Activity Rhythms and Masking Response in the Diurnal Fat Sand Rat Under Laboratory Conditions

Daily rhythms are heavily influenced by light in two major ways. One is through photic entrainment of a circadian clock, and the other is through a more direct process, referred to as masking. Whereas entraining effects of photic stimuli are quite similar in nocturnal and diurnal species, masking is very different. Laboratory conditions differ greatly from what is experienced by individuals in their natural habitat, and several studies have shown that activity patterns can greatly differ between laboratory environment and natural condition. This is especially prevalent in diurnal rodents. We studied the daily rhythms and masking response in the fat sand rat (Psammomys obesus), a diurnal desert rodent, and activity rhythms of Tristram’s jird (Meriones tristrami), a nocturnal member of the same subfamily (Gerbillinae). We found that most sand rats kept on a 12?h:12?h light-dark (LD) cycles at two light intensities (500 and 1000?lux) have a nocturnal phase preferences of general activity and higher body temperature during the dark phase. In most individuals, activity was not as stable that of the nocturnal Tritram’s jirds, which showed a clear and stable nocturnal activity pattern under the same conditions. Sand rats responded to a 6-h phase advance and 6-h phase delay as expected, and, under constant conditions, all tested animals free ran. In contrast with the nocturnal phase preference, fat sand rats did not show a masking response to light pulses during the dark phase or to a dark pulse during the light phase. They did, however, have a significant preference to the light phase under a 3.5?h:3.5?h LD schedule. Currently, we could not identify the underlying mechanisms responsible for the temporal niche switch in this species. However, our results provide us with a valuable tool for further studies of the circadian system of diurnal species, and will hopefully lead us to understanding diurnality, its mechanisms, causes, and consequences.     

Circadian Locomotor Activity Rhythms in the African Clawed Frog, Xenopus laevis: The Role of the Eye and the Hypothalamus

The effects of hypothalamic lesioning and removal of the eyes on locomotor activity rhythms of African clawed frog, Xenopus laevis were examined under light-dark cycles (LD12:12) and constant conditions. Frogs were kept individually and the activity rhythms at the bottom layer of water tank were recorded by means of the infrared photocells. Intact frogs displayed clear entrained nocturnal activity and expressed freerunning activity rhythms in constant darkness (DD), while some frogs did not freerun under co nstant dim light (dimLL) and constant light (LL). Freerunning periods in intact frogs were significantly shorter in dimLL than in DD. Although freerunning periods were shortened after blinding in same individuals, no significant changes in the freerunning periods were observed after blinding under dimLL and LL. When electrolytic lesions to the hypothalamus were performed, all frogs with more than 70% damage of the SCN abolished freerunning rhythms and in frogs with less than 70% damage, 57% of the animals became arrhythmic. In conclusion, (1) There is a circadian pacemaker somewhere outside the eyes, and it is probably situated in the hypothalamusincluding the SCN. (2) Both the eyes and the SCN are involved in the circadian system of the frogs.     

Circadian Locomotor Activity Rhythms in the African Clawed Frog,Xenopus laevis: The Role of the Eye and the Hypothalamus

The effects of hypothalamic lesioning and removal of the eyes on locomotor activity rhythms of African clawed frog, Xenopus laevis were examined under light-dark cycles (LD12:12) and constant conditions. Frogs were kept individually and the activity rhythms at the bottom layer of water tank were recorded by means of the infrared photocells. Intact frogs displayed clear entrained nocturnal activity and expressed freerunning activity rhythms in constant darkness (DD), while some frogs did not freerun under co nstant dim light (dimLL) and constant light (LL). Freerunning periods in intact frogs were significantly shorter in dimLL than in DD. Although freerunning periods were shortened after blinding in same individuals, no significant changes in the freerunning periods were observed after blinding under dimLL and LL. When electrolytic lesions to the hypothalamus were performed, all frogs with more than 70% damage of the SCN abolished freerunning rhythms and in frogs with less than 70% damage, 57% of the animals became arrhythmic. In conclusion, (1) There is a circadian pacemaker somewhere outside the eyes, and it is probably situated in the hypothalamusincluding the SCN. (2) Both the eyes and the SCN are involved in the circadian system of the frogs.     

Circadian system responses to nocturnal and diurnal hosts in the kissing bug,Triatoma infestans

ABSTRACT

Insects express diverse behavioral rhythms synchronized to environmental cycles. While circadian entrainment to light–dark cycles is ubiquitous in living organisms, synchronization to non-photic cycles may be critical for hematophagous bugs that depend on rhythmic hosts. The purpose was to determine whether Triatoma infestans are capable of synchronizing to the circadian rhythms of potential hosts with temporally distinct activity patterns; and, if so, if this synchronization occurs through masking or entrainment. Precise synchronization with the food source may be critical for the insects’ survival due to the specific predatory or defensive nature of each host. Kissing bugs were housed in a compartment in constant dark, air-flow-connected to another compartment with a nocturnal or a diurnal host; both hosts were synchronized to a light–dark cycle. The activity rhythms of kissing bugs were modulated by the daily activity rhythms of the vertebrates. Effects were a decrease in the endogenous circadian period, independent of the host being nocturnal or diurnal; in some cases relative coordination occurred and in others synchronization was clearly achieved. Moreover, splitting and bimodality arose, phenomena that were also affected by the host presence. The results indicate that T. infestans were able to detect the non-photic cycle of their potential hosts, an ability that surely facilitates feeding and hinders predation risk. Understanding triatomines behavior is of fundamental importance to the design of population control methods.     

Functional and anatomical variations in retinorecipient brain areas in Arvicanthis niloticus and Rattus norvegicus: implications for the circadian and masking systems

ABSTRACT

Daily rhythms in light exposure influence the expression of behavior by entraining circadian rhythms and through its acute effects on behavior (i.e., masking). Importantly, these effects of light are dependent on the temporal niche of the organism; for diurnal organisms, light increases activity, whereas for nocturnal organisms, the opposite is true. Here we examined the functional and morphological differences between diurnal and nocturnal rodents in retinorecipient brain regions using Nile grass rats (Arvicanthis niloticus) and Sprague-Dawley (SD) rats (Rattus norvegicus), respectively. We established the presence of circadian rhythmicity in cFOS activation in retinorecipient brain regions in nocturnal and diurnal rodents housed in constant dark conditions to highlight different patterns between the temporal niches. We then assessed masking effects by comparing cFOS activation in constant darkness (DD) to that in a 12:12 light/dark (LD) cycle, confirming light responsiveness of these regions during times when masking occurs in nature. The intergeniculate leaflet (IGL) and olivary pretectal nucleus (OPN) exhibited significant variation among time points in DD of both species, but their expression profiles were not identical, as SD rats had very low expression levels for most timepoints. Light presentation in LD conditions induced clear rhythms in the IGL of SD rats but eliminated them in grass rats. Additionally, grass rats were the only species to demonstrate daily rhythms in LD for the habenula and showed a strong response to light in the superior colliculus. Structurally, we also analyzed the volumes of the visual brain regions using anatomical MRI, and we observed a significant increase in the relative size of several visual regions within diurnal grass rats, including the lateral geniculate nucleus, superior colliculus, and optic tract. Altogether, our results suggest that diurnal grass rats devote greater proportions of brain volume to visual regions than nocturnal rodents, and cFOS activation in these brain regions is dependent on temporal niche and lighting conditions.     

Food availability affects circadian clock-controlled activity and Zugunruhe in the night migratory male blackheaded bunting (Emberiza melanocephala)

This study investigated the functional linkage between food availability and activity behavior in the Palaearctic Indian night migratory blackheaded bunting (Emberiza melanocephala) subjected to artificial light-dark (LD) cycles. Two experiments were performed on photosensitive birds. In the first one, birds were exposed to short days (LD 10/14; Experiment 1A), long days (LD 13/11; Experiment 1B), or increasing daylengths (8 to 13?h light/d; Experiment 1C) and presented with food either for the whole or a restricted duration of the light period. In Experiments 1A and 1B, illumination of the light and dark periods or of the dark period, alone, was changed to assess the influence of the light environment on direct and circadian responses to food cycles. In the second experiment, birds were exposed to LD 12/12 or LD 8/16 with food availability overlapping with the light (light and food presence in phase) or dark period (light and food presence in antiphase). Also, birds were subjected to constant dim light (LL(dim)) to examine the phase of the activity rhythms under synchronizing influence of the food cycles. Similarly, the presentation of food ad libitum (free food; FF) during an experiment examined the effects of the food-restriction regimes on activity rhythms. A continuous measurement of the activity-rest pattern was done to examine both the circadian and direct effects of the food and LD cycles. Measurement of activity at night enabled assessment of the migratory phenotype, premigratory restlessness, or Zugunruhe. The results show that (i) light masked the food effects if they were present together; (ii) birds had a higher anticipatory activity and food intake during restricted feeding conditions; and (iii) food at night alone reduced both the duration and amount of Zugunruhe as compared to food during the day alone. This suggests that food affects both the daily activity and seasonal Zugunruhe, and food cycles act as a synchronizer of circadian rhythms in the absence of dominant natural environmental synchronizers, such as the light-dark cycle.     

Light Cycles Given During Development Affect Freerunning Period of Circadian Locomotor Rhythm of period Mutants in Drosophila melanogaster

We reared wild type (Canton-S) and period mutant flies, i.e., per(S) and per(L), of Drosophila melanogaster in constant darkness, constant light or 24h light dark cycles with various light to dark ratios throughout the development from embryo to early adult. The locomotor activity rhythms of newly eclosed individuals were subsequently monitored in the lighting conditions, in which they had been reared, for several days and then in constant darkness. Circadian rhythms were clearly exhibited in constant darkness even in flies reared in constant light and constant darkness as well as flies reared in light-dark cycles, but the freerunning period differed among groups. The results suggest that the circadian clock is assembled without any cyclical photic information, and that the light influences the developing circadian clock of Drosophila to alter the freerunning period. The effects of light on the rhythm differed in some aspects between per(L) flies and the other two strains. Possible mechanisms through which light affects the developing circadian clock are discussed. Copyright 1997 Elsevier Science Ltd. All rights reserved     

Circadian Activity Rhythms in the Australian Platypus,Ornithorhynchus anatinus (Monotremata)

The Australian platypus, Ornithorhynchus anatinus, is one of three extant genera of the order monotremata. Given the divergent evolutionary lineage of monotremes in relation to more commonly studied animals, it was of interest to determine first, whether platypuses possess endogenous biological pacemakers and, second, general parameters of aquatic activity rhythms under artificial and natural light–dark (LD) cycles. Using a novel recording device, aquatic activity rhythms were measured in three platypuses: a paired male and female studied together, and a single female studied in isolation from other platypuses. Under a constant photic environment, some evidence was found for persistent and free-running rhythmicity, indicating the presence of an endogenous circadian pacemaker in the platypus. Under artificial LD cycles the paired animals exhibited a nocturnal pattern of entrainment, although in the single female considerable variability in entrained phase-relations was found under natural LD cycles. Evidence for a circadian pacemaker in the hypothalamic region of platypuses is also discussed.     

Ultradian and circadian CO2 emission variations in nocturnal and diurnal animals exposed to a light stimulus

1. Carbon dioxide emission (VCO2) has been continuously recorded in three laboratory animal species (Sprague-Dawley rats, Japanese quail, Hartley guinea-pigs) which differ by their nocturnal and diurnal activities. A 100 lux stimulus has been delivered at various time intervals. 2. A regular alternation of 12, 3 or 1.5 hr light (L) and darkness (D) gives VCO2 circadian and ultradian rhythms of 24, 6 or 3 hr periods, respectively, in quail and rats. 3. Such circadian and ultradian LD rhythms are not induced in all guinea-pigs. 4. The amplitudes of the VCO2 responses are greatest at D----L when the animals have a maximum diurnal activity and at L----D when their maximum activity is nocturnal. 5. Interactions between circadian and ultradian rhythms are seen in all LD experiments, as well as in continuous light (LL) or continuous dark (DD). 6. No more well-marked or even inverted VCO2 responses to the light stimuli may occur after several days of exposure to these LD alternations.     

Acute Behavioral Responses to Light and Darkness in Nocturnal Mus musculus and Diurnal Arvicanthis niloticus

The term masking refers to immediate responses to stimuli that override the influence of the circadian timekeeping system on behavior and physiology. Masking by light and darkness plays an important role in shaping an organism's daily pattern of activity. Nocturnal animals generally become more active in response to darkness (positive masking) and less active in response to light (negative masking), and diurnal animals generally have opposite patterns of response. These responses can vary as a function of light intensity as well as time of day. Few studies have directly compared masking in diurnal and nocturnal species, and none have compared rhythms in masking behavior of diurnal and nocturnal species. Here, we assessed masking in nocturnal mice (Mus musculus) and diurnal grass rats (Arvicanthis niloticus). In the first experiment, animals were housed in a 12:12 light-dark (LD) cycle, with dark or light pulses presented at 6 Zeitgeber times (ZTs; with ZT0 = lights on). Light pulses during the dark phase produced negative masking in nocturnal mice but only at ZT14, whereas light pulses resulted in positive masking in diurnal grass rats across the dark phase. In both species, dark pulses had no effect on behavior. In the 2nd experiment, animals were kept in constant darkness or constant light and were presented with light or dark pulses, respectively, at 6 circadian times (CTs). CT0 corresponded to ZT0 of the preceding LD cycle. Rhythms in masking responses to light differed between species; responses were evident at all CTs in grass rats but only at CT14 in mice. Responses to darkness were observed only in mice, in which there was a significant increase in activity at CT 22. In the 3rd experiment, animals were kept on a 3.5:3.5-h LD cycle. Surprisingly, masking was evident only in grass rats. In mice, levels of activity during the light and dark phases of the 7-h cycle did not differ, even though the same animals had responded to discrete photic stimuli in the first 2 experiments. The results of the 3 experiments are discussed in terms of their methodological implications and for the insight they offer into the mechanisms and evolution of diurnality.     

Locomotor Activity Rhythms in Cave Catfishes, Genus Taunayia, from Eastern Brazil (Teleostei: Siluriformes: Heptapterinae)

We studied the locomotor rhythmicity in heptapterine catfishes, genus Taunayia, under free-running conditions (DD) and LD cycles (12:12). Taunayia sp., anophthalmic and depigmented undescribed species from a cave in northeastern Brazil, is the fourth Brazilian troglobitic catfish studied with focus on circadian rhythms. Weak free-running rhythmicity, with absence of significant circadian components, was observed for this species when compared to the epigean, eyed relatives. On the other hand, the studied troglobitic catfishes in general presented significant circadian rhythms under LD cycles, with activity peaks in the night phase probably corresponding to nocturnal activity pattern inherited from their epigean ancestors. However, no residual oscillations were observed after transition from LD to DD. This indicates masking of activity by light-dark cycles. Regression of circadian rhythmicity in the stable, permanently dark subterranean habitat was also observed for other cave fishes. Such regression corroborates the notion that circadian rhythmicity is mainly selected in the epigean environment by ecological factors, namely daily cycles of light and/or temperature.     

Circadian rhythms of self-feeding and locomotor activity in zebrafish (Danio Rerio)

To investigate daily feeding rhythms in zebrafish, the authors have developed a new self-feeding system with an infrared photocell acting as a food-demand sensor, which lets small-size fish such as zebrafish trigger a self-feeder. In this paper, the authors used eight groups of 20 fish. Locomotor activity rhythms were also investigated by means of infrared sensors. Under a 12?h:12?h light (L)-dark (D) cycle, zebrafish showed a clear nocturnal feeding pattern (88.0% of the total daily food-demands occurring in the dark phase), concentrated during the last 4?h of the dark phase. In contrast, locomotor activity was mostly diurnal (88.2% of total daily activity occurring in the light phase). Moreover, both feeding and locomotor rhythms were endogenously driven, as they persisted under free-running conditions. The average period length (τ) of the locomotor and feeding rhythms was shorter (τ?=?22.9?h) and longer (τ?=?24.6?h) than 24?h, respectively. During the time that food availability was restricted, fish could only feed during ZT0-ZT12 or ZT12-ZT16. This resulted in feeding activity being significantly modified according to feeding time, whereas the locomotor activity pattern remained synchronized to the LD cycle and did not change during this trial. These findings revealed an independent phasing between locomotor and feeding activities (which were mostly nocturnal or diurnal, respectively), thus supporting the concept of multioscillatory control of circadian rhythmicity in zebrafish.     

Locomotor Activity Rhythms in Cave Catfishes,Genus Taunayia,from Eastern Brazil (Teleostei: Siluriformes: Heptapterinae)

We studied the locomotor rhythmicity in heptapterine catfishes, genus Taunayia, under free-running conditions (DD) and LD cycles (12:12). Taunayia sp., anophthalmic and depigmented undescribed species from a cave in northeastern Brazil, is the fourth Brazilian troglobitic catfish studied with focus on circadian rhythms. Weak free-running rhythmicity, with absence of significant circadian components, was observed for this species when compared to the epigean, eyed relatives. On the other hand, the studied troglobitic catfishes in general presented significant circadian rhythms under LD cycles, with activity peaks in the night phase probably corresponding to nocturnal activity pattern inherited from their epigean ancestors. However, no residual oscillations were observed after transition from LD to DD. This indicates masking of activity by light-dark cycles. Regression of circadian rhythmicity in the stable, permanently dark subterranean habitat was also observed for other cave fishes. Such regression corroborates the notion that circadian rhythmicity is mainly selected in the epigean environment by ecological factors, namely daily cycles of light and/or temperature.     

Circadian rhythm of locomotor activity in the Japanese honeybee, Apis cerana japonica

Abstract.  To reveal circadian characteristics and entrainment mechanisms in the Japanese honeybee Apis cerana japonica , the locomotor-activity rhythm of foragers is investigated under programmed light and temperature conditions. After entrainment to an LD 12 : 12 h photoperiodic regime, free-running rhythms are released in constant dark (DD) or light (LL) conditions with different free-running periods. Under the LD 12 : 12 h regime, activity offset occurs approximately 0.4 h after lights-off transition, assigned to circadian time (Ct) 12.4 h. The phase of activity onset, peak and offset, and activity duration depends on the photoperiodic regimes. The circadian rhythm can be entrained to a 24-h period by exposure to submultiple cycles of LD 6 : 6 h, as if the locomotive rhythm is entrained to LD 18 : 6 h. Phase shifts of delay and advance are observed when perturbing single light pulses are presented during free-running under DD conditions. Temperature compensation of the free-running period is demonstrated under DD and LL conditions. Steady-state entrainment of the locomotor rhythm is achieved with square-wave temperature cycles of 10 °C amplitude, but a 5 °C amplitude fails to entrain.     

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.     

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.     

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.     

Circadian locomotory rhythm and the influence of moulting in Australian field cricket nymphs

ABSTRACT. Evidence is presented for a circadian control of locomotory activity in the larval stadia of the cricket, Teleogryllus commodus Walker. Under light—dark cycles (LD), maximal activity occurs around the L/D transition and/or in the hours preceding it. Free-running rhythm patterns longer than 24 h are observed in constant light. Re-entrainment to phase advances in the LD cycle is also accompanied by several transient cycles. However, free-running rhythms under constant darkness or transients when exposed to LD cycle delays were not found. LD cycles during the eighth stadium set the phase of a free-running rhythm in the adult, even if the nymph does not show a rhythm. Nymphal activity is often erratic and is disrupted periodically by the moulting cycle, but moulting does not interrupt the operation of the circadian system. The daily timing of the moult itself is not under circadian control.     

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.     

Circadian Rhythms of Self-feeding and Locomotor Activity in Zebrafish (Danio Rerio)

To investigate daily feeding rhythms in zebrafish, the authors have developed a new self-feeding system with an infrared photocell acting as a food-demand sensor, which lets small-size fish such as zebrafish trigger a self-feeder. In this paper, the authors used eight groups of 20 fish. Locomotor activity rhythms were also investigated by means of infrared sensors. Under a 12?h:12?h light (L)-dark (D) cycle, zebrafish showed a clear nocturnal feeding pattern (88.0% of the total daily food-demands occurring in the dark phase), concentrated during the last 4?h of the dark phase. In contrast, locomotor activity was mostly diurnal (88.2% of total daily activity occurring in the light phase). Moreover, both feeding and locomotor rhythms were endogenously driven, as they persisted under free-running conditions. The average period length (τ) of the locomotor and feeding rhythms was shorter (τ?=?22.9?h) and longer (τ?=?24.6?h) than 24?h, respectively. During the time that food availability was restricted, fish could only feed during ZT0–ZT12 or ZT12–ZT16. This resulted in feeding activity being significantly modified according to feeding time, whereas the locomotor activity pattern remained synchronized to the LD cycle and did not change during this trial. These findings revealed an independent phasing between locomotor and feeding activities (which were mostly nocturnal or diurnal, respectively), thus supporting the concept of multioscillatory control of circadian rhythmicity in zebrafish. (Author correspondence: javisan@um.es)