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.     

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).     

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.     

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.     

Significance of Nonparametric Light Effects in Entrainment of Circadian Rhythms in Owl Monkeys (Aotus Lemurinus Griseimembra) by Light-Dark Cycles

In a total of 12 adult Colombian owl monkeys, Aotus lemurinus griseimembra, the significance of nonparametric light effects for the entrainment of the circadian system by light-dark (LD) cycles was studied by carrying out (a) phase-response experiments testing the phase-shifting effect of 30-min light pulses (LPs) of 250 lx applied at various phases of the free-running circadian activity rhythm (LL 0.2 lx) and (b) synchronization experiments testing the entraining effect of 24-h single LP photoperiods consisting of 30-min L of 80 lx and 23.5-h D of 0.5 lx (sP 0.5) and skeleton photoperiods consisting of two 30-min LPs of 80 lx, given against a background illuminance of 0.5 lx either symmetrically at 12-h intervals (PP 12:12) or asymmetrically at 9- and 15-h intervals (PP 9:15). The phase-response characteristics in Aotus, as evidenced by the phase-response curve, generally correspond to those of nocturnal rodents, proving that this neotropical simian primate chronobiologically is a genuine nocturnal species. When free-running with a spontaneous period close to 24 h (24.3 ± 0.1 h), the PP 12:12 produced entrainment in only two of five owl monkeys, whereas the sP 0.5 entrained four of them. The PP 9:15, however, brought about stable entrainment of the circadian rhythms of locomotor activity, feeding activity, and core temperature in all animals tested (n = 8). Changes in phase position of the activity time with the endogenous rhythm entrained by a PP 12:12, by an sP 0.5, or by a PP 9:15 give evidence that both LPs of a skeleton photoperiod contribute to the phase setting of the circadian system. When free-running with a considerably lengthened spontaneous period (τ ≥ 25.5 h), even the sP 0.5 and the PP 9:15 failed to entrain the owl monkeys' circadian rhythms, whereas a 24-h photoperiod with a very long LP of 3 h caused entrainment. The results indicate that in Aotus lemurinus griseimembra, in addition to the nonparametric light effects, parametric light effects play a significant role in the entrainment of circadian rhythms by LD cycles.     

Circadian and Seasonal Responses in Indian Weaver Bird: Subjective Interpretation of Day and Night Depends Upon Both Light Intensity and Contrast Between Illuminations

This study investigated whether changes in illumination modify perception of day and night conditions in a diurnal species, the Indian weaver bird. Birds were initially subjected to a 12-h light:12-h dark regime (12L:12D; L?=?20 lux, D =?0.5 lux). After every 2 wks, the combinations of light illumination in L and D phases were changed as follows: 20:2 lux, 20:5 lux, 20:10 lux, 20:20 lux, 20:100 lux, and 20:200 lux. Finally, birds were released into dim constant light (0.5 lux) for 2 wks to determine the phase and period of the circadian activity rhythm. They were also laparotomized at periodic intervals to examine the effects of the light regimes on the seasonal testicular cycle. All individuals showed a consistently similar response. As evident by the activity pattern under these light regimes, both in total activity during contrasting light phases and during the 2?h in the beginning and end of first light phase, birds interpreted the period of higher light intensity as day, and the period of lower intensity as the night. During the period of similar light intensity, i.e., under LL, birds free-ran with a circadian period (～24?h). In bright LL (20 lux), the activity rhythm was less distinct, but periodogram analysis revealed the circadian period for the group as 24.46?±?0.41?h (mean?±?SE). However, in dim LL at the end of the experiment, all birds exhibited a circadian pattern with average period of 25.52?±?0.70?h. All birds also showed testicular growth and regression during the 16-wks study. It is suggested that weaver birds interpret day and night subjectively based on both the light intensity and contrast between illuminations during two phases over the 24?h. (Author correspondence: drskumar7@yahoo.com)     

The Circadian Clock Machinery During Early Development of Senegalese sole (Solea senegalensis): Effects of Constant Light and Dark Conditions

Circadian rhythms are established very early during vertebrate development. In fish, environmental cues can influence the initiation and synchronization of different rhythmic processes. Previous studies in zebrafish and rainbow trout have shown that circadian oscillation of clock genes represents one of the earliest detectable rhythms in the developing embryo, suggesting their significance in regulating the coordination of developmental processes. In this study, we analyzed the daily expression of the core clock components Per1, Per2, Per3, and Clock during the first several days of Senegalese sole development (0–4 d post fertilization or dpf) under different lighting regimes, with the aim of addressing when the molecular clock first emerges in this species and how it is affected by different photoperiods. Rhythmic expression of the above genes was detected from 0 to 1 dpf, being markedly affected in the next few days by both constant light (LL) and dark (DD) conditions. A gradual entrainment of the clock machinery was observed only under light-dark (LD) cycles, and robust rhythms with increased amplitudes were established by 4 dpf for all clock genes currently studied. Our results show the existence of an embryonic molecular clock from the 1st d of development in Senegalese sole and emphasize the significance of cycling LD conditions when raising embryos and early larvae. (Author correspondence: munoz.cueto@uca.es; carlos.pendon@uca.es)     

Timekeeping Through Social Contacts: Social Synchronization of Circadian Locomotor Activity Rhythm in the Carpenter Ant Camponotus paria

In ant colonies a large proportion of individuals remain inside nests for most of their lives and come out only when necessary. It is not clear how, in a nest of several thousand individuals, information about local time is communicated among members of the colony. Central to this seem to be circadian clocks, which have an intrinsic ability to keep track of local time by entraining to environmental light-dark, temperature, and social cycles. Here, the authors report the results of their study aimed at understanding the role of cyclic social interactions in circadian timekeeping of a day-active species of carpenter ant Camponotus paria. The authors found that daily social interactions with visitors (worker ants) was able to synchronize the circadian locomotor activity rhythm of host worker ants and queens, in one-on-one (pair-wise) and multi-individual (group-wise) interactions. Interestingly, the outcome of cyclic social interactions was context specific; when visitor workers socially interacted with host workers one-on-one, host workers considered the time of interaction as subjective day, but when visitor workers interacted with a group of workers and queens, the hosts considered the time of interaction as subjective night. These results can be taken to suggest that members of the ant species C. paria keep track of local time by socially interacting with workers (foragers) who shuttle in and out of the colony in search of food. (Author correspondence: vsharma@jncasr.ac.in)     

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.     

Expression of circadian rhythmicity in Djungarian hamsters under constant light: Effects of light intensity and the circadian system's state

Summary Djungarian hamsters (Phodopus sungorus), were exposed to constant light with increasing intensities (20, 60, 350 lux), and wheel running activity was recorded. With increasing light intensity the percentage of hamsters showing a split in their daily activity pattern increased and the free running period was lengthened for both the unsplit and the split state. The fact that the free running period of both states depended on the light intensity together with the observation that the highest incidence of acircadian activity occurred under 350 lux, provoked the idea that the emergence of splitting or acircadian rhythmicity is a direct consequence of the light induced lengthening of the free running period. However, analysis of the data failed to support the idea that emergence of a split or acircadian activity is a threshold phenomenon with respect to the free running period.Due to differences in circadian function some Djungarian hamsters do not exhibit photoinduction following short day exposure. In these individuals splitting also occurred but required exposure to a higher light intensity than in photo-responsive hamsters. This observation is in accordance with the idea that the two phenotypes differ in the interaction of the two component oscillators underlying circadian rhythmicity.Abbreviations LD long day photoperiod - LL constant light - SD short day photoperiod - free running period     

Social stimuli fail to act as entraining agents of circadian rhythms in the golden hamster

Summary The ability of social stimuli to act as entraining agents of circadian rhythms was investigated in golden hamsters (Mesocricetus auratus). In a first experiment, pairs of male hamsters (one of them enucleated and the other intact) were maintained under a light-dark (LD) cycle with a period of 23.3 h. Running-wheel activity was recorded to determine the effect of social interaction on the free-running circadian rhythm of activity. In several pairs, general activity and body temperature were also recorded. In all pairs the intact animals entrained to the LD cycle, whereas the activity rhythms of the enucleated animals free-ran with periods of approximately 24 h and showed no apparent sign of synchronization or relative coordination with the other member of the pair. In a second experiment, male hamsters maintained in constant darkness received pulses of social interaction, which have been reported to induce phase shifts of the activity rhythm. Consistent phase shifts in the running-wheel activity rhythm were not induced by the social pulses in our experiment. These results suggest strongly that social stimuli are not effective entraining agents of circadian rhythms in the golden hamster.Abbreviations CT circadian time - LD light-dark     

Behavioral Responses to Combinations of Timed Light,Food Availability,and Ultradian Rhythms in the Common Vole (Microtus arvalis)

Light is the main entraining signal of the central circadian clock, which drives circadian organization of activity. When food is made available during only certain parts of the day, it can entrain the clock in the liver without changing the phase of the central circadian clock. Although a hallmark of food entrainment is a behavioral anticipation of food availability, the extent of behavioral alterations in response to food availability has not been fully characterized. The authors have investigated interactions between light and temporal food availability in the timing of activity in the common vole. Temporally restricted food availability enhanced or attenuated re-entrainment to a phase advance in light entrainment when it was shifted together with the light or remained at the same time of day, respectively. When light-entrained behavior was challenged with temporal food availability cycles with a different period, two distinct activity components were observed. More so, the present data indicate that in the presence of cycles of different period length of food and light, an activity component emerged that appeared to be driven by a free-running (light-entrainable) clock. Because the authors have previously shown that in the common vole altering activity through running-wheel availability can alter the effectiveness of food availability to entrain the clock in the liver, the authors included running-wheel availability as a parameter that alters the circadian/ultradian balance in activity. In the current protocols, running-wheel availability enhanced the entraining potential of both light and food availability in a differential way. The data presented here show that in the vole activity is a complex of individually driven components and that this activity is, itself, an important modulator of the effectiveness of entraining signals such as light and food. (Author correspondence: vanderveen@nd.edu)     

Daily Behavioral Rhythmicity and Organization of the Suprachiasmatic Nuclei in the Diurnal Rodent,Lemniscomys barbarus

Wheel‐running activity was recorded in Lemniscomys barbarus exposed to different lighting conditions. This rodent shows rhythmic locomotor activity under natural twilight‐light/dark (LD) as well as squared‐LD cycles. A mean of 77% of the activity occurred during the light phase. Under different controlled photoperiods, the quantity of daily locomotor activity was relatively stable except for a lower level in the shortest photoperiod tested (LD 06∶18). The duration of the active phase tended to increase with the duration of the light phase, especially in the longer photoperiods. Whatever the lighting conditions, Lemniscomys barbarus started running before lights‐on and stopped after lights‐off. The phase angle of activity offset relative to lights‐off was stable in each squared‐photoperiod, whereas the phase angle of activity onset relative to lights‐on was significantly the highest under the shortest photoperiods. Recording of activity under constant lighting conditions showed that the daily rhythm of locomotor activity is fundamentally circadian. The endogenous period was slightly<24 h (mean=23.8 h) in permanent darkness and>24 h (mean=24.5 h) in continuous light. Re‐entrainment of the locomotor activity rhythm after a 6 h phase advance or delay requires only four days on average. Moreover, the phase‐responses curve to a 30 min light pulse (200 lux) in Lemniscomys barbarus kept in constant dark reveals large phase shifts according to circadian times (CT). With CT0 being defined as the onset of daily activity, maximum phase delay and advance shifts were observed at CT11 (Δ Ψ=‐5.7 h±2.3 h) and CT21 (Δ Ψ =4.9±1.2 h), respectively. Interestingly, the phase‐response curve to light did not show any dead zone. Immunohistochemical staining of the suprachiasmatic nuclei indicates that arginine vasopressin‐immunoreactive cell bodies and fibers delimited a dorsal subregion that extends laterally and medially. The ventral subregion is rich in vasoactive intestinal peptide‐immunoreactive neurones overlapping a smaller area containing gastrin‐releasing peptide‐expressing cells and receives numerous fibers labeled with neuropeptide Y antibody. The results of this study clearly demonstrate that Lemniscomys barbarus is a diurnal species highly sensitive to the shifting effects of light. Overall, this rodent can be considered a new and interesting model for circadian rhythm neurobiology.     

Emphasizing Difficulties in the Detection of Rhythms with Lomb-Scargle Periodograms

The Lomb-Scargle periodogram was introduced in astrophysics to detect sinusoidal signals in noisy unevenly sampled time series. It proved to be a powerful tool in time series analysis and has recently been adapted in biomedical sciences. Its use is motivated by handling non-uniform data which is a common characteristic due to the restricted and irregular observations of, for instance, free-living animals. However, the observational data often contain fractions of non-Gaussian noise or may consist of periodic signals with non-sinusoidal shapes. These properties can make more difficult the interpretation of Lomb-Scargle periodograms and can lead to misleading estimates. In this letter we illustrate these difficulties for noise-free bimodal rhythms and sinusoidal signals with outliers. The examples are aimed to emphasize limitations and to complement the recent discussion on Lomb-Scargle periodograms.     

Gene-Dependent Effect of Lithium on Circadian Rhythms in Mice (Mus Musculus)

Lithium has been shown to lengthen free-running circadian periods in a variety of species. Here we show that lithium carbonate differentially lengthens the free-running period of a circadian wheel running rhythm in BALB/CByJ and C57BL/10Sn inbred mouse strains. This result supports previous evidence that lithium lengthens mammalian circadian rhythms, and also demonstrates that gene differences can mediate individual differences in response to lithium treatment.     

Seasonal variation in daily activity patterns of free-ranging European ground squirrels (Spermophilus citellus)

Daily aboveground activity of European ground squirrels (Spermophilus citellus) in their natural habitat was recorded with a visual scanning procedure during the active seasons of 1992 and 1993. Activity patterns were analyzed with respect to time of year and to the animal's reproductive state. Aboveground activity started on average 3.9 h (SD 0.6 h, n = 37 days) after civil twilight at dawn and ended on average 3.2 h (SD 0.9 h, n = 37 days) before civil twilight at dusk. Between onset and offset of activity, 54% was spent aboveground, of which 73% was spent foraging. Activity patterns were influenced by photoperiod, rainfall, and by reproductive state. During mating, reproductively active males started activity earlier than females and reproductively inactive males. For females, time spent foraging was high during lactation. The midpoint of daily activity was at 12:16 h (SD 0.37 h, n = 37 days). Activity patterns of European ground squirrels thus appear robustly positioned in the middle of the photoperiod.     

Light Synchronization of the Daily Spawning Rhythms of Gilthead Sea bream (Sparus aurata L) Kept under Different Photoperiod and after Shifting the LD Cycle

Reproduction in most fish is typically a seasonal process, as spawning takes place usually at a given time of the year, depending on the reproduction strategy of the species, to ensure maximal survival of offspring. Nevertheless, fish reproduction cannot be considered an exclusively annual phenomenon, because spawning may also show daily rhythmicity. In this study, we investigated the existence of a daily spawning rhythm in gilthead seabream (Sparus aurata L) exposed to different light‐dark (LD) cycles and at different times of the year using an automatic and programmable egg collector. Floatability and fertilization rates were analyzed at different times throughout the 24 h. The results showed a daily spawning rhythm with spanning occurring from 14:30 to 21:30 h, with the acrophase (peak time) being 18:29 and 18:08 h in fish exposed to an artificial photoperiod of 9L∶15D in winter and in spring, respectively. Nevertheless, in fish exposed to a natural photoperiod of 12L∶12D in spring, the acrophase of the rhythm was recorded later, at 21:28 h. The average fertilization rate was 87%, and no significant differences were found between the different hours of spawning. Moreover, when the LD cycle (9L∶15D) was shifted by 12 h, the daily spawning rhythm gradually re‐synchronized, resuming a stable phase‐relationship after 4–5 transient days, which is characteristic of a endogenous circadian rhythm. Our results clearly demonstrated the existence of a 24 h period of spawning in gilthead sea bream, with a peak anticipating the forthcoming night, and its capacity to gradually re‐synchronize after a 12 h shift in the LD cycle, pointing to the endogenous nature of this rhythm. These findings will be valuable for better understanding the reproductive physiology of this species and for optimizing the protocols of egg collection and larvae production in aquaculture. (Author correspondence: javisan@um.es)     

Effects of Prolonged Exposure to Darkness on Circadian Photic Responsiveness in the Mouse

Circadian rhythms in mammals are generated by an endogenous pacemaker but are modulated by environmental cycles, principally the alternation of light and darkness. Although much is known about nonparametric effects of light on the circadian system, little is known about other effects of photic stimulation. In the present study, which consists of a series of five experiments in mice, various manipulations of photic stimulation were used to dissect the mechanisms responsible for a variation in the magnitude of light-induced phase-shifts that results from prolonged exposure to darkness. The results confirmed previous observations that prolonged exposure to darkness causes an increase in the magnitude of phase shifts (both phase advances and phase delays) evoked by discrete light pulses. The results also indicated that the increase in responsiveness results from the lack of exposure to light per se and not from collateral effects of exposure to constant darkness such as the lack of previous entrainment. The lack of exposure to light causes the circadian system to undergo a process of dark adaptation similar to dark adaptation in the visual system but with a much slower temporal course. The results suggest that circadian dark adaptation may take place at the retinal level, but it is not clear whether it involves a change in the sensitivity or maximal responsiveness of the system.     

Ecological Aspects of Circadian Rhythms in Six Species of Omni-Seasonal Beetles (Coleoptera, Tenebrionidae) Inhabiting Kara Kum Desert (Turkmenistan)

Circadian rhythms of activity were compared in ground-dwelling Tenebrionid beetles from the Kara Kum sand desert: Trigonoscelis gigas Reitter, Trigonoscelis sublaevicollis Reitter, Pisterotarsa gigantea Fish.-W., Sternodes caspicus Pall, Blaps faustii Seidlitz, and Ocnera imbricata Fish.-W. For the observations, artificial pens (enclosures) 60 × 60 cm and 80 cm deep were arranged in field conditions and filled with various kinds of sand to simulate natural habitats as closely as possible. The activity of individually marked beetles was assessed visually and recorded every 30 min. The beetle species studied had different types of behavior, from strictly crepuscular to strictly nocturnal. Closely related species sharing one habitat differed in activity pattern — they occupied different time niches; hence their circadian rhythms contribute to reproductive isolation. In contrast, non-closely related species could have similar circadian patterns as a result of convergent evolution. In experiments with T. gigas, we failed to stimulate individual variability among beetles in circadian pattern by applying additional light and heat at night. Therefore, the circadian rhythms in field conditions were found to be remarkably stable, which can be explained by our model of the beetle circadian system consisting of two strongly coupled oscillators. However, in laboratory conditions, we previously observed a wide range of individual variability in free-running circadian rhythms. The strong coupling between two putative endogenous circadian oscillators is a crucial tool for beetles to survive in a harsh arid environment. Such organization of the circadian system ensures synchronous activity of beetles despite individual differences, thus preserving the variability of circadian clock properties in a population, which increases chances for survival of a population in a changing environment.