Entrainment to light of circadian activity rhythms in tench (Tinca tinca)

The present article analyzes locomotor activity rhythms in Tinca tinca. To that end, three different experiments were conducted on 24 animals (20 g body weight) kept in pairs in 60-liter aquaria fitted with infrared sensors connected to a computer to continuously record fish movements. The first experiment was designed to study the endogenous circadian clock under free-running conditions [ultradian 40:40 min LD pulses and constant dark (DD)] and after shifting the LD cycle. Our results demonstrate that tench has a strictly nocturnal activity pattern, an endogenous rhythm being evident in 45.8% of the fish analyzed. The second experiment was conducted to test the influence of different photoperiods (LD 6:18, 12:12, 18:6, and 22:2) on locomotor activity, the results showing that even under an extremely long photoperiod, tench activity is restricted to dark hours. The third experiment examined the effect of light intensity on locomotor activity rhythms. When fish were exposed to decreasing light intensities (from 300:0 lux to 30:0, 3:0, and 0.3:0 lux) while maintaining a constant photoperiod (LD 12:12), the highest percentage of locomotor activity was in all cases associated with the hours of complete darkness (0 lux). In short, our results clearly show that (a) tench is a species with a strictly nocturnal behavior, and (b) daily activity rhythms gradually entrain after shifting the LD cycle and persist under free-running conditions, pointing to their circadian nature. However, light strongly influences activity rhythms, since (c) the length of the active phase is directly controlled by the photophase, and (d) strictly nocturnal behavior persists even under very dim light conditions (0.3 lux). The above findings deepen our knowledge of tench behavior, which may help to optimize the aquacultural management of this species, for example, by adjusting feeding strategies to their nocturnal behavior.     

Light and dark rations and the photic entrainment of circadian locomotor activity patterns in the South American Silver Catfish (Rhamdia quelen,Quoy & Gaimard, 1824)

The aim of this study was to evaluate the daily rhythm of locomotor activity in Rhamdia quelen (R. quelen). A total of 30 fish were enrolled in the study and were equally divided in 10 groups and maintained in 100 liters tanks. The locomotor activity was measured in fish maintained under the LD 12:12 photoperiod regime; thereafter, the LD cycle was reversed to DL in order to study the resynchronization and to explore the endogenous pacemaker. Subsequently, the fish were subjected to constant conditions of light to test whether or not locomotor rhythms are regulated by the endogenous circadian clock. The effect of increasing light length and intensity was studied on daily rhythm of locomotor activity of fish. Our results showed that the R. quelen is a strictly diurnal species, the rhythm of locomotory activity resynchronized quickly after inverting the LD cycle and persist under free course LL, suggesting a circadian origin. The light showed a significant masking effect often blocking the expression of the biological rhythm. The strictly diurnal behavior is controlled directly by the photoperiod and maintained even under very dim light (30 lux).     

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 1,091 µW/cm²), and the effect of 1 h broad spectrum white light pulses of different intensities (3.3, 5.3, 10.5, 1,091.4 µW/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 µW/cm² range) and to 61.8±18.3 pg/ml (with the 1,091.4 µW/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).     

Circadian Rhythms of Locomotor Activity in the Nile Tilapia Oreochromis niloticus

Behavioral rhythms of the Nile tilapia were investigated to better characterize its circadian system. To do so, the locomotor activity patterns of both male and female tilapia reared under a 12:12 h light-dark (LD) cycle were studied, as well as in males the existence of endogenous rhythmicity under free-running conditions (DD and 45 min LD pulses). When exposed to an LD cycle, the daily pattern of activity differed between individuals: some fish were diurnal, some nocturnal, and a few displayed an arrhythmic pattern. This variability would be typical of the plastic circadian system of fish. Moreover, reproductive events clearly affected the behavioral rhythms of female tilapia, a mouth-brooder teleost species. Under DD, 50% (6 of 12) of male fish showed circadian rhythms with an average period (τ) of 24.1±0.2 h, whereas under the 45 min LD pulses, 58% (7 of 12) of the fish exhibited free-running activity rhythms with an average τ of 23.9±0.5 h. However, interestingly in this case, activity was always confined to the dark phase. Furthermore, when the LD cycle was reversed, a third of the fish showed gradual resynchronization to the new phase, taking 7–10 days to be completely re-entrained. Taken together, these results suggest the existence of an endogenous circadian oscillator that controls the expression of locomotor activity rhythms in the Nile tilapia, although its anatomical localization remains unknown.     

Light and temperature cycles as zeitgebers of zebrafish (Danio rerio) circadian activity rhythms

Light and temperature cycles are the most important synchronizers of biological rhythms in nature. However, the relative importance of each, especially when they are not in phase, has been poorly studied. The aim of this study was to analyze the entrainment of daily locomotor activity to light and/or temperature cycles in zebrafish. Under two constant temperatures (20°C and 26°C) and 12:12 light-dark (LD) cycles, zebrafish were most active during the day (light) time and showed higher total activity at the warmer temperature, while diurnalism was higher at 20°C than at 26°C (87% and 77%, respectively). Under thermocycles (12:12 LD, 26:20°C thermophase:chryophase or TC), zebrafish daily activity synchronized to the light phase, both when the thermophase and light phase were in phase (LD/TC) or in antiphase (LD/CT). Under constant dim light (3 lux), nearly all zebrafish synchronized to thermocycles (τ=24 h), although activity rhythms (60% to 67% of activity occurred during the thermophase) were not as marked as those observed under the LD cycle. Under constant dim light of 3 lux and constant temperature (22.5°C), 4 of 6 groups of zebrafish previously entrained to thermocycles displayed free-running rhythms (τ=22.9 to 23.6 h). These results indicate that temperature cycles alone can also entrain zebrafish locomotor activity.     

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).     

Circadian rhythm of motor activity of the Brazilian rock cavy (Kerodon rupestris) under artificial photoperiod

Kerodon rupestris, a Brazilian caviidae rodent, lives in dry stony places. In a first experiment, seven animals were kept in LD (250:0 lux and 400:0 lux) during 40 days in each condition. In the second, four animals were kept in LD (470 lux: red dim light) for 47 days, then in LL (470 lux) for 18 days and in DD (red dim light) for 23 days. Motor activity was continuously recorded by infrared sensors. Animals showed entrained rhythms to the LD cycle being light and dark active, with higher values in phase transitions. When the light intensity was increased, four animals increased and two reduced the activity. In LL, three animals expressed an endogenous tau of 24.4, 26.5 and 24.6 h and one was arrhythmic; in DD, two expressed tau of 23.6 and 23.7 h and one was arrhythmic. Results indicate that Kerodon rupestris circadian rhythm is affected by light intensity but it is not yet possible to determine its habit.     

Circadian rhythm of locomotor activity in individual workers of the wood ant Formica rufa

ABSTRACT. Individual worker ants isolated in an actograph exhibit circadian rhythms of locomotor activity. Entrainment occurs more readily in LD 18:6 h than in LD 12:12 h. The ants are either light-active or dark-active. Phase angle and duration of activity is influenced by photoperiod.     

Possible evidence for shift work schedules in the media workers of the ant species Camponotus compressus

The locomotor activity rhythm of the media workers of the ant species Camponotus compressus was monitored under constant conditions of the laboratory to understand the role of circadian clocks in social organization. The locomotor activity rhythm of most ants entrained to a 24 h light/dark (12:12 h; LD) cycle and free-ran under constant darkness (DD) with circadian periodicities. Under entrained conditions about 75% of media workers displayed nocturnal activity patterns, and the rest showed diurnal activity patterns. In free-running conditions these ants displayed three types of activity patterns (turn-around). The free-running period (τ) of the locomotor activity rhythm of some ants (10 out of 21) showed period lengthening, and those of a few (6 out of 21) showed period shortening, whereas the locomotor activity rhythm of the rest of the ants (5 out of 21) underwent large phase shifts. Interestingly, the pre-turn-around τ of those ants that showed nocturnal activity patterns during earlier LD entrainment was shorter than 24 h, which became greater than 24 h after 6-9 days of free-run in DD. On the other hand, the pre-turn-around τ of those ants, which exhibited diurnal patterns during earlier LD entrainment, was greater than 24 h, which became shorter than 24 h after 6-9 days of free-run in DD. The patterns of activity under LD cycles and the turn-around of activity patterns in DD regime suggest that these ants are shift workers in their respective colonies, and they probably use their circadian clocks for this purpose. Circadian plasticity thus appears to be a general strategy of the media workers of the ant species C. compressus to cope with the challenges arising due to their roles in the colony constantly exposed to a fluctuating environment.     

The effects of pinealectomy and blinding on the circadian locomotor activity rhythm in the Japanese newt,Cynops pyrrhogaster

We examined the effects of pinealectomy and blinding (bilateral ocular enucleation) on the circadian locomotor activity rhythm in the Japanese newt, Cynops pyrrhogaster. The pinealectomized newts were entrained to a light-dark cycle of 12 h light and 12 h darkness. After transfer to constant darkness they showed residual rhythmicity for at least several days which was gradually disrupted in prolonged constant darkness. Blinded newts were also entrained to a 12 h light/12 h dark cycle. In subsequent constant darkness they showed free-running rhythms of locomotor activity. However, the freerunning periods noticeably increased compared with those observed in the previous period of constant darkness before blinding. In blinded newts entrained to the light/dark cycle the activity rhythms were gradually disrupted after pinealectomy even in the presence of the light/dark cycle. These results suggest that both the pineal and the eyes are involved in the newt's circadian system, and also suggest that the pineal of the newt acts as an extraretinal photoreceptor which mediates the entrainment of the locomotor activity rhythm.Abbreviations circadian period - DD constant darkness - LD cycle, light-dark cycle - LD 12:12 light-dark cycle of 12 h light and 12 h darkness     

Light and Temperature Cycles as Zeitgebers of Zebrafish (Danio rerio) Circadian Activity Rhythms

Light and temperature cycles are the most important synchronizers of biological rhythms in nature. However, the relative importance of each, especially when they are not in phase, has been poorly studied. The aim of this study was to analyze the entrainment of daily locomotor activity to light and/or temperature cycles in zebrafish. Under two constant temperatures (20°C and 26°C) and 12:12 light-dark (LD) cycles, zebrafish were most active during the day (light) time and showed higher total activity at the warmer temperature, while diurnalism was higher at 20°C than at 26°C (87% and 77%, respectively). Under thermocycles (12:12 LD, 26:20°C thermophase:chryophase or TC), zebrafish daily activity synchronized to the light phase, both when the thermophase and light phase were in phase (LD/TC) or in antiphase (LD/CT). Under constant dim light (3 lux), nearly all zebrafish synchronized to thermocycles (τ=24 h), although activity rhythms (60% to 67% of activity occurred during the thermophase) were not as marked as those observed under the LD cycle. Under constant dim light of 3 lux and constant temperature (22.5°C), 4 of 6 groups of zebrafish previously entrained to thermocycles displayed free‐running rhythms (τ=22.9 to 23.6 h). These results indicate that temperature cycles alone can also entrain zebrafish locomotor activity.     

Two Oscillators Might Control the Locomotor Activity Rhythm of the High‐Altitude Himalayan Strain of Drosophila Helvetica

The properties of the pacemaker controlling the adult locomotor activity rhythm of the high‐altitude Himalayan (haH) strain (Hemkund Sahib, 4121 m above sea level) of Drosophila helvetica are strikingly different from those of the low‐altitude Himalayan (laH) strain (Birahi, 1132 m above sea level) of the same species. The haH strain has a unimodal activity pattern with a delayed peak occurring about 4.5 h after lights‐on of the entraining light‐dark (LD) cycle, while the laH strain has a bimodal activity pattern with the morning and evening peaks. It is rather unusual for a wild type strain of any Drosophila species to have a unimodal activity pattern during entrainment as observed in the haH strain. The single activity peak of the haH strain is regarded as a consequence of delayed morning peak merging with the evening one. Three experiments were performed to test this hypothesis. The first experiment examined whether the single activity peak could be dissociated into two components by LD cycles in which photoperiods varied from 10 to 16 h per 24 h. The haH strain again exhibited a unimodal activity pattern with a delayed peak in 10, 12, and 14 h photoperiods but a bimodal activity pattern in 16 h photoperiod. The laH strain had bimodality in 10 and 12 h photoperiods, unimodality in a 14 h photoperiod, but complete arrhythmicity in a 16 h photoperiod.

In the second experiment, the haH flies were transferred from LD 16∶8 to LL at 5 lux to confirm whether the bimodality of this strain in LD 16∶8 cycles was not the result of masking by the long photoperiod of 16 h. Bimodality of the haH strain persisted in LL too; moreover, the morning component free‐ran with period (τ) <24 h, while the evening component free‐ran with τ>24 h. The third experiment examined the LL‐induced splitting of activity peak of the haH strain. Flies were transferred from LD 12∶12 cycles to LL at 0, 1, 5, and 15 lux. The haH strain was rhythmic in LL at 0 and 1 lux with a unimodal activity pattern. It was also rhythmic in LL at 5 lux, but the single activity peak was split into two discrete components; the morning component free‐ran with τ<24 h, while the evening component free‐ran with τ>24 h. This strain, however, was completely arrhythmic in LL at 15 lux. The laH strain was uniformly arrhythmic in LL at all levels of light intensity. These results suggest that the single but late activity component of the haH strain during entrainment appears to be the consequence of merging the delayed morning peak with the evening one as an adaptation to the environmental conditions at the altitude of origin of this strain, where these flies begin activity in the forenoon owing to non‐permissible low temperature in the morning.     

Possible Evidence for Shift Work Schedules in the Media Workers of the Ant Species Camponotus compressus

The locomotor activity rhythm of the media workers of the ant species Camponotus compressus was monitored under constant conditions of the laboratory to understand the role of circadian clocks in social organization. The locomotor activity rhythm of most ants entrained to a 24 h light/dark (12:12 h; LD) cycle and free-ran under constant darkness (DD) with circadian periodicities. Under entrained conditions about 75% of media workers displayed nocturnal activity patterns, and the rest showed diurnal activity patterns. In free-running conditions these ants displayed three types of activity patterns (turn-around). The free-running period (τ) of the locomotor activity rhythm of some ants (10 out of 21) showed period lengthening, and those of a few (6 out of 21) showed period shortening, whereas the locomotor activity rhythm of the rest of the ants (5 out of 21) underwent large phase shifts. Interestingly, the pre-turn-around τ of those ants that showed nocturnal activity patterns during earlier LD entrainment was shorter than 24 h, which became greater than 24 h after 6–9 days of free-run in DD. On the other hand, the pre-turn-around τ of those ants, which exhibited diurnal patterns during earlier LD entrainment, was greater than 24 h, which became shorter than 24 h after 6–9 days of free-run in DD. The patterns of activity under LD cycles and the turn-around of activity patterns in DD regime suggest that these ants are shift workers in their respective colonies, and they probably use their circadian clocks for this purpose. Circadian plasticity thus appears to be a general strategy of the media workers of the ant species C. compressus to cope with the challenges arising due to their roles in the colony constantly exposed to a fluctuating environment.     

Daily rhythms of locomotor and demand-feeding activities in Schizothorax pelzami (Kessler, 1870)

A study was carried out to investigate the daily rhythms of locomotor and feeding activity of Khajoo, Schizothorax pelzami, a candidate species for freshwater aquaculture. Using self-feeder juvenile Khajoo were exposed to a 12/12 LD cycle to determine the rhythms of locomotor and feeding activity. The effects of feeding on locomotor and feeding activity of fish were also examined. Finally, the endogenous rhythmicity under different lighting condition tested. Fish displayed a strictly diurnal feeding and locomotor activities with 98% and 84% of the total activity occurred in the photophase, respectively. In scheduled feeding, both the L-group (fed in light) and the D-group (fed in the dark) showed a diurnal locomotor activity pattern. However, the L-group had a peak of locomotor activity near the feeding time, but the D-group had a scarce locomotor activity in the scatophase with no significant change at the mealtime. Most of the individuals display free-running rhythms when exposed to different lighting condition including, constant darkness, ultradian 45:45 min LD cycle and reversed DL photo cycle. Taken together the results of this study showed that both locomotor and feeding activity have diurnal rhythms in Khajoo S. pelzami, even fish feeding had taken place at night. Additionally, the free-running locomotor activity of the fish in the absence of external light stimuli, suggests the existence of an endogenous timing mechanism in this fish species.     

Circadian rhythms of demand-feeding and locomotor activity in rainbow trout

Under free-running conditions, most rainbow trout displayed circadian feeding rhythms, although the expression of circadian rhythmicity depended on the experimental condition: 16·7% of fish under constant dim light (LL dim), 66·1% under a 45 :45 min light-dark cycle (LD pulses), and 83·8% under constant light (LL). Under LD pulses, the period length of the free-running rhythms for feeding was significantly shorter (21·9 ± 0·7 h, n =8) than under LL (26·2 ± 0·3 h, n =10). Period length for locomotor activity under LL was 25·8 ± 0·6 h ( n =4). Under LD conditions, the daily demand-feeding profile was always confined to the light phase and chiefly composed of two main episodes, directly after lights on (light elicited) and in anticipation to lights off (endogenous). Contrasting to feeding, the diel locomotor activity profile varied remarkably: a diurnal activity pattern at the bottom, while a clearly nocturnal pattern at the surface. These results contribute to a better understanding of feeding and locomotor rhythms of rainbow trout, providing evidence for the existence of a biological clock involved in their circadian control. This finding contrasts with the previously recorded lack of an endogenous oscillator in the pineal organ driving the rhythmic secretion of melatonin, which suggests different locations from the pineal for the circadian pacemakers in this species.     

Circadian Pattern of Wheel‐Running Activity of a South American Subterranean Rodent (Ctenomys cf knightii)

Circadian rhythms are regarded as essentially ubiquitous features of animal behavior and are thought to confer important adaptive advantages. However, although circadian systems of rodents have been among the most extensively studied, most comparative biology is restricted to a few related species. In this study, the circadian organization of locomotor activity was studied in the subterranean, solitary north Argentinean rodent, Ctenomys knightii. The genus, Ctenomys, commonly known as Tuco‐tucos, comprises more than 50 known species over a range that extends from 12°S latitude into Patagonia, and includes at least one social species. The genus, therefore, is ideal for comparative and ecological studies of circadian rhythms. Ctenomys knightii is the first of these to be studied for its circadian behavior. All animals were wild caught but adapted quickly to laboratory conditions, with clear and precise activity‐rest rhythms in a light‐dark (LD) cycle and strongly nocturnal wheel running behavior. In constant dark (DD), the rhythm expression persisted with free‐running periods always longer than 24 h. Upon reinstatement of the LD cycle, rhythms resynchronized rapidly with large phase advances in 7/8 animals. In constant light (LL), six animals had free‐running periods shorter than in DD, and 4/8 showed evidence of “splitting.” We conclude that under laboratory conditions, in wheel‐running cages, this species shows a clear nocturnal rhythmic organization controlled by an endogenous circadian oscillator that is entrained to 24 h LD cycles, predominantly by light‐induced advances, and shows the same interindividual variable responses to constant light as reported in other non‐subterranean species. These data are the first step toward understanding the chronobiology of the largest genus of subterranean rodents.     

Photic entrainment of daily rhythm pattern of locomotor activity in sea bass (Dicentrarcus labrax)

The aim of this study was to evaluate the circadian rhythm of daily locomotor activity in sea bass exposed to three different artificial lighting regimes including 12/12, 24/0, and 0/24 L/D periods, and scheduled daytime feeding. Daily rhythm of locomotor activity during 12/12 L/D was observed, whereas locomotor activity recorded during 24/0 and 0/24 L/D resulted arrhythmic. Statistical differences in mesor values among the photoperiods and between light and dark phases of each photoperiod were found. During the 12/12 and 0/24 L/D, the fish were active mostly during the light phase. During 24/0 L/D, a phase inversion occurred. The results showed that photoperiod is a dominant synchronizer in sea bass, and that the appetite in this fish follows daily rhythms which does not match the imposed feeding schedule, suggesting the absence or the lability of internal process that couples feeding cycles and biological rhythms.     

Plasticity of Hamster Circadian Entrainment Patterns Depends on Light Intensity

The multiple oscillatory basis of the mammalian circadian pacemaker is adduced by, among other phenomena, the occurrence of split locomotor activity rhythms in rodents after prolonged exposure to constant light. More recently, split rhythms entrained to a 24h light:dark:light:dark cycle have been documented following scheduled access of hamsters to a novel running wheel or by photoperiod manipulations alone. Because the incidence of constant light-induced splitting depends on light intensity, the role of this variable was assessed in this new splitting paradigm. Male Syrian hamsters, entrained to a 14h light:10h dark cycle, were transferred to individual running wheel cages 7h after light onset. Transfer coincided with the beginning of the scotophase of a new photocycle alternating between 5h of relative dark and 7h of light. For four weeks bright photophases (～350 lux) were alternated with either dim (<0.1 lux) or completely dark (0 lux) scotophases. An additional group received moderate intensity photophases (～45 lux) paired with dim scotophase illumination. For an additional four weeks, all hamsters were exposed to the same bright:dim light:dark cycle. Dim light in the scotophase significantly increased the incidence of split activity rhythms relative to that observed with completely dark scotophases. Overall wheel-running rates and activity induced by a cage change were also increased in dim light-exposed animals. Group differences largely disappeared four weeks later when hamsters previously maintained in completely dark scotophases were exposed to dim scotophases. Photophase light intensity did not affect the overall incidence of splitting, but influenced the timing of activity in the afternoon scotophase. The effects of dim illumination may be mediated in part via enhanced locomotor responses to transfer to a new cage or by changes in coupling interactions between component oscillators.     

Melatonin effects on food intake and activity rhythms in two fish species with different activity patterns: Diurnal (goldfish) and nocturnal (tench)

Melatonin has several known physiological functions, the main one being synchronization of daily and seasonal rhythms. In addition, melatonin has been reported to influence food intake and behavioral rhythms with varying results depending on the species. The aim of this research was to evaluate the effects of intraperitoneal melatonin injection on food intake and locomotor activity in two different fish species: goldfish (diurnal) and tench (nocturnal), under different light regimes: constant light (LL) conditions or LD 12:12, with melatonin administration at mid-light (ML), mid-dark (MD), and after a 1-h light pulse at MD. In addition to these acute tests, in the case of goldfish we also investigated the effects of daily melatonin administration for 1 week. Our results indicated that acute melatonin administration significantly decreased goldfish food intake (16-52% inhibition, depending on the light regime) and locomotor activity (55-100%), with the chronic treatment inducing a similar total food intake inhibition that persisted for 7 days. In tench, a nocturnal fish species, acute melatonin administration at MD and ML reduced food intake (37% and 29%, respectively), while locomotor activity was not affected at MD and slightly increased at ML. Taken together, these results indicated that melatonin reduced food intake in both species, while its effects on locomotor activity depended on the time of administration (light or dark phase) and the activity patterns of the species.     

Circadian Rhythms and time Measurement in Locomotor Activity of the Scorpion Leiurus Quinquestriatus (Buthidae)

The circadian rhythms of locomotor activity of the scorpion Leiurus quinqueslriatus were examined under different light-dark cycles and in free-running conditions. The circadian rhythm is bimodal in LD 12:12 with alternating cycles of temperature (35°-25°C) with high intensity (1300 lux) or in LD 12: 12 with constant temperature 35° C with 300 lux. In LD 12:12 (1300 lux), in long or in short light spans with constant temperature, the bimodal pattern is slightly changed with the appearance of a third minor peak of activity. In free-running conditions, the bimodal rhythm of locomotor activity persists in DD with T about 24 hr, but in LL the rhythm becomes unimodal with T about 24 hr. Cosinor and power spectrum analysis showed the presence of more than one periodic component. It seems that there is a correlation between the range of light regimens, temperature, light intensity and the coincidence of these components. These components are independently entrained by the environmental light cycle. The mechanism of entrainment of components is discussed.