Daily rhythms of blood glucose differ in diurnal and nocturnal European sea bass (Dicentrarchus labrax L.) undergoing seasonal phase inversions.

Sea bass change their feeding rhythms from diurnal to nocturnal in winter, returning to diurnal feeding in spring. Despite behavioral data, the physiological changes that take place during such changes remain unexplored. In this paper, blood glucose rhythms of European sea bass with diurnal/nocturnal self-feeding rhythms were investigated during phase inversions of their feeding behavior (in winter and spring) when both diurnal and nocturnal fish coexist. Blood glucose showed daily variations in both seasons (ANOVA, p < 0.03), fitting a cosine function (COSINOR, p < 0.05) in all cases, except in diurnal fish in spring. The average blood glucose levels of nocturnal fish in winter (2.67 ± 0.09 mmol/l, mean ± SEM) were significantly (t test, p < 0.01) higher than in spring (2.20 ± 0.08 mmol/l), while they were similar (~2.25 mmol/l) in diurnal fish in both seasons. These findings revealed for the first time insights into the seasonal physiological changes that accompany changes in behavioral rhythms in diurnal and nocturnal sea bass.

Does feeding time affect fish welfare?

Increased aquaculture production has raised concerns about managing protocols to safeguard the welfare of farmed fish, as consumers demand responsible aquaculture practices to provide 'welfare friendly' products. Feeding is one of the largest production cost in a fish farm and can be one of the biggest stressors for fish. Under farming conditions, fish are challenged with artificial diets and feeding regimes, and inadequate feeding conditions cause stress, alteration of normal behavioural patterns, poor performance and eventually diseases and death, which are by no means acceptable neither economically nor ethically. This review aims to highlight the impact of feeding rhythms and feeding time upon physiological and behavioural welfare indicators, which show circadian rhythms as well. Therefore, all these variables should be considered when designing feeding strategies in farming conditions and assessing the welfare state of cultured fish.

Effects of cadmium on locomotor activity rhythms of the amphipod Gammarus aequicauda.

Behavioural responses are linked to complex biochemical and physiologic changes and may act as sensitive indicators of the sublethal effects of pollutants. This article investigates changes in the locomotor activity rhythms of the amphipod Gammarus aequicauda exposed to cadmium (Cd) as a model to study the effect of pollutants on an ecologically important species. Under a 12:12 h light-to-dark cycle, G. aequicauda showed a strict nocturnal rhythm, with 90.2±0.4% of their total daily activity occurring during the night. Under constant darkness, circadian rhythms persisted for 10 days, with a mean periodicity of 24.32 h, thus confirming endogenous control. Exposure to sublethal concentrations of Cd (0.16, 0.20, 0.24, and 0.28 mg l(-1)) did not change the nocturnal activity patterns of G. aequicauda, although their swimming activity during the night was significantly decreased by exposure to concentrations of 0.24 and 0.28 mg Cd l(-1). In conclusion, locomotor activity bioassays using the amphipod G. aequicauda appeared to be a sensitive indicator of Cd contamination, and sensitivity and tolerance to Cd in short-term bioassays may depend on the time of the day tests are carried out. These results provide further support for the idea that behavioural end points in amphipods are useful indicators of pollutant exposure and that future studies should take circadian rhythms into consideration.

Daily rhythms of locomotor activity, feeding behavior and dietary selection in Nile tilapia (Oreochromis niloticus).

Fish do not feed at any time of the day and on whatever food item they encounter in the wild, but they show daily rhythms of feeding activity and dietary selection. The aim of this research was to investigate the daily rhythms of behavior in Nile tilapia self-fed with plant-based diets supplemented with different levels of exogenous phytase (an enzyme that hydrolyzes non-digestible phytate and improves the nutritional value of the diet). To this end, ten males were individually kept in 50-L tanks, each equipped with two self-feeders and one infrared photo-cell connected to a computer. The selection factors investigated were the level of phytase supplementation (0 IU kg(-1) vs 1500 IU kg(-1); 1500 IU kg(-1) vs 4000 IU kg(-1)) or sodium phytate (1% phytate vs 1% phytate+1500 IU kg(-1) phytase). The results revealed that 66.7% of total daily activity occurred during the day, while feeding was strictly nocturnal, with 93.0% of the daily food demands occurring at night. Tilapia preferred the diet with 1500 IU kg(-1) phytase rather than the control or 4000 IU kg(-1) diets. When exogenous sodium phytate was added to the diet, tilapia preferred the phytase diet. In conclusion tilapia self-feed at night (although locomotor activity was mostly diurnal) and chose plant-diets containing phytase, which should be taken into account when designing feeding strategies and practical diets for tilapia aquaculture.

Glucose tolerance in fish: Is the daily feeding time important?

Daily rhythms in glucose tolerance have been reported for several species of mammals, which seem to be linked to factors such as daily rhythms of insulin resistance and feeding habits. In this paper, we studied entrainment to a daily scheduled meal of blood glucose after carbohydrate intake (dextrin or glucose). After a meal containing dextrin, blood glucose showed different maximum concentrations (Cmax) and time to peak (Tmax) depending on mealtime, the greatest differences in Cmax being found 2 h after feeding. The highest Cmax (8.22 mmol/l) was obtained when mealtime was set in the middle of the light phase (ML), and the lowest Cmax (3.46 mmol/l) when goldfish were fed in the middle of the dark phase (MD). Cosinor analysis revealed a significant daily rhythm in dextrin tolerance with its acrophase around ML, amplitude of 1.99 mmol/l, and a mesor of 5.49 mmol/l. However, when the carbohydrate source in the meal was glucose, an inverse pattern was observed: higher blood glucose in goldfish fed at MD compared with ML (6.98 vs 4.32 mmol/l, respectively). Similar results were obtained when glucose was administered intraperitoneally, with higher values at MD than at ML (13.82 vs 9.54 mmol/l, respectively). Finally, no differences in amylase activity were observed in the gut after dextrin load at ML and MD (522 vs 446 U/mg protein), discarding the effect of digestive factors in the daily rhythm of tolerance. In conclusion, our results described for the first time a daily rhythm in tolerance to carbohydrate in a teleost fish, highlighting the impact of the time of day in glucose metabolism.

Effects of water salinity on melatonin levels in plasma and peripheral tissues and on melatonin binding sites in European sea bass (Dicentrarchus labrax).

Sea bass is an euryhaline fish that lives in a wide range of salinities and migrates seasonally from lagoons to the open sea. However, to date, the influence of water salinity on sea bass melatonin levels has not been reported. Here, we evaluated the differences in plasma and tissue melatonin contents and melatonin binding sites in sea bass under four different salinity levels: seawater (36 per thousand), isotonic water (15 per thousand), brackish water (4 per thousand) and freshwater (0 per thousand). The melatonin content was evaluated in plasma, whole brain, gills, intestine and kidney, while melatonin binding sites were analyzed in different brain regions and in the neural retina. Plasma melatonin levels at mid-dark varied, the lowest value occurring in seawater (102 pg/mL), and the highest in freshwater (151 pg/mL). In gills and intestine, however, the highest melatonin values were found in the seawater group (209 and 627 pg/g tissue, respectively). Melatonin binding sites in the brain also varied with salinity, with the highest density observed at the lower salinities in the optic tectum, cerebellum and hypothalamus (30.3, 13.0, and 8.0 fmol/mg protein, respectively). Melatonin binding sites in the retina showed a similar pattern, with the highest values being observed in freshwater. Taken together, these results reveal that salinity influences melatonin production and modifies the density of binding sites, which suggests that this hormone could play a role in timing seasonal events in sea bass, including those linked to fish migration between waters of different salinities for reproduction and spawning.

Lunar and daily spawning rhythms of Senegal sole Solea senegalensis.

A periodicity of 29 days was observed in spawning rhythms in Senegal sole Solea senegalensis, with an acrophase around the last quarter and the new moon. In both spring and autumn, a very marked nocturnal spawning rhythm was registered, with spawning beginning after dusk and the acrophase occurring around 2300 hours. When the photoperiod was artificially extended (from 10L:14D to 14L:10D), S. senegalensis synchronized to the new photoperiod: spawning took place after the new 'dusk', the beginning gradually shifting from 2100 to 2300 hours and the acrophase from 2325 to 0032 hours. Under continuous light conditions, fish sustained rhythmicity for 2 days, with an acrophase at 2249 hours, which suggested the existence of an endogenous pacemaker controlling the daily spawning rhythm. These findings provided new insights for better understanding the reproductive physiology of this species and for optimizing the timing protocols of egg collection and larvae production in S. senegalensis aquaculture.

Role of cholecystokinin and its antagonist proglumide on macronutrient selection in European sea bass Dicentrarchus labrax, L.

Teleost fish are able to adjust their energy intake when fed on pure macronutrient sources, although the exact mechanisms regulating macronutrient selection remain unknown. Since cholecystokinin (CCK) has been reported to modify macronutrient selection patterns in mammals, we explored the effect of CCK administered orally to European sea bass on the selection of separately encapsulated macronutrients. CCK doses of 0.05, 0.15 and 0.25 mg/kg BW administered in gelatine capsules for 5 consecutive days produced a significant inhibition of total food intake (21, 28 and 51%, respectively) at highest doses, evenly reducing the quantity of all the macronutrients ingested and, without affecting their relative proportions in the diet. Oral administration of proglumide, a non-specific CCK receptor antagonist, at doses of 5, 15 and 25 mg/kg BW, induced a quantitative total food intake increase of 2, 18 and 44%, respectively, and an increase of 52% in CH and 43% in P quantity ingested at highest dose. Co-administration of proglumide (25 mg/kg BW) and CCK (0.25 mg/kg BW) in a single preload capsule blocked the effects observed with CCK alone. In conclusion, orally administered CCK induced an anorexigenic effect on both total food and single macronutrient intake, an effect that is counteracted by the CCK antagonist proglumide.

Endogenous modification of macronutrient selection pattern in sea bass (Dicentrarchus labrax, L.).

The use of a single diet with a well defined composition to feed fish throughout their life cycle is an oversimplification that probably does not respond to their metabolic requirements. For example, the seasonal reproduction that characterizes most fish species demands changes in nutritional requirements. Bearing this in mind, the macronutrient selection pattern was studied from January to August in twelve individually housed sea bass exposed to a constant photoperiod (12L:12D h) and temperature (23+/-0.5 degrees C). The endogenous "seasonal" effect on food and energy intake regulation and macronutrient selection was determined, using protein (P), carbohydrate (CH), and fat (F) packaged separately into gelatine capsules, a method that prevents the diet chemosensory properties at oropharyngeal level from interfering with macronutrient selection. Energy intake changed monthly, the highest values being recorded in May and June and the lowest values in March and April. The preliminary results illustrated "seasonal" changes in the sea bass macronutrient selection pattern with, which showed a predominantly proteinic selection during April (53% P, 21% CH, 25% F) and lipidic in July (35% P, 19% CH, 42% F); the increase in fat selection from May to July being statistically significant. This is the first evidence supporting the existence of an endogenous rhythm in the "seasonal" energy regulation and macronutrient selection in fish through post-ingestive mechanisms and probably involving chemosensory detection in the gut and/or post-absorptive mechanisms, although the exact mechanisms involved have yet to be clarified.

Feeding entrainment of locomotor activity rhythms, digestive enzymes and neuroendocrine factors in goldfish.

The existence of food anticipatory activity (FAA) in animals subjected to daily feeding schedules seems to be mediated by a feeding-entrainable oscillator (FEO). Such an FEO may help in anticipating meal time and so optimizing food acquisition and nutrient utilization. In this study we investigated the existence of FAA and whether digestive enzymes, plasma cortisol, hypothalamic NPY and gastrointestinal tract (GIT) and plasma melatonin were entrained by periodic feeding in goldfish. We observed that periodically fed goldfish showed FAA in locomotor activity as well as in amylase and NPY. Alkaline protease and GIT melatonin were higher after feeding, whereas plasma cortisol levels were reduced. Plasma melatonin remained unmodified before and after meal time. These results suggested that scheduled feeding entrained both behavioral and certain physiological patterns in goldfish, FAA being of adaptive value to anticipate a meal and prepare the digestive physiology of fish.

Effect of restricted feeding schedule on seasonal shifting of daily demand-feeding pattern and food anticipatory activity in European sea bass (Dicentrarchus labrax L.).

The effect of restricted feeding schedule was investigated on the seasonal shifting of daily demand-feeding pattern and food anticipatory activity in European sea bass (Dicentrarchus labrax) held under natural environmental conditions in an outdoor laboratory. To that end, demand-feeding behavior was continuously monitored for approximately one year in four groups of 15 fish each exposed to natural fluctuations of water temperature (from 13.2 degrees C to 27.4 degrees C) and photophase (from 9.5 h to 14.5 h of light). When the animals were subjected to a time-restricted feeding schedule, the demand-feeding rhythm rapidly synchronized to the three periods of food availability: the first meal (FM) from 08:00 to 09:00 h, the second meal (SM) from 16:00 to 17:00 h, and the third meal (TM) from 00:00 to 01:00 h. The occurrence of demand-feeding activity into the three periods of food availability displayed a double seasonal shift: fish that self-fed mostly during the daytime periods of feeding availability (FM and SM) in summer and autumn changed to nocturnal feeding (TM) from December to April, returning to diurnal preferences in April. Food-demands appeared to be predominantly associated with feed availability, reaching its maximum levels during the hours of reward. In addition, feeding anticipatory activity (FAA) was observed. A relationship was detected between the duration of FAA and feeding-time, with shortest FAA (30-60 min) when mealtime occurred just after sunrise (FM) or sunset (TM). These findings demonstrate the ability of sea bass to self-feed under time-restricted schedules, and show a seasonal-phase inversion in demand-feeding activity in spite of the restrictions in their feeding availability. Sea bass can use external signals as reference to anticipate the time of feed availability. This information may be useful for designing new feeding strategies for European sea bass fish farming.

Oral serotonin administration affects the quantity and the quality of macronutrients selection in European sea bass Dicentrarchus labrax L.

Teleost fish are able to regulate their energy intake selecting from pure macronutrients sources, but the regulatory mechanisms involved in macronutrients selection remain unknown. Serotonin (5-HT) reduces food intake in mammals and fish and modifies the macronutrients selection pattern in mammals; however, no information is available about its role on macronutrients selection in fish. The aim was to determine the effect of orally administered 5-HT (0.1, 0.5 and 2.5 mg kg BW(-)(1)) into gelatine capsules on the subsequent macronutrient selection of sea bass, using for this purpose gelatine capsules including carbohydrates, protein, or lipids separately. The voluntary ingested 5-HT was released into the plasma of fish, reaching a level two times greater than the controls, 45 min after the ingestion of a capsule containing 2.5 mg kg BW(-1) of 5-HT. The indoleamine, at doses of 0.1, 0.5 and 2.5 mg kg BW(-1), produced a reduction in total food intake of 31%, 49% and 37%, respectively, compared to the baseline, modifying the macronutrient selection pattern. The percentage of fat selected was significantly reduced whereas the percentage of protein significantly increased after administration of highest dose, but no changes were observed in the proportion of carbohydrate for any 5-HT doses. In conclusion, oral administration of 5-HT affected both amount of food intake and pattern of macronutrients selected. This is the first evidence supporting a role of 5-HT as a neurohumoral mediator involved in macronutrients selection in fish.

Influence of nutrient preload on encapsulated macronutrient selection in European sea bass.

The sea bass is a teleost that is able to regulate its energy intake by selecting from pure macronutrient sources, although the regulatory mechanisms involved in this selection are unknown. Nutrient preloads are known to reduce food intake and modify macronutrient selection patterns in mammals, but no information is available on its effects in fish. The aim of the present work was to determine the effect of orally administered macronutrient preloads of protein (P), fat (F) or carbohydrate (CH) on the subsequent macronutrient selection, using for the purpose feed consisting of CH, P or F packaged separately in gelatin capsules. The macronutrient preloads left the total food intake unaltered, but caused differential changes in the pattern of macronutrient selection. The CH preload increased the selection of CH (39%) and decreased that of P (20%), independently of the fish's previous nutritional preferences. The F preload induced an F increase (32%) and a P decrease (18%) in P-preferring fish, but not in F-preferring fish in which the macronutrient selection pattern remained unaffected. The P preload stimulated F selection by 42% in P-preferring fish, but left the macronutrient selection pattern unchanged in F-preferring fish. In conclusion, oral macronutrient preloads affected the pattern of macronutrient selection in fish, acting by post-ingestive mechanisms. The effect was influenced by the fish's previous nutritional preference and/or status, which could depend on its metabolic capacity.

Locomotor, feeding and melatonin daily rhythms in sharpsnout seabream (Diplodus puntazzo).

Sharpsnout seabream is a marine teleost of increasing interest for Mediterranean aquaculture, but there is still a lack of information regarding its circadian organization. In this study, we have investigated sharpsnout seabream locomotor activity, feeding and plasma melatonin daily rhythms under a 12:12-h LD cycle, as well as the persistence of locomotor activity circadian rhythmicity under constant light (LL) conditions. When submitted to an LD cycle, most sharpsnout seabream displayed a diurnal locomotor pattern, with an average 74% of activity recorded during daytime. However, along the experiment 40% of fish spontaneously changed their locomotor rhythm phasing and became nocturnal. Feeding behaviour, nevertheless, remained strictly diurnal in all cases, with 97% of food demands being made during the light period. Free-running locomotor rhythms were recorded in one third of the fish kept under LL. Daily plasma melatonin levels displayed a rhythmic profile, with low daytime values (111 pg/ml) and high nighttime concentrations (791 pg/ml). Taken together, these results evidence a high degree of plasticity for sharpsnout seabream activity patterns, as well as phasing independence of locomotor and feeding rhythms. Finally, the existence of a well-defined daily rhythm of plasma melatonin was found.

Effects of melatonin administration on oxidative stress and daily locomotor activity patterns in goldfish.

Melatonin has a number of physiological functions in addition to light-dark transduction. In recent years, many in vivo and in vitro studies in rodents have revealed an important antioxidant activity of melatonin, both directly and indirectly. Nevertheless, the potential effects of melatonin as an antioxidant in fish remain unknown. The aim of this research was to evaluate the capacity of melatonin injections (3 mg/kg) to attenuate oxidative damage after submitting goldfish to oxidative stress caused directly by hydrogen peroxide (H2O2) baths and indirectly by hypoxia and subsequent reoxygenation, as well as the locomotor activity. The results revealed that melatonin decreased lipid damage in muscle after hypoxia/reoxygenation (1.22 vs. 2.27 nmoles lipid peroxides/g tissue), but not in liver. Mortality caused by oxidative stress was not attenuated by melatonin. Surprisingly, melatonin caused an increase of mortality (50 vs. 95%) when administered before hypoxia. Locomotor activity was also affected by melatonin but not by the administration of the vehicle, suggesting a sedative effect of melatonin in goldfish. In conclusion, melatonin administration provoked slight effects on lipid peroxidation and mortality resulting from oxidative stress, with reduction of locomotor activity in relation to the vehicle.

Daily rhythms in the somatotropic axis of Senegalese sole (Solea senegalensis): The time of day influences the response to GH administration.

Growth factors in vertebrates display daily rhythms, which, while widely described in mammals, are still poorly understood in teleost fish. Here, we investigated the existence of daily rhythms in the somatotropic axis of the flatfish Solea senegalensis. In a first experiment, daily rhythms of the expression of pituitary adenylate cyclase-activating polypeptide (pacap), growth hormone (gh), insulin-like growth factor 1 (igf1) and its receptor (igf1r) were analyzed under a 12:12 h light:dark cycle. All genes displayed daily rhythms with the acrophases of pacap, gh and igf1 located in the second half of the dark phase (ZT 20:28-0:04 h), whereas the acrophase of igf1r was located around mid-light (ZT 5:33 h). In a second experiment, the influence of the time of day (mid-light, ML, versus mid-darkness, MD) of GH administration on the expression of these factors and on plasma glucose levels was tested. The response observed depended on the time of injection: the strongest effects were observed at MD, when GH administration significantly reduced pituitary gh and enhanced liver igf1 expression. These results provide the first evidence of daily rhythms and differential day/night effects in growth factors in S. senegalensis, suggesting new insights for investigating the physiology of growth and possible applications to improve fish aquaculture.

Fish macronutrient selection through post-ingestive signals: effect of selective macronutrient deprivation.

Recent reports describe teleosts as being able to regulate energy intake by selecting from pure macronutrient sources, although the regulatory mechanisms involved in this selection remain unknown. The aim of the present work was to determine the effect of selective macronutrient deprivation on energy regulation and macronutrient selection, using for this purpose carbohydrate (CH), protein (P), and fat (F) packaged separately into gelatin capsules, a method that prevents the diet chemosensory properties at oropharyngeal level from interfering with macronutrient selection. Twenty-four individually housed sea bass (Dicentrarchus labrax) were subjected to two experiments: (a) two-macronutrient deprivation, and (b) one-macronutrient deprivation. In two-macronutrient deprivation, fish were fed sequentially with P, CH, or F, and in one-macronutrient deprivation, they were fed sequentially with two separately packaged macronutrients (P and CH, CH and F, or P and F). There was a rapid reduction of macronutrient intake in two-macronutrient deprivation, reaching 80% inhibition after 4, 5, and 7 days of P, CH, and F intake, respectively. In one-macronutrient deprivation, the energy intake was significantly reduced during selective F deprivation, but not with P or CH deprivation. Although the fish were being fed with only two macronutrients, the relative proportions of these macronutrients in each selective deprivation phase were the same as the baseline. These results show that in deprivation studies fish need at least F plus one other macronutrient to regulate their energy intake, and that their macronutrient selection is stable even when one is absent. In summary, fish seem to regulate energy and macronutrient selection through post-ingestive mechanisms probably involving chemosensory detection in the gut, and/or post-absorptive mechanisms.

Effects of salinity on food intake and macronutrient selection in European sea bass.

Salinity is one of the most relevant environmental parameters in regards to fish physiology, modifying food intake and growth performance in many fish species; however, its possible effects on macronutrient selection are still unknown. The aim of this study was to determine the effects of three salinity levels (25 per thousand, 7 per thousand, and 0 per thousand) on total food intake and encapsulated macronutrient selection in a euryhaline teleost, European sea bass. A total of 40 fish (five per tank) with an average body weight of 52.4 +/- 7.1 g were used. Lowering the salinity level from 25 per thousand to 7 per thousand and 0 per thousand reduced food intake by 27% and 42%, respectively. Regarding macronutrient selection, these salinity changes significantly decreased the percentage of CH intake by 31% and 27%, while increasing that of P by 30% and 25%, respectively. Fat selection remained unaltered, with an average value of 22% for all tested salinities. Specific growth rate (SGR) and feed conversion efficiency (FCE) were affected by macronutrient selection pattern, which in turn was salinity-dependent. These results indicate a strong influence of salinity on European sea bass food intake and macronutrient selection.

Demand-feeding rhythms and feeding-entrainment of locomotor activity rhythms in tench (Tinca tinca).

Tench (Tinca tinca) has been described as a strictly nocturnal species whose locomotor activity rhythms, albeit strongly synchronised by light, have an endogenous nature. Aside from light, a number of other environmental factors, such as mealtime, can act as circadian system synchronisers in fish; however, there is a scarcity of information on tench feeding rhythms. This study describes daily self-feeding rhythms in tench, and analyses the role of feeding time on synchronisation of locomotor activity rhythms. Tench were able to operate string sensor-activated self-feeders, and they displayed a strictly nocturnal behavior, both under indoor and outdoor conditions. Locomotor activity remained strictly nocturnal irrespective of whether tench were fed only during the scotophase (D-feeding) or the photophase (L-feeding). However, no statistically significant differences were detected between both groups in terms of food intake or growth performance. Furthermore, unlike L-feeding, D-feeding elicited a clear anticipatory activity (FAA). When tench were given the possibility of feeding at both times of the day, they showed a clear preference for D-feeding. Finally, in fish exposed to constant darkness (DD), feeding time acted as a true zeitgeber and FAA was observed. When animals were fasted under DD conditions, locomotor activity free-run and 6 out of 12 individuals yielded significant results in the periodogram analysis. Under DD, fish resynchronised when regular food was resumed, with some tench displaying FAA. The obtained results indicated the existence of a feeding-entrainable oscillator (FEO) in tench.

Influence of light intensity on plasma melatonin and locomotor activity rhythms in tench.

Melatonin production by the pineal organ is influenced by light intensity, as has been described in most vertebrate species, in which melatonin is considered a synchronizer of circadian rhythms. In tench, strict nocturnal activity rhythms have been described, although the role of melatonin has not been clarified. In this study we investigated daily activity and melatonin rhythms under 12:12 light-dark (LD) conditions with two different light intensities (58.6 and 1091 microW/cm2), and the effect of I h broad spectrum white light pulses of different intensities (3.3, 5.3, 10.5, 1091.4 microW/cm2) applied at middarkness (MD) on nocturnal circulating melatonin. The results showed that plasma melatonin in tench under LD 12:12 and high light conditions displayed rhythmic variation, where values at MD (255.8 +/- 65.9 pg/ml) were higher than at midlight (ML) (70.7 +/- 31.9 pg/ml). Such a difference between MD and ML values was reduced in animals exposed to LD 12: 12 and low light intensity. The application of 1 h light pulses at MD lowered plasma melatonin to 111.6 +/- 3.2 pg/ml (in the 3.3-10.5 microW/cm2 range) and to 61.8 +/- 18.3 pg/ml (with the 1091.4 microW/cm2 light pulse) and totally suppressed nocturnal locomotor activity. These results show that melatonin rhythms persisted in tench exposed to low light intensity although the amplitude of the rhythm is affected. In addition, it was observed that light pulses applied at MD affected plasma melatonin content and locomotor activity. Such a low threshold suggests that the melatonin system is capable of transducing light even under dim conditions, which may be used by this nocturnal fish to synchronize to weak night light signals (e.g., moonlight cycles).