Circadian components of semilunar and lunar timing mechanisms

The timing of semilunar as well as lunar reproductive rhythms has been analyzed in different geographic populations of the intertidal chironomid Clunio. In stocks of three populations differing in period and phase relationship with the lunar month, these long-term rhythms were synchronized in the laboratory by using artificial moonlight cycles of 30 days in otherwise 24-hr light-dark (LD) cycles (0.4 lux during 4 successive nights every 30 days in LD 12:12). In LD cycles of various periods, a strong synchronization was only possible in LD 12:12 and LD 11:11, whereas in LD 10:10 and LD 15:15 the synchronization by the 30-"day" moonlight cycle was weak or even absent. The study demonstrates a limited range of circadian periods for entrainment of the long-term rhythms. It is concluded that an LD cycle with a period near 24 hr is an essential zeitgeber condition for semilunar and lunar timing in this marine insect. Further, it is suggested that the underlying physiological timing mechanism of Clunio consists of a circadian function for the perception of the monthly moonlight zeitgeber cycles that entrain the endogenous, temperature-compensated oscillator of the circasemilunar (or circalunar) period. The long-term oscillator triggers the metamorphosis of the insect, and thereby determines the time of its eclosion and reproduction on the shorelines, in correlation with days of spring tides recurring about every 14-15 days.     

The effect of photoperiod on daily rhythms of Oxygen consumption in the tropical toad,Bufo marinus

Summary Oxygen consumption was measured with an automatic continuously recording electrolysis system in toads acclimated at 15° C and under photoperiods of LD 816, 1212, 168, 66, 231 and LL and DD. All groups exposed to LD had pronounced significant daily rhythms with a trough near the onset of the photophase and a peak at the beginning of the scotophase (Figs. 1–3, 9, 10), while no rhythms were detected in animals exposed under free-running conditions of constant light (Figs. 6–7) or constant darkness (Fig. 8), even on the first day under LL or DD. These rhythms are thus shown to be non-circadian, since they do not persist in freerunning conditions of up to 45 days. Some preliminary evidence from studies on locomotor activity (Figs. 12, 13) indicates that the daily rhythm in metabolic rate may be independent of the locomotor activity cycle. The difference in the rate of oxygen consumption during peak and low hours of each daily cycle is defined as the daily routine metabolic scope, which may be more useful in ecological studies of animal energetics than the difference between the minimum and a forced maximum metabolic rate. No correlations were found between daily changes in atmospheric pressure and cycles of oxygen consumption.This work was supported by a grant from the National Science Foundation (GB-3574) and a University of Rhode Island Research Committee Grant-in-aid. We are grateful to Robert Cubert for aid in designing and constructing the electrolysis system and for assistance with computer programs.     

Rhythmicity of swimming activity in an axenic population of Paramecium multimicronucleatum

A unicellular organism, Paramecium, exhibits circadian rhythm activities in many physiological phenomena, i.e., mating reactivity, photoaccumulation in Paramecium bursaria and mating type reversals in Paramecium multimicronucleatum. In this study, we used an image-processing system to analyze swimming activity in a population of Paramecium multimicronucleatum cultured axenically under 12 h-light/12 h-dark cycles (LD 1212). Swimming behavior was recorded both under LD 1212 and constant darkness and images tracing the tracks of Paramecium were produced every 4 min. Swimming activity was represented by the occupied area by the tracks relative to the total observed area. It is high during daytime and low at night and exhibits a freerunning rhythm in constant darkness. Furthermore, criteria for two major components of swimming behavior, straight and circle swimming, were established and analyzed. The results indicate that swimming behavior alters depending on the time of day: straight swimming increased during the day and circling was dominant around dusk both under LD 1212 and constant darkness.Abbreviations ZT Zeitgeber Time - LD 1212 12h- light/12h-dark cycles - TF transversing frequency     

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

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

Effect of hydration,photoperiod and temperature on diapause termination in eggs ofPetrobia (Tetranychina) harti (Acari: Tetranychidae)

Petrobia harti (Ewing) displays a facultative summer diapause in the egg stage. An adult female will lay only either diapause or non-diapause eggs throughout her life. In the laboratory, diapause eggs are laid by females which develop on detachedOxalis articulata leaves under long-day photoperiods and a relatively low temperature of 19±1°C.Diapause occurs in a stage of advanced embryonic development, in which the embryo appears U-shaped when observed from the egg's ventral side. Embryonic development ceased at this stage, and no further growth occurred when the eggs were kept under a relative humidity of about 70% in various photoperiod and temperature conditions. However, when the eggs were hydrated by placing them on wet cotton wool, development in some embryos (apparently in those which had completed their diapause development) proceeded beyond the U-stage at a rate similar to that in non-diapause embryos and the eggs hatched.Under LD 168 and 19±1°C or 26±1°C, the later from oviposition the period of egg hydration started, the higher the percentage of diapause termination. Under LD 168 and 26±1°C, diapause termination occurred mostly during the first week of hydration, while at 19±1°C mostly during the second and third week.At 26±1°C, in eggs hydrated 15 days but not 30 days from oviposition, the percentage of diapause termination was higher under a long-day than under a short-day photoperiod.Under LD 168, when the eggs were hydrated continuously from oviposition or starting 15, 30 and 45 days from it, the percentage of diapause termination was higher at 26±1°C than at 19±1°C.The percentage of diapause-laying adult females and the intensity of egg diapause were higher when the pre-imaginal mites grew at LD 1212 and 19±1°C, than when they grew at LD 168 and 26±1°C. This maternal effect on egg diapause intensity was expressed when the eggs were maintained at LD 1212 and 19±1°C but not at LD 168 and 26±1°C.     

A quantitative study of synaptic ribbons in pinealocytes of adult Chinese hamsters (Cricetulus griseus) under different photoperiodic conditions

Summary Synaptic ribbons (SR) in pinealocytes of adult (120–130 day-old) male Chinese hamsters (Cricetulus griseus) were classified into types 1, 2 and 3; these have a central dense structure showing rod-like, various and ringlike profiles, respectively. The central structure of the type-2 SR usually appeared as round, oval or comma-like bodies, and occasionally as plates showing various profiles or clubshaped bodies. The quantity of each type of SR, expressed as the SR index, was determined over a 24-h period under a light/dark regime (LD) 1212 or LD 1410. On comparing the results obtained from adults with previously published data from young (60–70-days-old) animals under LD 1212, it was found that, in both young and adult animals, the type-1 and type-3 SR indices exhibited different 24-h variations, whereas the type-2 SR index remained constant over a 24-h period. In addition, the indices of the type-2 SR, but not those of the other SR types, were found to be significantly larger in adult than in young animals. In adult animals, the effects of the photoperiod were different between the three types of SR. A nocturnal increase in the type-1 SR index was observed under both LD 1212 and LD 1410, its time course being different for each of these photoperiods. Under LD 1410, the type-2 SR index showed a significant 24-h rhythm with larger values during the dark period; this was not observed under LD 1212. The type-3 SR index was almost the same under LD 1212 and LD 1410. The results suggest that pinealocyte SR of the Chinese hamster may be composed of three types of SR, each with a different functional role.     

Influence of photoperiod,temperature and host plant on the production of diapause eggs inPetrobia (Tetranychina) harti (Acari: Tetranychidae)

Petrobia harti (Ewing) diapauses in the egg stage. Adult females lay either diapause or nondiapause eggs. On the University of Thessaloniki campus (41°N), the mite was found to develop on leaves ofOxalis corniculata L. throughout the year, while no mites were found on leaves ofOxalis articulata Savigny growing in the same area. In the laboratory the mite could be maintained equally well on detached leaves of both plant species, kept on wet cotton-wool.Forty to 90% females laying diapause eggs (dlf) were produced when the mites developed under LD 1212 and 19±1 °C, or LD 168 and 19±1 °C or 25±1 °C on leaves ofO. articulata detached from plants grown in the open in various seasons. Under the same conditions, a very low to zero percentage ofdlf was produced onO. corniculata. By rearing certain feeding stages on one of these twoOxalis hosts, and the other feeding stages on the other host, various percentages ofdlf were obtained. These percentages were the net effect of the antagonistic action of the twoOxalis species.By rearing the mites at LD 8.515.5, LD 1212 or LD 168 and a temperature of 19±1 °C onO. articulata leaves renewed every 3 days, or every 16–18 days, or not at all, it could be shown that diapause induction or aversion is caused by the direct effect of photoperiod on the mites, and not by an effect through the host leaves.When wholeO. articulata plants were grown under LD 168 and 19±1 °C in the laboratory, or developed in the open during April and May, flowers were produced, while under LD 1212 no flowering occurred. In the laboratory under diapause-inducing conditions, higher percentages ofdlf were produced on leaves detached from flowering plants than on leaves detached from plants not flowering.OnO. articulata leaves at 20 °C, photoperiods with photophases equal to or longer than 12 h induced from 70 to 80%dlf, while photoperiods with photophases equal to or shorter than 10.9 h induced very low to zero percentages. By transferring different chrysalis stages from a diapause-inducing (LD 1212) to a diapause-averting (LD 8.515.5) photoperiod, and vice versa, it was found that the nymphochrysalis through deutonymph stages were sensitive to photoperiod, the deutochrysalis and deutonymph being the most sensitive.Under an LD 1212 photoperiod, a temperature of 20 °C induced diapause, whereas 25 °C, 30 °C, or a daynight thermoperiod of 25 °C18 °C suppressed it.     

Light-dark cues entrain oviposition time in the chicken hen

Single Comb White Leghorn hens, entrained to a 24-h light-dark cycle (LDC), were maintained under 16L8D and 20L4D LDC for studying LD cues affecting oviposition time. A shift in the onset of darkness (74% response) had two times the effect on timing the oviposition in a LDC compared to a shift in the onset of light (35–38% response). An effect of over 94% response shift in time of oviposition was found when both LD cues were shifted together. A shift in oviposition time followed the same direction of a displacement of the dark period in a LDC. The longer dark period in a LDC delayed the time of oviposition when counted from the onset of darkness (9.32–9.58 h for 20L4D and 10.50–10.98 h for 16L8D cycles). Data indicated that the onset of light and of darkness, the daily position and length of the dark period all determined the time of oviposition during a LDC.Contribution from the Missouri Agricultural Experiment Station. Journal Series Number10042.     

Circadian organization in the ruin lizard Podarcis sicula: the role of the suprachiasmatic nuclei of the hypothalamus

The effects of electrolytic lesions to the suprachiasmatic nuclei of the hypothalamus (SCN) on circadian rhythms of locomotor activity were examined in ruin lizards Podarcis sicula maintained in constant darkness and constant temperature (29°C). All lizards (N=15) in which the lesion damaged 80% or more of the SCN became behaviorally arrhythmic. On the contrary, locomotor rhythms persisted in all cases (N=11) when the SCN remained intact and lesions were confined to neighbouring regions of the preoptic area. Taken together with previous work which demonstrates that the pineal and the retinae are not essential for the persistence of circadian locomotor rhythmicity in Podarcis sicula and with recent evidence showing the homology between the SCN of lizards and those of mammals the present results strongly support the view that the SCN of Podarcis sicula contain the primary pacemaker(s) for locomotor rhythms.Abbreviations DD constant darkness - LL constant light - SCN suprachiasmatic nuclei of the hypothalamus - PH nucleus periventricularis hypothalami - OC optic chiasm - te length of circadian activity - freerunning circadian period     

Periodicity of recurrent hypothermia during hibernation in the pocket mouse,Perognathus longimembris

Summary The timing of bouts of torpor was continuously monitored in 36 pocket mice,Perognathus longimembris, for as long as 11 months. Animals were kept at 8 °C or 18 °C, with light cycles of LD 1212, 1014, 159, or continuous darkness. At 8 °C, torpor duration had a polymodal frequency distribution, and the near absence of bouts of torpor lasting exactly 24, 48, 72, and 96 h divided the bouts into discrete classes (one-day, two-day, etc.). The maximum duration of torpor was 2.5 times greater at 8 °C than 18 °C.A circadian rhythmicity in the timing of all classes of torpor bouts was indicated because, when entrained to a 24-h photocycle, the intervals between successive arousals were multiples of 24 h. This rhythmicity persisted in continuous darkness, demonstrating its endogenous origin.There were two different free-running periods () for the torpor rhythm of each animal in constant darkness at 8 °C. In general, series of multi-day torpor bouts had a period less than 24 h, while series of one-day bouts showed a period near to or greater than 24 h. The transition between the two frequencies of the torpor rhythm commonly coincided with a phase shift involving one or more transient cycles.The general circadian oscillator model of Aschoff and Wever was modified to describe the timing and the changes in duration of torpor. The data may be viewed as arising from an asymmetrical oscillation which is synchronized by the lights-on transition of the photocycle, and whose shape is correlated with the period of the rhythm. The different classes of torpor (one-day, two-day, etc.) were also timed differently with respect to the entraining conditions of a light-dark cycle, and these differences in timing are consistent with oscillator theory.This study was conducted while the author was in receipt of a U.S. Atomic Energy Commission fellowship. Instrumentation and logistic support were provided by National Science Foundation grant #GB32947 to G.A. Bartholomew. I wish to thank George A. Bartholomew for suggestions throughout the course of this study, and also James Enright, Karl Hamner, and G.J. Kenagy for numerous helpful suggestions.     

Circadian oscillation of the multiple unit activity in the guinea pig suprachiasmatic nucleus

Summary In the guinea pig with chronically implanted electrodes, neuronal multiple unit activity (MUA) was recorded inside and outside the suprachiasmatic nucleus (SCN). Long-term recording of the SCN indicated distinct daily rhythms with a daytime peak in MUA during a 24-h light-dark (LD 1212) cycle. On the other hand, MUA recorded from adjacent hypothalamic regions outside the SCN showed a phase reversal with a nighttime peak, similarly to the rat. The amplitude of the rhythms recorded outside the SCN was much smaller (one-half to one-quarter) than that inside the SCN. These rhythms persisted during constant darkness indicating characteristics of endogenous circadian rhythmicity. When the external lightdark cycle was delayed abruptly for 12 h, MUA rhythms showed a gradual phase shift taking 7–10 days for complete reentrainment. Overt behavior including sleep-wakefulness did not show significant and consistent daily or circadian rhythms in spite of the distinct oscillation in neuronal activity inside the SCN.Abbreviations SCN suprachiasmatic nucleus - MUA multiple unit activity     

Seasonal effects on the freerunning rhythm of circadian activity of longnose dace (Rhinichthys cataractae)

Synopsis Freerunning circadian rhythms of locomotor activity in individual longnose dace sampled from a population at 41°N latitude were recorded under constant darkness throughout the year. There was an annual cycle in the length of the circadian period, with maximum and minimum lengths of mean period of 23.6 and 21.6 h recorded during summer (June) and winter (December), respectively. These annual changes in period length may have resulted from seasonal changes in entrainment by natural light-dark cycles and their after-effects on endogenous circadian rhythms. The possibility of an endogenous circannual rhythm was also considered.     

Variable thermal sensitivity as output of a circadian clock controlling the bimodal rhythm of temperature choice in the ant Camponotus mus

Along a thermal gradient and under a LD 1212 h cycle, nurse workers of the ant Camponotus mus select for the brood two different temperatures daily: 30.8°C at the middle of the light period (circadian phase = 90°), and 27.5°C 8 h later, during the dark period (CP = 210°). Brood-carrying activity proved to be self-sustained, running its two daily bursts free with a similar period of 23.5 h, under both LL and DD. The LD alternation acted as a strong Zeitgeber. A phase-delay of the LD 1212 h cycle reset the overt rhythm at once, being both daily events locked-on to the delayed light: dark transition. However, changes in expression, non-occurrence, or even splitting of the two daily brood-carrying events during resetting depended on the phase of the delayed DL transition. By comparing the occurrence of activity with predictions based on a threshold curve of thermal sensitivity, results indicated that an immediate resetting of the involved pacemaker actually takes place. Nurse workers do not directly control the total time spent by the brood at the selected temperature. Instead, the endogenously-driven thermal sensitivity triggers their thermal-searching behavior at two critical times of the day, when environmental temperature is expected to reach its maximum and minimum.     

Entrainment and phase shifting of the circadian rhythm of cell division by light in cultures of the achlorophyllous ZC mutant ofEuglena gracilis

The photosynthesis-deficient ZC mutant ofEuglena gracilis Klebs (strain Z) was cultured at 16°C on an aerated, magnetically stirred, mineral medium containing 0.1% ethanol (pH 7.0). Cell division could be entrained by a 12: 12 light: dark cycle (LD: 12, 12) or even by a one-pulse skeleton photoperiod (LD: 1,23) The rhythm free-ran in DD for at least 8 days with a circadian period (=25.5 h) in populations that had been previously entrained by LD. The freerunning rhythm could be phase-shifted by a single 1-h light pulse (3000 lx). The strong (Type 0) phase-response curve derived from the resetting effects of such signals given at different circadian times was similar to that for the photosynthetic wild-type strain. These results demonstrate that the presence of a functional chloroplast compartment is not necessary for the circadian clock to function inEuglena and suggest that phase resetting of the circadian clock by light occurs via a similar pathway in both photosynthetic and non-photosynthetic cell types.     

The role of the pineal and the retinae in the expression of circadian locomotor rhythmicity in the ruin lizard,Podarcis sicula

Summary A marked interspecific variability in the role played by the pineal and the retinae characterizes the circadian system of lizards. I examined the role played by these structures in a new model species, the ruin lizard, Podarcis sicula. In constant temperature and darkness pinealectomy as well as bilateral removal of the retinae produced significant changes (both lengthening and shortening) in the freerunning period of locomotor rhythms. Circadian activity time was also affected by pinealectomy. Circadian locomotor rhythmicity persisted in all cases even when both operations were combined in the same individuals. This demonstrates in Podarcis sicula the existence of an oscillatory system outside the pineal and the retinae which can drive locomotor rhythms. The period changes recorded after pinealectomy as well as after bilateral removal of the retinae specifically suggest that both the pineal and the retinae play a modulating role on circadian oscillators located elsewhere in the system, with the final effect of stabilizing the overt rhythms.Abbreviations DD constant darkness - LL constant light - PIN-X pinealectomy - RET-X bilateral removal of the retinae - SHAM sham pinealectomy - circadian activity time - freerunning circadian period     

Circadian rhythms of corneal mitotic rate,retinal melatonin and immunoreactive visual pigments,and the effects of melatonin on the rhythms in the Japanese quail

We investigated circadian ocular rhythms in the Japanese quail, Coturnix coturnix japonica. The birds were placed under light-dark cycles (LD 1212), constant light (LL) and constant darkness (DD), and the retinas were dissected out at four-hour intervals throughout 24 h. Following measurements were performed. (1) Melatonin content in the retina was measured by radioimmunoassay. It was low in light and several folds higher in darkness under LD 1212. The rhythm continued in DD, but disappeared in LL. (2) Mitotic figures in the corneal epithelium were counted. Similar rhythms to the melatonin content were observed in the corneal mitotic rate with a slight phase delay. (3) The retinas were fixed at 4-h intervals and immunostained with anti-bovine rhodopsin serum and anti-chicken iodopsin monoclonal antibodies. The outer segments of photoreceptor cells were stained intensively throughout 24 h in LD 1212, LL and DD. In contrast, the stainability of the locus close to the outer limiting membrane where the Golgi apparatus exists changed diurnally. Scores showing the ratio of cells with positive staining indicated high values from 4 h after the onset of light to the beginning of dark phase under LD 1212. The values were high throughout 24 h in LL and intermediate or low in DD. (4) To investigate the effect of melatonin on the corneal mitotic rate and visual pigments at the Golgi region, melatonin was injected into one eye and saline into the contralateral eye. Melatonin induced a phase advance in the corneal mitotic rate under LD 1212, but did not induce a rhythm under LL. The ratio of photoreceptor cells with positive staining to anti-visual pigment antibodies at the Golgi region was not affected by melatonin injection.Abbreviations ANOVA analysis of variance - BSA bovine serum albumin - DD constant darkness - Io-mAb monoclonal antibodies against chicken iodospin - LD light-dark - LL constant light - mRNA messenger ribonucleic acid - PBS phosphate buffer solution - Rh-As antiserum against bovine rhodopsin - SCN suprachiasmatic nucleus - T transducin - T transducin      

Circadian organization of an animal lacking a pineal organ; the young American alligator,Alligator mississippiensis

Summary Circadian rhythms were examined in young American alligators,Alligator mississippiensis, an animal that naturally lacks a pineal organ. Individual alligators displayed persistent, stable, free-running circadian activity rhythms under both constant darkness and constant illumination. Free-running activity was entrained by 24 h light-dark cycles. The circadian period length was also temperature compensated, with Q₁₀'s of 1.0–1.40 obtained for a 22–32 °C interval. The presence of this quite normal circadian rhythmicity in alligators indicates that the pineal organ is not always essential for the expression of circadian rhythms in lower vertebrates.This research was supported by grant NS 12257 for N.I.H. to C.L. Ralph. Alligators were obtained through the courtesy of C.R. Tracy and T. Joanen of the Rockefeller Wildlife Refuge, Grand Chenier, Louisiana.     

Circadian locomotor rhythms in the desert iguana

Summary Desert iguanas, Dipsosaurus dorsalis, displaying freerunning circadian locomotor rhythms in conditions of constant darkness and temperature received electrolytic lesions to the hypothalamus. The locomotor activity of those lizards (N = 9) which sustained 80% or more damage to the suprachiasmatic nucleus (SCN) became arrhythmic whereas all animals that sustained less than 35% damage to the SCN remained rhythmic, even though they sustained significant damage to nearby regions of the hypothalamus and preoptic area. These results suggest strongly that the SCN plays a role in the regulation of circadian rhythms in the desert iguana. Taken together with other evidence, they support the view that this structure is homologous to the mammalian SCN, which acts as a pacemaker in the circadian system.Abbreviations SCN suprachiasmatic nucleus - freerunning circadian period     

Circadian organization in the lizardAnolis carolinensis: a multioscillator system

Summary The locomotor activity rhythm of normal anoles freerunning in conditions of constant darkness or constant dim light typically shows a bimodal pattern. Pinealectomy of freerunning anoles renders the lizards arrhythmic (continuously active). Pinealectomy of anoles entrained to light-dark (LD) cycles often causes a large phase advance of the lizards' daily activity onsets relative to light onsets. Also, the activity onsets of individual pinealectomized anoles entrained to LD cycles often alternate between two preferred phase relationships relative to the LD cycle. These data support the hypothesis that at least two circadian oscillators (or sets of oscillators) drive the overt rhythm of locomotor activity and the pineal acts either as a circadian pacemaker or as a coupling device for these oscillators.Abbreviation SCN suprachiasmatic nuclei     

Geographical variation in circadian eclosion rhythm and photoperiodic adult diapause inDrosophila littoralis

Summary The time measuring system ofDrosophila littoralis strains originating between 40–70° N was found to be highly variable and latitude dependent. The critical daylength for photoperiodic adult diapause varied from 12 h or no diapause response in the south to 20 h in north. The median timing of pupal eclosion rhythm varied correspondingly from 21 h to 12 h from lights off in LD 321, and the period of free-running rhythm of eclosion from 24 h to 19 h. The phase of the free-running rhythm was also variable, and correlated with the phase of the entrained rhythm. Latitudinal variation in the entrained rhythm of eclosion and in diapause is adaptive, leading to eclosion early in the morning and to overwintering at the adult stage. In some strains with a late phase of eclosion, strong transient cycles were seen following the transition from LL to DD. A total damping of the free-running eclosion rhythm within 2–7 days was common to all strains. This damping was more pronounced in the northern strains. The phase and period of eclosion rhythms were statistically independent. Diapause was not correlated with any parameters of the eclosion rhythm in the analysis. Diapause may still be influenced by the period of the eclosion rhythm, even though its minor contribution may be masked by a more variable, eclosion rhythm independent system in the determination of diapause.Abbreviations, symbols and terms LD Light/dark; as in LD 321 meaning a cycle of 3 h light21 h darkness - LL Continuous light - DD Continuous darkness - T Period of a Zeitgeber cycle - Natural period of eclosion rhythm in constant conditions - _EL Phase of the free-running rhythm of eclosion - A Amplitude of the free-running rhythm of eclosion; possible range is from 4.17% (no rhythmicity) to 20% (the daily eclosion peaks 2–6 within 5 h each) - P Persistence of the free-running rhythm of eclosion; the number of daily eclosion peaks where the mean for five highest hourly percentages still exceed 6% - A phase shift, expressed in h; a re-setting of a rhythm; either as an advance shift (i.e. earlier= +), or as a delay shift (i.e. later = –) - PRC Phase-response curve - _LD Phase of entrained rhythm of eclosion; e.g. _{LD 321} is the median hour of eclosion peak from lights off at LD 321 - SD _ecl Amplitude of the entrained rhythm of eclosion; the smaller SD_ecl the higher the amplitude - PPRC Photoperiodic response curve; proportion of females in diapause displayed as a function of daylength - CDL Critical photoperiod; the photoperiod in the 24 h LD cycle at which 50% of the population studied diapauses - SD _diap Accuracy of diapause response of a strain; the smaller the SD_diap the more accurate the response - Cdl The main locus controlling CDL inD. littoralis