Circadian rhythms of locomotor activity and temperature selection in sleepy lizards, <Emphasis Type="Italic">Tiliqua rugosa</Emphasis>

This study examined whether the daily rhythms of locomotor activity and behavioural thermoregulation that have previously been observed in Australian sleepy lizards (Tiliqua rugosa) under field conditions are true circadian rhythms that persist in constant darkness (DD) and whether these rhythms show similar characteristics. Lizards held on laboratory thermal gradients in the Australian spring under the prevailing 12-hour light : dark (LD) cycle for 14 days displayed robust daily rhythms of behavioural thermoregulation and locomotor activity. In the 13-day period of DD that followed LD, most lizards exhibited free-running circadian rhythms of locomotor activity and behavioural thermoregulation. The predominant activity pattern displayed in LD was unimodal and this was retained in DD. While mean levels of skin temperature and locomotor activity were found to decrease from LD to DD, activity duration remained unchanged. The present results demonstrate for the first time that this species’ daily rhythm of locomotor activity is an endogenous circadian rhythm. Our results also demonstrate a close correlation between the circadian activity and thermoregulatory rhythms in this species indicating that the two rhythms are controlled by the same master oscillator(s). Future examination of seasonal aspects of these rhythms, may, however, cause this hypothesis to be modified.     

Developmental plasticity of the locomotor activity rhythm of Drosophila melanogaster

We used four replicate outbred populations of Drosophila melanogaster to investigate whether the light regimes experienced during the pre-adult (larval and pupal) and early adult stages influence the free-running period (τ_DD) of the circadian locomotor activity rhythm of adult flies. In a series of two experiments four different populations of flies were raised from egg to eclosion in constant light (LL), in light/dark (LD) 12:12 h cycle, and in constant darkness (DD). In the first experiment the adult male and female flies were directly transferred into DD and their locomotor activity was monitored, while in the second experiment the locomotor activity of the emerging adult flies was first assayed in LD 12:12 h for 15 days and then in DD for another 15 days. The τ_DD of the locomotor activity rhythm of flies that were raised in all the three light regimes, LL, LD 12:12 h and in DD was significantly different from each other. The τ_DD of the locomotor activity rhythm of the flies, which were raised in DD during their pre-adult stages, was significantly shorter than that of flies that were raised as pre-adults in LL regime, which in turn was significantly shorter than that of flies raised in LD 12:12 h regime. This pattern was consistent across both the experiments. The results of our experiments serve to emphasise the fact that in order to draw meaningful inferences about circadian rhythm parameters in insects, adequate attention should be paid to control and specify the environment in which pre-adult rearing takes place. The pattern of pre-adult and early adult light regime effects that we see differs from that previously observed in studies of mutant strains of D. melanogaster, and therefore, also points to the potential importance of inter-strain differences in the response of circadian organisation to external influences.     

Parametric and nonparametric entrainment of circadian locomotor rhythm in the Japanese honeybee Apis cerana japonica

The circadian rhythm of locomotor activity in the Japanese honeybee Apis cerana japonica was studied to determine the involvement of parametric and/or nonparametric entrainment. The rhythm was entrained to a skeleton photoperiod in which a 1-h first light pulse was imposed in the morning along with a second light pulse in the evening, as well as to a complete photoperiodic regime (LD 12:12). However, the timing of peak activity relative to the lights-off in the evening in the skeleton photoperiod was earlier than that in the complete photoperiod. A single daily light pulse in the evening entrained the rhythm, whereas a daily light pulse in the morning allowed free-running as in constant darkness. The free-running period (τ) of locomotor activity in constant light became longer as the light intensity increased. A Winfree's type I phase response curve of the locomotor activity rhythm was obtained using a single 1-h light pulse. The results suggest that both parametric and nonparametric entrainment are involved in the circadian rhythm of individual locomotor activity in this honeybee.     

Locomotor activity rhythm in four wrasse species under varying light conditions

Under controlled laboratory conditions, the locomotor activity rhythms of four species of wrasses (Suezichthys gracilis, Thalassoma cupido, Labroides dimidiatus andCirrhilabrus temminckii) were individually examined using an actograph with infra-red photo-electric switches in a dark room at temperatures of 21.3–24.3°C, for 7 to 14 days. The locomotor activity ofS. gracilis occurred mostly during the light period under a light-dark cycle regimen (LD 12:12; 06:00-18:00 light, 18:00-06:00 dark). The locomotor activity commenced at the beginning of the light period and continued until a little before the beginning of dark period. The diel activity rhythm of this species synchronizes with LD. Under constant illumination (LL) this species shows distinct free-running activity rhythms varying in length from 23 hrs. 39 min. to 23 hrs. 47 min. Therefore,S. gracilis appears to have a circadian rhythm under LL. However, in constant darkness (DD), the activity of this species was greatly suppressed. All the fish showed no activity rhythms in DD conditions. After DD, the fish showed the diel activity rhythm with the resumption of LD, but this activity began shortly after the beginning of light period. The fish required several days to synchronize with the activity in the light period. Therefore,S. gracilis appeared to continue the circadian rhythm under DD. InT. cupido, the locomotor activity commenced somewhat earlier than the beginning of the light period and continued until the beginning of the dark period under LD. The diel activity rhythm of this species synchronizes with LD. Under LL, four of the five specimens of this species tested showed free-running activity rhythms for the first 5 days or longer varying in length from 22 hrs. 54 min. to 23 hrs. 39 min. Although the activity of this species was suppressed under DD, two of five fish showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 38 min. to 23 hrs. 50 min. under DD. Therefore, it was ascertained thatT. cupido has a circadian rhythm. InL. dimidiatus, the locomotor activity rhythm under LD resembled that observed inT. cupido. The diel activity rhythm of this species synchronizes with LD. Under LL, four of seven of this species showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 07 min. to 25 hrs. 48 min. Although the activity of this species was suppressed under DD, three of five fish showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 36 min. to 23 hrs. 41 min. under DD. Therefore, it was ascertained thatL. dimidiatus has a circadian rhythm. Almost all locomotor activity of C.temminckii occurred during the light period under LD. The diel activity rhythm of this species coincides with LD. Under LL, two of four of this species showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 32 min. to 23 hrs. 45 min. Although the activity of this species was suppressed under DD, one of the four fish showed free-running activity rhythms throughout the experimental period. The length of the free-running period was 23 hrs. 21 min. under DD. Therefore,C. temminckii appeared to have a circadian rhythm. According to field observations,S. gracilis burrows and lies in the sandy bottom whileT. cupido, L. dimidiatus, andC. temminckii hide and rest in spaces among piles of boulders or in crevices of rocks during the night. It seems that the differences in nocturnal behavior among the four species of wrasses mentioned above are closely related to the intensity of endogenous factors in their locomotor activity rhythms.     

A peripheral pacemaker drives the circadian rhythm of synaptic boutons in Drosophila independently of synaptic activity

Circadian rhythms in the morphology of neurons have been demonstrated in the fly Drosophila melanogaster. One such rhythm is characterized by changes in the size of synaptic boutons of an identified flight motor neuron, with larger boutons during the day compared with those at night. A more detailed temporal resolution of this rhythm shows here that boutons grow at a time of increased locomotor activity during the morning but become gradually smaller during the day and second period of increased locomotor activity in the evening. We have experimentally manipulated the synaptic activity of the fly during short periods of the day to investigate whether changes in bouton size might be a consequence of the different levels of synaptic activity associated with the locomotion rhythm of the fly. In the late night and early morning, when the flies normally have an intense period of locomotion, the boutons grow independently of whether the flies are active or completely paralyzed. Bouton size is not affected by sleep-deprivation during the early night. The cycle in bouton size persists for 2 days even in decapitated flies, which do not move, reinforcing the notion that it is largely independent of synaptic activity, and showing that a pacemaker other than the main biological clock can drive it.     

Early- and late-emerging Drosophila melanogaster fruit flies differ in their sensitivity to light during morning and evening

The authors derived early and late populations of fruit flies showing increased incidence of emergence during morning or evening hours by imposing selection for timing of emergence under 12:12 h light/dark (LD) cycles. From previous studies, it was clear that the increased incidence of adult emergence during morning and evening hours in early and late populations was a result of evolution of divergent and characteristic emergence waveforms in these populations. Such characteristic waveforms are henceforth referred to as "evolved emergence waveforms" (EEWs). The early and late populations also evolved different circadian clocks, which is evident from the divergence in their clock period (τ) and photic phase response curve (PRC). Although correlation between emergence waveforms and clock properties suggests functional significance of circadian clocks, τ and PRCs do not satisfactorily explain the early and late emergence phenotypes. In order to understand the functional significance of the PRC for early and late emergence phenotypes, the authors investigated whether circadian clocks of these flies exhibit any difference in photosensitivity under entrained conditions. Such differences would suggest that the light requirement for circadian entrainment of the emergence rhythm in early and late populations is different. To test this, they examined if early and late flies differ in their light utilization behavior, first by assaying their emergence rhythm under complete photoperiod and then in three different skeleton photoperiods. The results showed that early and late populations require different durations of light during the morning and evening to achieve their EEWs, suggesting that for the circadian entrainment of the emergence rhythm, early and late flies utilize light from different parts of the day.     

Daily activity rhythm of desert omni-seasonal Tenebrionid beetles, <Emphasis Type="Italic">Trigonoscelis gigas</Emphasis> and <Emphasis Type="Italic">T. sublaevicollis</Emphasis> (Coleoptera,Tenebrionidae) in constant darkness and under alternating 1-hour pulses of light and darkness

In the Karakum Desert (Turkmenistan) the beetles Trigonoscelis gigas are only active in the morning and evening while T. sublaevicollis are strictly nocturnal, regardless of the season and weather. The daily activity rhythm of T. gigas and T. sublaevicollis was studied in the laboratory according to the following pattern: 5 days under a light-darkness cycle of 15: 9 h (LD 15: 9), then 10 days in constant darkness (DD), and then 10 more days under alternating 1-h pulses of light and darkness (LD 1: 1). The temperature was 25°C in all the modes. At LD 15: 9, beetles of both species maintained a 24-h period and a natural pattern of the activity rhythm. In DD, the circadian rhythm ran with a period of 23.5 ± 0.3 h (n = 40) in T. gigas and 23.6 ± 0.4 h (n = 40) in T. sublaevicollis. At DD, the morning and evening activity peaks of T. gigas merged to form a rhythm with only one peak. Under LD 1: 1, both T. gigas and T. sublaevicollis recovered a 24-h period of the rhythm, while the rhythm of T. gigas regained the two-peak structure. Our research confirmed the assumption of Tshernyshev (1980) about the 24-h period of the free-running endogenous rhythm and the distorting effect of constant conditions on this rhythm.     

Circadian Locomotor Activity Under Artificial Light in the Freshwater Crab Pseudothelphusa americana

Long-term recordings of locomotor activity were obtained from intact freshwater crabs, Pseudothelphusa americana in constant darkness (DD), constant light (LL) and different light-dark (LD) protocols. Bimodal rhythms were typically observed in this crab when subjected to DD or LD, with bouts of activity anticipating lights-on and lights-off, respectively. Freerunning circadian rhythms were expressed in both DD and LL for longer than 30 days. In DD, we observed that some animals presented different period lengths for each activity component. During LL, activity was primarily unimodal, however spontaneous splitting of the rhythms were observed in some animals. When activity was recorded under artificial long days, the morning bouts maintained their phase relationship but the evening bouts changed their phase relationship with the Zeitgeber. Our results indicate that, bimodal locomotor activity rhythm in the crab Pseudothelphusa americana is variable among organisms. The characteristics of phase relationship with LD and responses to LL for morning and evening bouts, suggest that, locomotor activity could be driven by multiple oscillators, and that coupling between these oscillators may be regulated by light.     

Circadian Locomotor Activity Under Artificial Light in the Freshwater Crab Pseudothelphusa americana

Long-term recordings of locomotor activity were obtained from intact freshwater crabs, Pseudothelphusa americana in constant darkness (DD), constant light (LL) and different light-dark (LD) protocols. Bimodal rhythms were typically observed in this crab when subjected to DD or LD, with bouts of activity anticipating lights-on and lights-off, respectively. Freerunning circadian rhythms were expressed in both DD and LL for longer than 30 days. In DD, we observed that some animals presented different period lengths for each activity component. During LL, activity was primarily unimodal, however spontaneous splitting of the rhythms were observed in some animals. When activity was recorded under artificial long days, the morning bouts maintained their phase relationship but the evening bouts changed their phase relationship with the Zeitgeber. Our results indicate that, bimodal locomotor activity rhythm in the crab Pseudothelphusa americana is variable among organisms. The characteristics of phase relationship with LD and responses to LL for morning and evening bouts, suggest that, locomotor activity could be driven by multiple oscillators, and that coupling between these oscillators may be regulated by light.     

Possible evidence for morning and evening oscillators in Drosophila melanogaster populations selected for early and late adult emergence

In this paper, we report the results of our study aimed at a systematic analysis of the circadian phenotypes of fruit flies Drosophila melanogaster selected for early and late adult emergence, in light of the "morning and evening oscillator" (M and E) model for circadian clocks. We monitored adult emergence and activity/rest rhythms in these flies under light/dark (LD) cycles with short (8:16 h), normal (12:12 h) and long (16:8 h) photoperiods, as well as under constant darkness (DD). Across all the three LD cycles, the early populations displayed a morning phenotype with peak of emergence and activity occurring earlier than the controls and greater anticipation to "lights-on" and weak anticipation to "lights-off", while the late populations showed an evening phenotype with peak of emergence and activity occurring later than the controls and greater anticipation to lights-off and weak anticipation to lights-on. The gate of adult emergence and duration of activity in the early populations was narrower than the controls, while those of the late populations were wider than the controls. In addition, the circadian periodicities of adult emergence and activity/rest rhythms of the early flies were significantly shorter than the controls, while those of the late flies were significantly longer than the controls. In summary, the circadian phenotypes indicate that the early populations have evolved a dominant M oscillator, while the late populations have evolved a dominant E oscillator, thus providing an empirical support for the M and E model in Drosophila.     

Early- and Late-Emerging Drosophila melanogaster Fruit Flies Differ in Their Sensitivity to Light During Morning and Evening

The authors derived early and late populations of fruit flies showing increased incidence of emergence during morning or evening hours by imposing selection for timing of emergence under 12:12?h light/dark (LD) cycles. From previous studies, it was clear that the increased incidence of adult emergence during morning and evening hours in early and late populations was a result of evolution of divergent and characteristic emergence waveforms in these populations. Such characteristic waveforms are henceforth referred to as “evolved emergence waveforms” (EEWs). The early and late populations also evolved different circadian clocks, which is evident from the divergence in their clock period (τ) and photic phase response curve (PRC). Although correlation between emergence waveforms and clock properties suggests functional significance of circadian clocks, τ and PRCs do not satisfactorily explain the early and late emergence phenotypes. In order to understand the functional significance of the PRC for early and late emergence phenotypes, the authors investigated whether circadian clocks of these flies exhibit any difference in photosensitivity under entrained conditions. Such differences would suggest that the light requirement for circadian entrainment of the emergence rhythm in early and late populations is different. To test this, they examined if early and late flies differ in their light utilization behavior, first by assaying their emergence rhythm under complete photoperiod and then in three different skeleton photoperiods. The results showed that early and late populations require different durations of light during the morning and evening to achieve their EEWs, suggesting that for the circadian entrainment of the emergence rhythm, early and late flies utilize light from different parts of the day. (Author correspondence: vsharma@jncasr.ac.in or vksharmas@gmail.com)     

Aging alters properties of the circadian pacemaker controlling the locomotor activity rhythm in males of Drosophila nasuta

Effects of aging on the circadian rhythm of locomotor activity in males of Drosophila nasuta were investigated. The adult life of males was divided in 1-3 stages according to spontaneous changes in free-running period x in constant darkness (DD): stage 1, days 1-19; stage 2, days 20-36; stage 3, days 37-43. Stage 1 was characterized by a bimodal activity pattern with a short light-induced morning peak and a prolonged evening peak when the flies were entrained to light-dark cycles of 12 hours of light, 12 hours of darkness (LD 12:12). The morning peak had a phase angle difference Ψ_m (Ψ, the time from lights on in LD 12:12 cycles to the onset of morning peak) of about 0.1h, while Ψ_e (Ψ of evening peak) was about 9h at stage 1. The transient morning peak was curtailed at the end of stage 1. At stage 2, the Ψ_e was about 10h, and the activity end was delayed by an addition of about 3h of activity in the scotophase. The changes in W during DD free runs were determined in two groups of flies: flies reared in LD 12:12 and flies reared in DD. In both groups, W increased from about 23h at stage 1 to about 25h at stage 2. Stage 3 was characterized by arrhythmicity associated with highest mean activity level (total number of passes/fly/day) in the entrained and both free-running groups. The mean activity level increased significantly from stage 1 to stage 3 in all three groups of flies.     

Circadian phenotypes of Drosophila fragile x mutants in alternative genetic backgrounds

Drosophila FMR1 mutants are models of human fragile X syndrome. They show a loss of locomotor activity rhythm and severe degradation of eclosion timing. We analyzed the circadian behavior of FMR1 mutants (dfmr1(B55)) in two genetic backgrounds, yellow white (yw) and Canton S (CS). The arrhythmic phenotype of circadian locomotor activity in constant darkness (DD) did not significantly change in either genetic background. Surprisingly, eclosion timing was completely restored by backcrossing dfmr1(B55) with yw or CS flies. Morphological analysis of the small ventrally located lateral neurons of FMR1 mutants revealed that the dorsal-projection area was significantly larger in arrhythmic than rhythmic flies. In addition, dfmr1(B55) mutants in both genetic backgrounds had a significantly lower evening peak in the light-dark (LD) cycle. These results indicate that lack of FMR1 does not affect eclosion timing, but alters locomotor activity patterns in both LD and DD conditions by affecting the arborization of small ventrally located lateral neurons. Thus, the FMR1 gene may regulate the circadian-related locomotor activity of Drosophila.     

Entrainment of Eclosion Rhythm in Drosophila melanogaster Populations Reared for More Than 700 Generations in Constant Light Environment

In this paper, we report the results of our extensive study on eclosion rhythm of four independent populations of Drosophila melanogaster that were reared in constant light (LL) environment of the laboratory for more than 700 generations. The eclosion rhythm of these flies was assayed under LL, constant darkness (DD) and three periodic light‐dark (LD) cycles (T20, T24, and T28). The percentage of vials from each population that exhibited circadian rhythm of eclosion in DD and in LL (intensity of approximately 100 lux) was about 90% and 18%, respectively. The mean free‐running period (τ) of eclosion rhythm in DD was 22.85 ± 0.87 h (mean ± SD). Eclosion rhythm of these flies entrained to all the three periodic LD cycles, and the phase relationship (ψ) of the peak of eclosion with respect to “lights‐on” of the LD cycle was significantly different in the three periodic light regimes (T20, T24, and T28). The results thus clearly demonstrate that these flies have preserved the ability to exhibit circadian rhythm of eclosion and the ability to entrain to a wide range of periodic LD cycles even after being in an aperiodic environment for several hundred generations. This suggests that circadian clocks may have intrinsic adaptive value accrued perhaps from coordinating internal metabolic cycles in constant conditions, and that the entrainment mechanisms of circadian clocks are possibly an integral part of the clockwork.     

Effect of temperature on circadian rhythm controlling the crepuscular activity of the burying beetle Nicrophorus quadripunctatus Kraatz (Coleoptera: Silphidae)

Locomotor activity rhythm was examined at various temperatures under a 16 h light : 8 h dark photoperiod (LD 16:8) or LD 12:12 using adults of the burying beetle Nicrophorus quadripunctatus. At 20°C, the locomotor activity of the beetles showed a bimodal daily pattern with two peaks around lights on and lights off under both photoperiods. This bimodal activity rhythm persisted under constant darkness; therefore, the activity of adult N. quadripunctatus is controlled by a circadian clock. Adults showed a bimodal activity pattern for temperatures ranging from 15 to 25°C. The evening peak of the daily activity was earlier at lower temperatures. These findings suggest that in the field, N. quadripunctatus shows crepuscular activity, and is active earlier in the afternoon in cooler seasons. In this species, therefore, temperature appears to play an important role in the determination of daily activity patterns.     

Temporal Organization in Locomotor Activity of the Hypogean Loach, Nemacheilus Evezardi, and its Epigean Ancestor

Locomotor activity rhythm in the hypogean population of Nemacheilus evezardi was recorded first under light-to-dark (LD) 12 : 12 h cycle and then DD. The results were compared with that of its epigean counterpart held under comparable regimes. In LD 12 : 12, while hypogean loach exhibited a distinct bimodality in its locomotor activity rhythm, it was altogether absent in the case of epigean population. In hypogean loach, dark-to-light transition peak in LD was observed to free-run under DD. The same was not discernible in case of epigean loach. The locomotor activity rhythm in epigean fish was noticed to free-run in DD either from the dawn peak or dusk peak in LD. It is hypothesized that the hypogean fish still possesses a functional oscillator underlying its overt circadian rhythm in locomotor activity. The ecophysiological significance of these findings is yet to be fully understood.     

Circadian Rhythms of Locomotor Activity in Lycosa tarentula (Araneae,Lycosidae) and the Pathways of Ocular Entrainment

Lycosa tarentula is a ground-living spider that inhabits a burrow where it awaits the appearance of prey or conspecifics. In this study, circadian rhythms of locomotor activity were examined as well as the ocular pathway of entrainment. Thirty-three adult virgin females were examined under constant darkness (DD); all of them exhibited robust circadian rhythms of locomotor activity with a period averaging 24.1h. Fourteen of these spiders were studied afterwards under an LD 12:12 cycle; they usually entrained to in the first or second day, even when the light intensity was as low as 1 lx. During the LD cycle, locomotor activity was generally restrained to the darkness phase, although several animals showed a small amount of diurnal activity. Ten males were also examined under LD; they were also nocturnal, but were much more active than the females. Seven females were examined under constant light (LL); under this they became arrhythmic. Except for the anterior median eyes (OMAs), all the eyes were capable of entraining the locomotor activity to an LD cycle. These results demonstrate that under laboratory conditions and low light intensities locomotor activity of Lycosa tarentula is circadian and in accordance with Aschoff's 'rule'. Only OMAs are unable to entrain the rhythm; the possible localization of circadian clock is therefore discussed.     

Diel activity of the pink shrimp Penaeus duorarum

Twelve juvenile pink shrimp, Penaeus duorarum Burkenroad, were tested individually for 3-day periods in electronic shuttleboxes to determine their diel patterns of locomotor activity, in relation to a natural summer photoperiod. Nocturnal activity was twice that exhibited during the daytime; however, a bimodal pattern was evident with crepuscular peaks occurring at dawn and dusk. The dusk peak was more pronounced, with activity increasing markedly before sunset (indicative of an endogenous circadian rhythm component), and continuing during the initial hours of darkness, gradually declining later during the night with a secondary peak at dawn falling off to minimal activity during daylight. Crepuscular activity (mean of dawn and dusk) was twice the nocturnal average.     

Separate sets of cerebral clock neurons are responsible for light and temperature entrainment of Drosophila circadian locomotor rhythms

The fruit fly Drosophila melanogaster shows a bimodal circadian locomotor rhythm with peaks at lights-on and before lights-off, which are regulated by multiple clocks in the brain. Even under light-dark cycles, the timing of the evening peak is highly dependent on temperature, starting earlier under lower ambient temperature but terminating almost at the same time. In the present study, using behavioral and immunohistochemical assays, the authors show that separate groups of clock neurons, either light-entrainable or temperature-entrainable, form a functional system driving the locomotor rhythm. When subjected to a light cycle combined with a temperature cycle advanced by 6 h relative to the light cycle, the dorsally located neurons (DNs) and lateral posterior neurons (LPNs) shifted their phase of TIMELESS expression, but the laterally located protocerebral neurons (LNs) basically maintained their original phase. Thus, the LNs seem to be preferentially light-entrainable and the DNs and LPNs to be primarily temperature-entrainable. In pdf(01) mutant flies that lack the neuropeptide PDF in the ventral groups of LNs, the onset of the evening peak was greatly advanced even under synchronized light and temperature cycles and was shifted even more than in wild-type flies in response to a 6-h phase shift of the temperature cycle, suggesting that ventral LNs have a strong impact on the phase of the other cells. It seems likely that the 2 sets of clock cells with different entrainability to light and temperature, and the coupling between them, enable Drosophila to keep a proper phase relationship of circadian activity with respect to the daily light and temperature cycles.     

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).