methyltransferase. However, its physiology and function in invertebrate models are less understood. In this work, we studied daily variations in melatonin synthesis and AA-NAT activity in the nematode Caenorhabditis elegans. Under light-dark conditions (LD), a rhythmic pattern of melatonin levels was observed, with higher levels toward the middle of the night, peaking at zeitgeber time (ZT) 18, and with a minimum value around ZT0-6. AA-NAT activity showed a diurnal and circadian fluctuation with higher levels of activity during the early night, both under LD and constant darkness conditions. A peak was found around ZT12 and circadian time (CT) 12. In addition, we investigated whether this nocturnal AA-NAT activity is inhibited by light. Our results show that both white and blue light pulses significantly inhibited AA-NAT activity at ZT18. This work demonstrates the daily fluctuation of melatonin synthesis and AA-NAT activity in the adult nematode C. elegans. In summary, this study takes additional advantage of an extremely useful invertebrate model system, which has only recently been exploited for circadian studies.

Melatonin: neuritogenesis and neuroprotective effects in crustacean x-organ cells

Melatonin has both neuritogenic and neuroprotective effects in mammalian cell lines such as neuroblastoma cells. The mechanisms of action include receptor-coupled processes, direct binding and modulation of calmodulin and protein kinase C, and direct scavenging of free radicals. While melatonin is produced in invertebrates and has influences on their physiology and behavior, little is known about its mechanisms of action. We studied the influence of melatonin on neuritogenesis in well-differentiated, extensively-arborized crustacean x-organ neurosecretory neurons. Melatonin significantly increased neurite area in the first 24h of culture. The more physiological concentrations, 1 nM and 1 pM, increased area at 48 h also, whereas the pharmacological 1 μM concentration appeared to have desensitizing effects by this time. Luzindole, a vertebrate melatonin receptor antagonist, had surprising and significant agonist-like effects in these invertebrate cells. Melatonin receptors have not yet been studied in invertebrates. However, the presence of membrane-bound receptors in this population of crustacean neurons is indicated by this study. Melatonin also has significant neuroprotective effects, reversing the inhibition of neuritogenesis by 200 and 500 μM hydrogen peroxide. Because this is at least in part a direct action not requiring a receptor, melatonin's protection from oxidative stress is not surprisingly phylogenetically-conserved.

The effects of UV radiation on the visual system of the crab Neohelice granulata: a protective role of melatonin

The first and main target-structure of ultraviolet (UV) radiation in animals is the body surface, including the skin and eyes. Here, we investigated cell damage in the visual system of the crab Neohelice granulata acclimated to constant light and exposed to UVA or UVB at 12:00 h for 30 min. The reactive oxygen species (ROS) production, antioxidant capacity against peroxyl radicals (ACAP), lipid peroxidation (LPO) damage, catalase (CAT) activity, and the melatonin immunohistochemical reactivity in the eyestalks were evaluated. The animals that received melatonin and were exposed to UVA and UVB radiation showed a decreased ROS concentration (p<0.05).The ACAP test showed a decrease (p<0.05) in their values when the animals received 2 pmol/crab of melatonin (physiological dose) before the exposure to UVA radiation. The animals exposed to UVB radiation after receiving the same dose of melatonin showed an increase (p<0.05) in the ACAP test compared with the animals exposed to UVB radiation after receiving only crab physiological saline. The CAT activity increased (p<0.05) in the animals that received melatonin and were exposed to UVA and UVB radiation. Animals exposed to UVA and UVB displayed an increase (p<0.05) in the LPO levels, whereas animals treated with melatonin showed lower (p<0.05) LPO levels when irradiated. The results indicate that the specific oxidative parameters altered by UV radiation can be modulated by a physiological dose of melatonin. Moreover, the melatonin regularly produced by virtually all eyestalk cells suggests that it may function to modulate the noxious effects of radiation, at least in the crab N. granulata.

Influence of the dark/light rhythm on the effects of UV radiation in the eyestalk of the crab Neohelice granulata

Crustaceans are interesting models to study the effects of ultraviolet (UV) radiation, and many species may be used as biomarkers for aquatic contamination of UV radiation reaching the surface of the Earth. Here, we investigated cell damage in the visual system of crabs Neohelice granulata that were acclimated to either 12L:12D, constant light, or constant dark, and were exposed to UVA or UVB at 12:00h (noon). The production of reactive oxygen species (ROS), antioxidant capacity against peroxyl radicals (ACAP), lipid peroxidation (LPO) damage, catalase activity, and pigment dispersion in the eye were evaluated. No significant differences from the three groups of controls (animals acclimated to 12L:12D, or in constant light, or not exposed to UV radiation) were observed in animals acclimated to 12L:12D, however, crabs acclimated to constant light and exposed to UV radiation for 30min showed a significant increase in ROS concentration, catalase activity, and LPO damage, but a decrease in ACAP compared with the controls. Crabs acclimated to constant darkness and exposed to UV for 30min showed a significantly increased ROS concentration and LPO damage, but the ACAP and catalase activity did not differ from the controls (animals kept in the dark while the experimental group was being exposed to UV radiation). Pigment dispersion in the pigment cells of eyes of animals acclimated to constant light was also observed. The results indicate that UVA and UVB alter specific oxidative parameters; however, the cell damage is more evident in animals deviated from the normal dark/light rhythm.

Effect of melatonin in the antioxidant defense system in the locomotor muscles of the estuarine crab Neohelice granulata (Decapoda, Brachyura)

In vertebrates, many studies verified different effects of melatonin in the antioxidant defense system (ADS). In crustaceans, few studies have been conducted to verify this possibility. We verified the melatonin effects in the crab Neohelice granulata using low (0.002 and 0.02 pmol/crab) and high (2.0 and 20.0 pmol/crab) melatonin dosages in short-term (0.5h) and long-term (9.5h) experiments. We analyzed the antioxidant capacity against peroxyl radicals (ACAP), reactive oxygen species (ROS) concentration, levels of by products of lipid peroxidation (LPO), oxygen consumption (VO(2)), the activity of glutamate cysteine ligase (gamma-GCL) and catalase (CAT) and glutathione content (GSH). Finally, the effects of exogenous melatonin were verified in terms of melatonin and N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK) content in the muscles of N. granulata. In short-term experiment and low dosages, melatonin increased the VO(2), gamma-GCL activity and GSH content (p<0.05) and decreased melatonin content (p<0.05) without effects in ROS, ACAP and LPO (p>0.05). Possibly, melatonin is acting in the ADS increasing its efficiency and/or acting in mitochondrial activity and/or through signaling muscles to increase its consumption. AFMK was only detected in the eyestalk and cerebroid ganglia. In high dosages melatonin effects decreased, possibly by the desensitization of their receptors. In long-term experiment, melatonin decreased ACAP (p<0.05), and CAT activity (p<0.05) in low dosages. In high dosages melatonin reduced VO(2) (p<0.05) and increased ACAP (p<0.05), possibly stimulating others components of the ADS. In conclusion, melatonin in the locomotor muscles of N. granulata affects the antioxidant/pro-oxidant balance in a time and dosage dependent manner.

Effects of melatonin in connection with the antioxidant defense system in the gills of the estuarine crab Neohelice granulata

Numerous studies have shown that melatonin exerts some influence on the antioxidant defense system (ADS) in vertebrates, but for crustaceans no such effect has been demonstrated till now. However, earlier reports did show a similar profile of daily variations in the ADS of the gills and the melatonin content of the eyestalk in the crab Neohelice granulata and, thus, the aim of this study was to take a closer look at the effects of melatonin in the gill ADS of N. granulata. Gill ADS is to a minor extent modulated by reactive oxygen species (ROS), because only the nonproteic sulfhydryl (NP-SH) content increases (p<0.05) in the presence of hydrogen peroxide (H(2)O(2)). No significant differences (p>0.05) were observed in the melatonin content of the hemolymph between intact and eyestalkless crabs. Gills from intact and eyestalkless crabs injected with physiological saline showed a daily variation in the total peroxyl radical scavenging capacity (TPRSC) (p<0.05) with two peaks, one at the photophase and another at the scotophase. However, in the gills of eyestalkless crabs injected with melatonin (2 x 10(-12)mol crab(-1)), the daily variation in TPRSC values was abolished (p>0.05). This molecule did not change the NP-SH content (p>0.05) in vitro, but decreased (p<0.05) the oxygen consumption in gills when incubated for 120 min. In the in vivo experiments melatonin also decreased (p<0.05) the oxygen consumption in eyestalkless crabs after 390 min. The results suggest that melatonin does not act directly on the ADS of the gills of N. granulata, but decreases the aerobic metabolism possibly involved in variations of tissue ADS.