Rhythms In Fish Research Articles

Circadian patterns and photoperiodic modulation of clock gene expression and neuroendocrine hormone secretion in the marine teleost Larimichthys crocea

ABSTRACT The light/dark cycle, known as the photoperiod, plays a crucial role in influencing various physiological activities in fish, such as growth, feeding and reproduction. However, the underlying mechanisms of this influence are not fully understood. This study focuses on exploring the impact of different light regimes (LD: 12 h of light and 12 h of darkness; LL: 24 h of light and 0 h of darkness; DD: 0 h of light and 24 h of darkness) on the expression of clock genes (LcClocka, LcClockb, LcBmal, LcPer1, LcPer2) and the secretion of hormones (melatonin, GnRH, NPY) in the large yellow croaker, Larimichthys crocea. Real-time quantitative PCR (RT-qPCR) and enzyme-linked immunosorbent assays were utilized to assess how photoperiod variations affect clock gene expression and hormone secretion. The results indicate that changes in photoperiod can disrupt the rhythmic patterns of clock genes, leading to phase shifts and decreased expression. Particularly under LL conditions, the pineal LcClocka, LcBmal and LcPer1 genes lose their rhythmicity, while LcClockb and LcPer2 genes exhibit phase shifts, highlighting the importance of dark phase entrainment for maintaining rhythmicity. Additionally, altered photoperiod affects the neuroendocrine system of L. crocea. In comparison to the LD condition, LL and DD treatments showed a phase delay of GnRH secretion and an acceleration of NPY synthesis. These findings provide valuable insights into the regulatory patterns of circadian rhythms in fish and may contribute to optimizing the light environment in the L. crocea farming industry.

Transcriptomic Analysis Reveals Circadian Rhythm Homeostasis in Pearl Gentian Grouper under Acute Hypoxia

Oxygen level is an important environmental factor affecting the circadian rhythm. However, little is known about the molecular mechanism by which clock genes regulate the circadian rhythm in fish under hypoxia. To explore changes in the transcription and expression of clock genes and related molecular regulatory mechanisms in pearl gentian grouper under hypoxia, liver transcriptome data were analyzed after exposure to acute hypoxic stress (dissolved oxygen 0.5 mg/L) for 1, 3, 6, and 9 h. miR-210 and m0044-5p inhibited the expression of period3 (per3) and casein kinase 1 delta b (csnk1db) in the core loop of the circadian clock, respectively. The nuclear receptor subfamily 1 group d member 1 (nr1d1) and RAR-related orphan receptor b (rorb) genes in the auxiliary loop were jointly up-regulated by three miRNAs (miR-144-3p/5p, miR-361-5p, and miR-133) and the transcription factor nuclear receptor subfamily 1 group d member 2 (Nr1d2). The pearl gentian grouper maintains the stability of circadian clock systems and normal physiological metabolism under hypoxic stress by regulating the transcriptional expression of these genes via miRNAs and transcription factors to improve hypoxic tolerance. These findings provide important basic data for future research on hypoxic tolerance in pearl gentian grouper and provide new insights into the interaction between hypoxia and the circadian rhythm in fish.

Widespread psychoactive pollutant augments daytime restfulness and disrupts diurnal activity rhythms in fish

Pharmaceutical pollution is a major driver of global change, with the capacity to alter key behavioural and physiological traits in exposed animals. Antidepressants are among the most commonly detected pharmaceuticals in the environment. Despite well-documented pharmacological effects of antidepressants on sleep in humans and other vertebrates, very little is known about their ecologically relevant impacts as pollutants on non-target wildlife. Accordingly, we investigated the effects of acute 3-day exposure of eastern mosquitofish (Gambusia holbrooki) to field-realistic levels (nominal concentrations: 30 and 300 ng/L) of the widespread psychoactive pollutant, fluoxetine, on diurnal activity patterns and restfulness, as indicators of disruptions to sleep. We show that exposure to fluoxetine disrupted diel activity patterns, which was driven by augmentation of daytime inactivity. Specifically, unexposed control fish were markedly diurnal, swimming farther during the day and exhibiting longer periods and more bouts of inactivity at night. However, in fluoxetine-exposed fish, this natural diel rhythm was eroded, with no differences in activity or restfulness observed between the day and night. As a misalignment in the circadian rhythm has been shown to adversely affect fecundity and lifespan in animals, our findings reveal a potentially serious threat to the survival and reproductive success of pollutant-exposed wildlife.

Tributyltin exposure disrupted the locomotor activity rhythms in adult zebrafish (Danio rerio) and the mechanism involved

The fish circadian rhythm system might be an emerging target of tributyltin (TBT), however, the mechanism by which TBT interferes with the circadian rhythm is poorly understood. Therefore, in the present study, zebrafish were used to assess the effects of TBT at environmental concentrations (1 and 10 ng/L) on locomotor activity rhythm. Furthermore, we focused on the visual system to explore the potential mechanism involved. After 90 d of exposure, TBT disturbed the locomotor activity rhythms in zebrafish, which manifested as: (1) low activities and lethargy during the arousing period; (2) inability to fall asleep quickly and peacefully during the period of latency to sleep; and (3) no regular “waves” of locomotor activities during the active period. After TBT exposure, the histological structure of the eyes significantly changed, the boundary between layers became blurred, and the melanin concentrations significantly decreased. Using KEGG and GSEA pathway analyses, the differentially expressed genes in the eyes screened by transcriptomics were significantly enriched in the tyrosine metabolism pathway and retinol metabolism pathway. Furthermore, a decrease in melanin and disruption of retinoic acid were found after TBT exposure, which would affect the reception of phototransduction, and then interfere with the circadian rhythm in fish. The disruption of the circadian rhythm of fish by aquatic pollutants would decrease their ecological adaptability, which should be considered in future research.

Nuclear Receptors (PPARs, REV-ERBs, RORs) and Clock Gene Rhythms in Goldfish (Carassius auratus) Are Differently Regulated in Hypothalamus and Liver.

The circadian system is formed by a network of oscillators located in central and peripheral tissues that are tightly linked to generate rhythms in vertebrates to adapt the organism to the cyclic environmental changes. The nuclear receptors PPARs, REV-ERBs and RORs are transcription factors controlled by the circadian system that regulate, among others, a large number of genes that control metabolic processes for which they have been proposed as key genes that link metabolism and temporal homeostasis. To date it is unclear whether these nuclear receptors show circadian expression and which zeitgebers are important for their synchronization in fish. Therefore, the objective of this study was to investigate whether the two main zeitgebers (light-dark cycle and feeding time) could affect the synchronization of central (hypothalamus) and peripheral (liver) core clocks and nuclear receptors in goldfish. To this aim, three experimental groups were established: fish under a 12 h light-12 h darkness and fed at Zeitgeber Time 2; fish with the same photoperiod but randomly fed; and fish under constant darkness and fed at Circadian Time 2. After one month, clock genes and nuclear receptors expression in hypothalamus and liver and circulating glucose were studied. Clock genes displayed daily rhythms in both tissues of goldfish if the light-dark cycle was present, with shifted-acrophases of negative and positive elements, as expected for proper functioning clocks. In darkness-maintained fish hypothalamic clock genes were fully arrhythmic while the hepatic ones were still rhythmic. Among studied nuclear receptors, in the hypothalamus only nr1d1 was rhythmic and only when the light-dark cycle was present. In the liver all nuclear receptors were rhythmic when both zeitgebers were present, but only nr1d1 when one of them was removed. Plasma glucose levels showed significant rhythms in fish maintained under random fed regimen or constant darkness, with the highest levels at 1-h postprandially in all groups. Altogether these results support that hypothalamus is mainly a light-entrained-oscillator, while the liver is a food-entrained-oscillator. Moreover, nuclear receptors are revealed as clear outputs of the circadian system acting as key elements in the timekeeping of temporal homeostasis, particularly in the liver.

Light conditions for commercial hatching success in Nile tilapia (Oreochromis niloticus)

Light plays an important role in the development of biological rhythms in fish. Previous studies on tilapia has revealed that spawning rhythms and juvenile growth are significantly influenced by photoperiod. However, the effect of light conditions on the hatching rhythms are unexamined thus far. Therefore, the aim of this study was to investigate the impact of the light conditions on the hatching in Nile tilapia. To this end, the central composite experimental design and response surface methodology were utilized to study the combined effects of photoperiod and light intensity on the hatching rate as well as the growth of early fry. Based upon the results of trial tests, regimes of photoperiod and light intensity ranged from 2 to 24 h and from 100 to 2000 lx (1.26 to 25.20 μmol m−2 s−1), respectively. The results demonstrated that within the factorial space developmental rhythms at the egg stage were significantly influenced by light conditions, signaling that the timing of hatching in Nile tilapia could be regulated by ambient light conditions. The more unsuitable the light conditions, the longer the eggs took to hatching (or the stronger inhibitory effect of light conditions on hatching), underlining the importance and necessity of affording optimized photoenvironment during commercial incubation as well as early fry development of tilapia. The effect of light intensity was greater than that of photoperiod during early ontogeny. The interaction between photoperiod and light intensity was found to be present, implying that the effect of only photoperiod upon reproduction and growth of Nile tilapia in all previous studies might be modified. Relationship of hatching to light conditions also could be reliably modelled. The optimal photoperiod/intensity combinations, 13.45 h/1020 lx (12.85 μmol m−2 s−1), were derived via model optimization, the optimality of which had been verified under commercial hatching conditions. These findings emphasize that the light conditions are essential and should be taken into account in optimizing the protocols of the commercial incubation in hatcheries of Nile tilapia.

Circadian rhythms and environmental disturbances - underexplored interactions.

Biological rhythms control the life of virtually all organisms, impacting numerous aspects ranging from subcellular processes to behaviour. Many studies have shown that changes in abiotic environmental conditions can disturb or entrain circadian (∼24 h) rhythms. These expected changes are so large that they could impose risks to the long-term viability of populations. Climate change is a major global stressor affecting the fitness of animals, partially because it challenges the adaptive associations between endogenous clocks and temperature - consequently, one can posit that a large-scale natural experiment on the plasticity of rhythm-temperature interactions is underway. Further risks are posed by chemical pollution and the depletion of oxygen levels in aquatic environments. Here, we focused our attention on fish, which are at heightened risk of being affected by human influence and are adapted to diverse environments showing predictable changes in light conditions, oxygen saturation and temperature. The examined literature to date suggests an abundance of mechanisms that can lead to interactions between responses to hypoxia, pollutants or pathogens and regulation of endogenous rhythms, but also reveals gaps in our understanding of the plasticity of endogenous rhythms in fish and in how these interactions may be disturbed by human influence and affect natural populations. Here, we summarize research on the molecular mechanisms behind environment-clock interactions as they relate to oxygen variability, temperature and responses to pollutants, and propose ways to address these interactions more conclusively in future studies.

Influence of artificially induced light pollution on the hormone system of two common fish species, perch and roach, in a rural habitat.

Almost all life on earth has adapted to natural cycles of light and dark by evolving circadian and circannual rhythms to synchronize behavioural and physiological processes with the environment. Artificial light at night (ALAN) is suspected to interfere with these rhythms. In this study we examined the influence of ALAN on nocturnal melatonin and sex steroid blood concentrations and mRNA expression of gonadotropins in the pituitary of European perch (Perca fluviatilis) and roach (Rutilus rutilus). In a rural experimental setting, fish were held in net cages in drainage channels experiencing either additional ALAN of ~15 lx at the water surface or natural light conditions at half-moon. No differences in melatonin concentrations between ALAN and natural conditions were detected. However, blood concentration of sex steroids (17β-estradiol; 11-ketotestosterone) as well as mRNA expression of gonadotropins (luteinizing hormone, follicle stimulating hormone) was reduced in both fish species. We conclude that ALAN can disturb biological rhythms in fish in urban waters. However, impacts on melatonin rhythm might have been blurred by individual differences, sampling methods and moonlight. The effect of ALAN on biomarkers of reproduction suggests a photo-labile period around the onset of gonadogenesis, including the experimental period (August). Light pollution therefore has a great potential to influence crucial life history traits with unpredictable outcome for fish population dynamics.

Influence of light intensity and spectral composition of artificial light at night on melatonin rhythm and mRNA expression of gonadotropins in roach Rutilus rutilus.

In this study we investigated the influence of artificial light at night (ALAN) of different intensities (0, 1, 10, 100lx) and different colours (blue, green, red) on the daily melatonin rhythm and mRNA expression of gonadotropins in roach Rutilus rutilus, a ubiquitous cyprinid, which occur in standing and moderately flowing freshwater habitats of central Europe. Melatonin concentrations were significantly lowered under nocturnal white light already at 1lx. Low intensity blue, green and red ALAN lowered the melatonin levels significantly in comparison to a dark control. We conclude that ALAN can disturb melatonin rhythms in roach at very low intensities and at different wavelengths and thus light pollution in urban waters has the potential to impact biological rhythms in fish. However, mRNA expression of gonadotropins was not affected by ALAN during the period of the experiments. Thus, suspected implications of ALAN on reproduction of roach could not be substantiated.

Effect of antidepressants on circadian rhythms in fish: Insights and implications regarding the design of behavioural toxicity tests.

Selective Serotonin Reuptake Inhibitors (SSRIs) and Serotonin and Norepinephrine Reuptake Inhibitors (SNRIs) are widely prescribed for the treatment of depression and anxiety disorders. Consequently, these compounds are frequently identified in global waterways where they may pose a hazard to aquatic biota. Evidence demonstrates these compounds to be capable of influencing the behaviour of fish, but the relevance of many reported behavioural endpoints is unclear and the value of some findings has been questioned. Since these compounds act on neuroendocrine-mediated pathways in vertebrates, the present study explored how exposure to two representative SSRIs (fluoxetine and sertraline) and an SNRI (venlafaxine) affect circadian rhythms in fish. Male mosquitofish (Gambusia holbrooki) were exposed to 1, 10 and 100μg/L concentrations of these compounds individually and when present as a full mixture, for a period of one week. Neither fluoxetine nor sertraline had an impact on diurnal activity patterns when fish were exposed to these compounds alone at any concentration, whereas venlafaxine significantly disrupted normal circadian rhythmicity but only at 100μg/L. When fish were exposed to the full mixture, significantly altered diurnal activity patterns were rapidly observed at nominal concentrations of 1 and 100μg/L, but there was no effect at 10μg/L. This sort of non-monotonic dose relationship is not altogether unusual for fish exposed to antidepressants, but it poses a problem when attempting to evaluate potential risks to the aquatic environment. To evaluate the possibility for misinterpretation when collecting behavioural data over short temporal scales, the data for each day of the experiment was analysed separately. The outcomes demonstrate the importance of longer periods of data collection, which may be necessary to capture the full range of natural behavioural variability that exists both amongst and within individual fish. More importantly, these findings may help reveal why discrepancies are commonly being reported in the literature with regards behavioural effects in fish exposed to antidepressants. It is thus suggested that research be aimed at documenting behavioural variability in fish species used in toxicity testing, to establish guidelines for quality control and where possible inform the development of standardised methodologies so that behavioural analysis can be more appropriately applied to the broad field of aquatic toxicology.

Environmental Progestins Progesterone and Drospirenone Alter the Circadian Rhythm Network in Zebrafish (Danio rerio).

Progestins alter hormone homeostasis and may result in reproductive effects in humans and animals. Thus far, studies in fish have focused on the hypothalamic-pituitary-gonadal (HPG)-axis and reproduction, but other effects have little been investigated. Here we report that progesterone (P4) and drospirenone (DRS) interfere with regulation of the circadian rhythm in fish. Breeding pairs of adult zebrafish were exposed to P4 and DRS at concentrations between 7 and 13 650 ng/L for 21 days. Transcriptional analysis revealed significant and dose-dependent alterations of the circadian rhythm network in the brain with little effects in the gonads. Significant alterations of many target transcripts occurred even at environmental relevant concentrations of 7 ng/L P4 and at 99 ng/L DRS. They were fully consistent with the well-described circadian rhythm negative/positive feedback loops. Transcriptional alterations of the circadian rhythm network were correlated with those in the HPG-Liver-axis. Fecundity was decreased at 742 (P4) and 2763 (DRS) ng/L. Dose-dependent alterations in the circadian rhythm network were also observed in F1 eleuthero-embryos. Our results suggest a potential target of environmental progestins, the circadian rhythm network, in addition to the adverse reproductive effects. Forthcoming studies should show whether the transcriptional alterations in circadian rhythm translate into physiological effects.

Pheromones as time cues for circadian rhythms in fish

Almost all living organisms, including fish, exhibit circadian rhythms. They synchronize their circadian clocks with the solar clock through the mediation of photic or non-photic zeitgebers. Pheromones are the major chemicals responsible for communication among the conspecifics. Pheromones are secreted by a sender and perceived by receiver (s), thus evoking a species specific response and in turn affecting the behavior of receiver (s). Can a pheromone act as a zeitgeber in fish? In this mini review attempts have been made to address the question.

Spotlight on fish: Light pollution affects circadian rhythms of European perch but does not cause stress

Flora and fauna evolved under natural day and night cycles. However, natural light is now enhanced by artificial light at night, particularly in urban areas. This alteration of natural light environments during the night is hypothesised to alter biological rhythms in fish, by effecting night-time production of the hormone melatonin. Artificial light at night is also expected to increase the stress level of fish, resulting in higher cortisol production. In laboratory experiments, European perch (Perca fluviatilis) were exposed to four different light intensities during the night, 0lx (control), 1lx (potential light level in urban waters), 10lx (typical street lighting at night) and 100lx. Melatonin and cortisol concentrations were measured from water samples every 3h during a 24hour period. This study revealed that the nocturnal increase in melatonin production was inhibited even at the lowest light level of 1lx. However, cortisol levels did not differ between control and treatment illumination levels. We conclude that artificial light at night at very low intensities may disturb biological rhythms in fish since nocturnal light levels around 1lx are already found in urban waters. However, enhanced stress induction could not be demonstrated.

Food consumption of five deep-sea fishes in the Balearic Basin (western Mediterranean Sea): are there daily feeding rhythms in fishes living below 1000 m?

Predation and food consumption of five deep-sea fish species living below 1000 m depth in the western Mediterranean Sea were analysed to identify the feeding patterns and food requirements of a deep-sea fish assemblage. A feeding rhythm was observed for Risso's smooth-head Alepocephalus rostratus, Mediterranean grenadier Coryphaenoides mediterraeus and Mediterranean codling Lepidion lepidion. Differences in the patterns of the prey consumed suggest that feeding rhythms at such depths are linked with prey availability. The diets of those predators with feeding rhythms are based principally on active-swimmer prey, including pelagic prey known to perform vertical migrations. The diets of Günther's grenadier Coryphaenoides guentheri and smallmouth spiny eel Polyacanthonotus rissoanus, which did not show any rhythm in their feeding patterns, are based mainly on benthic prey. Food consumption estimates were low (<1% of body wet mass day(-1) ). Pelagic feeding species showing diel feeding rhythms consumed more food than benthic feeding species with no feeding rhythms.

Leptin expression is rhythmic in brain and liver of goldfish (Carassius auratus). Role of feeding time

Daily rhythms of feeding regulators are currently arousing research interest due to the relevance of the temporal harmony of endocrine regulators for growth and welfare in vertebrates. However, it is unknown the leptin circadian pattern in fish. The aim of this study is to investigate if leptin (gLep-aI and gLep-aII) expression is rhythmic in goldfish (Carassius auratus) liver and brain, and if such rhythms are driven by feeding time through a food entrainable oscillator. Fish maintained under 12-h light:12-h dark photoperiod and a scheduled feeding time showed 24-h locomotor activity and glycaemia rhythms. Moreover, hepatic gLep-aI and brain gLep-aI and gLep-aII expression were rhythmic with different daily profiles, showing a postprandial increase of leptin expression in the liver but not in the brain. Under constant light and different feeding regimes (scheduled fed at 10:00, 22:00 or randomly fed), feeding time synchronized daily rhythms in locomotor activity, glycaemia and clock gene expression (gPer1a, gPer3 and gCry3), but the rhythmic expression of hepatic gLep-aI and brain gLep-aII only remained in fed fish at 10:00. In summary, daily rhythms of leptin expression in goldfish are differently regulated at central and peripheral level, and they are not directly driven by clock genes. The role of food entrained oscillators on leptin expression rhythms in fish remains to be demonstrated.

Effects of retinal light input on circadian rhythm genes in the yellowtail clownfish (Amphiprion clarkii) as determined using LED light spectra

The objective of this study was to test the effects of ophthalmectomy on the circadian rhythms in response to various wavelengths in yellowtail clownfish. The responses were evaluated by measuring rhodopsin (RH) and melatonin receptor 1 (MT1) mRNA and protein expression, and plasma melatonin levels. The RH, MT1 mRNA, and protein expression in ophthalmectomized fish were significantly lower than in intact fish. However, RH and MT1 mRNA and protein expression levels in green and blue light-emitting diode groups were similar between intact and ophthalmectomized fish. Furthermore, we found that the number of the RH-immunoreactive cells observed in the retina was almost in agreement with mRNA and protein expression patterns. These findings suggest that circadian rhythm involves the perception of light by the eye and the transduction of this signal through the circadian rhythm axis. Furthermore, short wavelengths lights may be most suitable for control of the circadian rhythm in fish.

Circadian Rhythms of Embryonic Development and Hatching in Fish: A Comparative Study of Zebrafish (Diurnal), Senegalese Sole (Nocturnal), and Somalian Cavefish (Blind)

During early development, most organisms display rhythmic physiological processes that are shaped by daily changes in their surrounding environment (i.e., light and temperature cycles). In fish, the effects of daily photocycles and their interaction with temperature during early developmental stages remain largely unexplored. We investigated the existence of circadian rhythms in embryonic development and hatching of three teleost species with different daily patterns of behavior: diurnal (zebrafish), nocturnal (Senegalese sole), and blind, not entrained by light (Somalian cavefish). To this end, fertilized eggs were exposed to three light regimes: 12 h of light: 12 h of darkness cycle (LD), continuous light (LL), or continuous darkness (DD); and three species-appropriate temperature treatments: 24°C, 28°C, or 32°C for zebrafish and cavefish and 18°C, 21°C, or 24°C for sole. The results pointed to the existence of daily rhythms of embryonic development and hatching synchronized to the LD cycle, with different acrophases, depending on the species: zebrafish embryos advanced their developmental stage during the light phase, whereas sole did so during the dark phase. In cavefish, embryogenesis occurred within 24 h post fertilization (hpf) at the same pace during day or night. The hatching rhythms appeared to be controlled by a clock mechanism that restricted or “gated” hatching to a particular time of day/night (window), so that embryos that reached a certain developmental state by that time hatch, whereas those that have not wait until the next available window. Under LL and DD conditions, hatching rhythms and the gating phenomenon persisted in cavefish, in zebrafish they split into ultradian bouts of hatching occurring at 12–18-h intervals, whereas in sole DD and LL produced a 24-h delay and advance, respectively. Hatching rates were best under the LD cycle and the reported optimal temperature for each species (95.2 ± 2.7% of the zebrafish and 83.3 ± 0.1% of the cavefish embryos hatched at 28°C, and 93.1 ± 2.9% of the sole embryos hatched at 21°C). In summary, these results revealed that hatching rhythms in fish are endogenously driven by a time-keeping mechanism, so that the day and time of hatching are determined by the interplay between the developmental state (temperature-sensitive) and the circadian clock (temperature-compensated), with the particular phasing being determined by the diurnal/nocturnal behavior of the species. (Author correspondence: javisan@um.es)

Ritmos biológicos e fotoperíodo em peixes

Os ritmos biológicos são definidos como qualquer evento que se repete de maneira regular em um organismo, sendo um evento cíclico caracterizado por um ambiente do qual o animal pode se adaptar. Os peixes, quando submetidos ao ciclo diário de luz/escuro do fotoperíodo demonstram um padrão de atividade locomotora que os podem classificar como diurnos, noturnos e crepusculares. Este ciclo de luz/escuro tem sido considerado um dos mais importantes fatores ambientais sincronizadores do ritmo biológico, sendo o fator chave para sincronização do ritmo de atividade em peixes. O ciclo alimentar também atua como um potente sincronizador sobre o controle da atividade locomotora. Este é demonstrado através da atividade alimentar antecipatória como ritmo biológico, isto é, a capacidade em que os peixes conseguem prever com habilidade e antecipar um recorrente evento que é a alimentação. Estas respostas antecipatórias à alimentação provavelmente funcionam sob um controle endógeno, onde os peixes precisam otimizar a captura do alimento, assim como os processos digestivos e metabólicos, para poder concentrar a ingestão de alimento em um menor intervalo de tempo, melhorando, portanto, a utilização dos nutrientes. O controle do relógio biológico em peixes é considerado como um multifotorreceptor e sistema multioscilador. A existência de um oscilador circadiano tem sido sugerida através da pineal, retina e o cérebro, estando estas estruturas envolvidas na transdução do sinal fótico para estabelecer um rítmo circadiano em peixes. O acoplamento entre essas estruturas pode variar em cada indivíduo de acordo com as condições fisiológicas e ambientais, resultando em uma plasticidade no sistema circadiano de peixes teleósteos. A manipulação do fotoperíodo com objetivo de aprimorar o crescimento dos peixes tem se tornado cada vez mais frequente dentro da produção de várias espécies de interesse comercial. O fotoperíodo, dentre outros fatores ambientais, é o que apresenta maior influência sobre o relógio biológico dos peixes ao afetar o ganho de peso, a ingestão de alimento, a eficiência alimentar, o gasto de energia, a atividade locomotora, a reprodução, bem como outros parâmetros fisiológicos relacionados ao estresse. Portanto, o controle e o conhecimento fisiológico deste ritmo biológico torna-se fundamental para otimização da produção de peixes.

Effectiveness of the anaesthetic MS-222 in gilthead seabream, Sparus aurata: Effect of feeding time and day–night variations in plasma MS-222 concentration and GST activity

Feeding time is a potent zeitgeber capable of synchronising behavioural and physiological daily rhythms in fish. However, the effect of feeding time on the daily rhythm of drugs toxicity and/or effectiveness remains unexplored to date. In this paper we investigated the day/night variations in the effectiveness of an anaesthetic commonly used in fish (Tricaine, MS-222) in a teleost of great chronobiological and aquaculture interest (gilthead seabream). To this end, fish were kept under LD 12:12 and fed at mid-light (ML), mid-darkness (MD) or random times (RD). The time needed to induce anaesthesia (reduction of locomotor activity) during MS-222 exposure (65mg/L) as well as the recovery period were investigated at ML and MD in the three experimental groups using specialised video tracking software. In addition, daily rhythms of GST activity in the liver (as an indicator of detoxification processes) and plasma MS-222 concentration (related to uptake) were determined. The results revealed that MS-222 effectiveness in the ML group was higher during the day than at night (significant reduction of activity after 3min vs. 5min) whereas in the MD group, the daily variation of MS-222 effectiveness was inverted (significant reduction of activity after 7min at ML vs. 2min at MD), suggesting that feeding time can shift the day–night variations in the effectiveness of MS-222. Hepatic GST also seemed to be affected by feeding time: in fish fed at MD or RD this enzyme activity showed significant differences during the day, and the highest levels were found at different times of the day in each group. Plasma MS-222 concentrations were higher at ML (142.4±12.8ng/ml) than at MD (96.3±10.9ng/ml) (t-Student, p<0.05). These results suggest that the daily variation in MS-222 concentration following exposure might be involved, among other factors, in the existence of day–night variations in the effectiveness of this anaesthetic. Furthermore, manipulation of the feeding schedule can be used to modify the daily variations in MS-222 effectiveness, which has basic as well applied implications for optimising anaesthesia protocols in fish aquaculture.

Daily rhythms of digestive physiology, metabolism and behaviour in the European eel (Anguilla anguilla)

Circadian rhythms are common in a wide variety of physiological processes. Our knowledge of biological rhythms in fish can be exploited to improve some key aspects of aquaculture such as feed optimization and fish welfare. The aim of this paper was to investigate daily rhythms in behavioural and physiological variables of the European eel. To this end, we first analysed the daily rhythms in locomotor activity measured by means of infrared photocells and feeding activity by means of self-feeders. The eels were then submitted to scheduled feeding (single meals once a day at a fixed time), and the levels of cortisol, glucose and digestive enzymes were analysed. Locomotor activity was predominantly nocturnal (63 % of daily activity occurring at night) with peaks located around light transitions, whereas feeding activity was less clearly defined. Cortisol and glucose showed daily rhythms, with acrophases located in the middle and at the beginning of the light phase, respectively, which should be carefully considered when using these parameters as welfare indicators. Significant variations were found during the day in acid protease levels, which increased in anticipation of mealtime. Taken together, these results describe for the first time a picture of the circadian physiology of the European eel, providing hints to improve feeding methods and welfare assessment.