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)     

Lunar and daily spawning rhythms of Senegal sole Solea senegalensis

A periodicity of 29 days was observed in spawning rhythms in Senegal sole Solea senegalensis , with an acrophase around the last quarter and the new moon. In both spring and autumn, a very marked nocturnal spawning rhythm was registered, with spawning beginning after dusk and the acrophase occurring around 2300 hours. When the photoperiod was artificially extended (from 10L:14D to 14L:10D), S. senegalensis synchronized to the new photoperiod: spawning took place after the new 'dusk', the beginning gradually shifting from 2100 to 2300 hours and the acrophase from 2325 to 0032 hours. Under continuous light conditions, fish sustained rhythmicity for 2 days, with an acrophase at 2249 hours, which suggested the existence of an endogenous pacemaker controlling the daily spawning rhythm. These findings provided new insights for better understanding the reproductive physiology of this species and for optimizing the timing protocols of egg collection and larvae production in S. senegalensis aquaculture.     

Effects of different photoperiods and handling stress on spawning and reproductive performance of pikeperch Sander lucioperca

The objective of this study was to control the reproductive cycle of pikeperch (Sander lucioperca) through determining the effects of different photoperiods and handling stress on the reproduction quality, timing and quality of spawning, fertilization, sex steroids, and cortisol concentrations. In this study, 72 pikeperch broodstocks with an average weight of 1367 ± 55.3 g were exposed to different photoperiods including constant light (24L:0D), constant darkness (0L:24D), and 12 h of light, 12 h of darkness (12L:12D) for 40 days. Half of the broodstocks of each photoperiod treatment were exposed to handling stress at a specific time of the day. Applying different photoperiods caused changes in the timing of broodstocks' spawning, so that fish under 24L:0D spawned earlier than those of other photoperiods, and stressed fish of the 0L:24D photoperiod had a delayed spawning compared to others. Also, the spawning of the broodstocks at different photoperiods which were exposed to handling stress was either delayed or did not occur at all. The highest and lowest spawnings were observed in the morning and at night, respectively. Fertilization percentage, number of eggs per gram, sex steroids including estradiol, progesterone, and testosterone, as well as cortisol and calcium concentrations did not show any significant difference in different photoperiods and handling stress. In stressed males of the 24L:0D photoperiod, there only was a significant decrease of testosterone concentration compared to the beginning of the experiment. Results indicated that the spawning performance of pikeperch broodstocks could be considerably stimulated using an effective photoperiod. Similarly, pikeperch broodstocks in culture systems are usually affected by handling stress, and this stress could lead to a poor reproductive performance and inhibition of spawning.     

Daily rhythms of clock gene expression and feeding behavior during the larval development in gilthead seabream,Sparus aurata

Light is the main environmental time cue which synchronizes daily rhythms and the molecular clock of vertebrates. Indeed, alterations in photoperiod have profound physiological effects in fish (e.g. reproduction and early development). In order to identify the changes in clock genes expression in gilthead seabream larvae during ontogeny, three different photoperiods were tested: a regular 12L:12D cycle (LD), a continuous light 24L:0D (LL) and a two-phases photoperiod (LL?+?LD) in which the photoperiod changed from LL to LD on day 15 after hatching (dph). Larvae were sampled on 10, 18, 30 and 60 days post-hatch (dph) during a 24?h cycle. In addition to the expression of clock genes (clock, bmal1, cry1 and per3), food intake was measured. Under LD photoperiod, larvae feed intake and clock genes expression showed a rhythmic pattern with a strong light synchronization, with the acrophases occurring at the same hour in all tested ages. Under LL photoperiod, the larvae also showed a rhythmic pattern but the acrophases occurred at different times depending on the age, although at the end of the experiment (60 dph) clock genes expression and feed intake rhythms were similar to those larvae exposed to LD photoperiod. Moreover, the expression levels of bmal1 and cry1 were much lower than in LD photoperiod. Under the LL?+?LD photoperiod, the 10 dph larvae showed the same patterns as LL treatment while 18 and 30 dph larvae showed the same patterns as LD treatment. These results revealed the presence of internal factors driving rhythmic physiological responses during larvae development under constant environmental conditions. The LL?+?LD treatment demonstrates the plasticity of the clock genes expression and the strong effect of light as synchronizer in developing fish larvae.     

Photoperiodic history determines the reproductive response of rainbow trout to changes in daylength

The mechanisms underlying the photoperiodic entrainment of the endogenous circannual rhythm of maturation in the rainbow trout were investigated by subjecting December-spawning fish to abrupt changes in daylength which varied in their timing or magnitude. These protocols advanced spawning by up to 4 months. Maturation occurred in sequence in fish maintained on 18L:6D from January and February, and in fish exposed to 18L:6D from December, January and February, followed by 6L:18D in May, indicating that the abrupt increases in daylength were effective entraining cues. `Long' photoperiods of between 12 and 22 h applied in January, followed by shorter photoperiods of between 3.5 and 13.5 h from May, were equally effective in advancing maturation. Maturation was also advanced, though to a lesser extent, in fish maintained on photoperiods of 8.5 or 10 h from January, followed by a photoperiod of 1.5 h from May. In contrast, maturation was delayed in fish maintained under a constant 8.5-h photoperiod from January, and these fish also exhibited a desynchronization of spawning times characteristic of endogenous circannual rhythms in free-run. Collectively, these results indicate that photoperiodic history determines the reproductive response of rainbow trout to changes in daylength. Accepted: 7 August 1998     

Does lighting manipulation during incubation affect hatching rhythms and early development of sole?

Light plays a key role in the development of biological rhythms in fish. Previous research on Senegal sole has revealed that both spawning rhythms and larval development are strongly influenced by lighting conditions. However, hatching rhythms and the effect of light during incubation are as yet unexplored. Therefore, the aim of this study was to investigate the impact of the light spectrum and photoperiod on Solea senegalensis eggs and larvae until day 7 post hatching (dph). To this end, eggs were collected immediately after spawning during the night and exposed to continuous light (LL), continuous darkness (DD), or light-dark (LD) 12L:12D cycles of white light (LD(W)), blue light (LD(B); λ(peak)?=?463?nm), or red light (LD(R); λ(peak)?=?685?nm). Eggs exposed to LD(B) had the highest hatching rate (94.5%?±?1.9%), whereas LD(R) and DD showed the lowest hatching rate (54.4%?±?3.9% and 48.4%?±?4.2%, respectively). Under LD conditions, the hatching rhythm peaked by the end of the dark phase, but was advanced in LD(B) (zeitgeber time 8 [ZT8]; ZT0 representing the onset of darkness) in relation to LD(W) and LD(R) (ZT11). Under DD conditions, the same rhythm persisted, although with lower amplitude, whereas under LL the hatching rhythm split into two peaks (ZT8 and ZT13). From dph 4 onwards, larvae under LD(B) showed the best growth and quickest development (advanced eye pigmentation, mouth opening, and pectoral fins), whereas larvae under LD(R) and DD had the poorest performance. These results reveal that developmental rhythms at the egg stage are tightly controlled by light characteristics, underlining the importance of reproducing their natural underwater photoenvironment (LD cycles of blue wavelengths) during incubation and early larvae development of fish.     

Daily rhythms of lipid metabolic gene expression in zebra fish liver: Response to light/dark and feeding cycles

Despite numerous studies about fish nutrition and lipid metabolism, very little is known about the daily rhythm expression of lipogenesis and lipolysis genes. This research aimed to investigate the existence of daily rhythm expressions of the genes involved in lipid metabolism and their synchronization to different light/dark (LD) and feeding cycles in zebra fish liver. For this purpose, three groups of zebra fish were submitted to a 12:12?h LD cycle. A single daily meal was provided to each group at various times: in the middle of the light phase (ML); in the middle of the dark phase (MD); at random times. After 20 days of acclimation to these experimental conditions, liver samples were collected every 4?h in one 24-h cycle. The results revealed that most genes displayed a significant daily rhythm with an acrophase of expression in the dark phase. The acrophase of lipolytic genes (lipoprotein lipase – lpl, peroxisome proliferator-activated receptor – pparα and hydroxyacil CoA dehydrogenase – hadh) was displayed between ZT 02:17?h and ZT 18:31?h. That of lipogenic genes (leptin-a – lepa, peroxisome proliferator-activated receptor – pparγ, liver X receptor – lxr, insulin-like growth factor – igf1, sterol regulatory element-binding protein – srebp and fatty acid synthase – fas) was displayed between ZT 15:25?h and 20:06?h (dark phase). Feeding time barely influenced daily expression rhythms, except for lxr in the MD group, whose acrophase shifted by about 14?h compared with the ML group (ZT 04:31?h versus ZT 18:29?h, respectively). These results evidence a strong synchronization to the LD cycle, but not to feeding time, and most genes showed a nocturnal acrophase. These findings highlight the importance of considering light and feeding time to optimize lipid metabolism and feeding protocols in fish farming.     

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

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

Effects of different photoperiods on the reproduction of the barbel, Barbus barbus (L.), reared at constant temperature

The role of photoperiod as an environmental factor controlling reproduction, particularly the duration and ending of the breeding season, in the barbel was investigated by tank experiments. The experiments used a population of barbel matured in captivity which were stripped of eggs at frequent intervals: 10–15 'spawnings'for each female were obtained at 15-day intervals.
A decreasing photoperiod (16·5 light: 7·5 dark→8L: 16D), for an annual cycle contracted to 6 months duration, inhibited the spawning of both female and male fish. This allowed two periods of reproduction (February-May and September-November) within one year. Under natural or constant (10L: 14D) photoperiod conditions, spawning came to an end spontaneously. The rate (accelerated, slow or natural) of daylength increase did not affect the onset of spawning.     

Seasonal effects of pinealectomy on testicular recrudescence and secretory activity of seminal vesicles in the catfish Heteropneustes fossilis (Bloch)

The effects of pinealectomy on testicular activity and secretory activity of seminal vesicles were examined in the catfish Heteropneustes fossilis under various combinations of photoperiod and temperature during different periods of the annual reproductive cycle. Pinealectomy had no effect on gonadal activity during the preparatory, prespawning and spawning periods of the reproductive cycle. However, during the postspawning period, under long (LD 14:10) or short (LD 9:15) photoperiod at 25° C or at gradually increasing ambient temperature, pinealectomy accelerated testicular recrudescence and secretory activity of the seminal vesicles. Nevertheless. during this period the presence of the pineal facilitated the recrudescence of testes and seminal vesicles in catfish exposed to continuous light (LL), continuous darkness (DD) and 12 hL:l2 hD (LD) at 25° C. These findings suggest that the role of the pineal in catfish reproduction is variable and depends upon the photoperiod and temperature regimes to which the fish are exposed, as well as on the time of the year and the state of the reproductive cycle. The results also suggest that the effects of pinealectomy in catfish are mediated through an influence on the hypothalamo-hypophyseal gonadal axis.     

Metamorphosis induces a light-dependent switch in Senegalese sole (Solea senegalensis) from diurnal to nocturnal behavior

Light plays a key role in the development of biological rhythms in fish. Recent research in Senegal sole has revealed that spawning and hatching rhythms, larval development, and growth performance are strongly influenced by lighting conditions. However, the effect of light on the daily patterns of behavior remains unexplored. Therefore, the aim of this study was to investigate the impact of different photoperiod regimes and white, blue, and red light on the activity rhythms and foraging behavior of Solea senegalensis larvae up to 40 days posthatching (DPH). To this end, eggs were collected immediately after spawning during the night and exposed to continuous white light (LL), continuous darkness (DD), or light-dark (LD) 12L:12D cycles of white (LD(W)), blue (LD(B), λ(peak) = 463 nm), or red light (LD(R), λ(peak) = 685 nm). A filming scenario was designed to video record activity rhythms during day and night times using infrared lights. The results revealed that activity rhythms in LD(B) and LD(W) changed from diurnal to nocturnal on days 9 to 10 DPH, coinciding with the onset of metamorphosis. In LD(R), sole larvae remained nocturnal throughout the experimental period, while under LL and DD, larvae failed to show any rhythm. In addition, larvae exposed to LD(B) and LD(W) had the highest prey capture success rate (LD(B) = 82.6% ± 2.0%; LD(W) = 75.1% ± 1.3%) and attack rate (LD(B) = 54.3% ± 1.9%; LD(W) = 46.9% ± 3.0%) during the light phase (ML) until 9 DPH. During metamorphosis, the attack and capture success rates in these light conditions were higher during the dark phase (MD), when they showed the same nocturnal behavioral pattern as under LD(R) conditions. These results revealed that the development of sole larvae is tightly controlled by light characteristics, underlining the importance of the natural underwater photoenvironment (LD cycles of blue wavelengths) for the normal onset of the rhythmic behavior of fish larvae during early ontogenesis.     

Monthly day/night changes and seasonal daily rhythms of sexual steroids in Senegal sole (Solea senegalensis) under natural fluctuating or controlled environmental conditions

In this paper we attempted to investigate the existence of daily fluctuations on plasma sexual steroids (17beta-estradiol, E(2) and testosterone, T) in Senegal sole (Solea senegalensis) females. We described the monthly day/night concentrations and seasonal daily rhythms in animals reared under natural photo- and thermo-period. In addition, the influence of the natural annual fluctuation of the water temperature on the plasma concentration of these steroids was investigated, using one group of Senegal sole under a natural photoperiod, but with an attenuated thermal cycle (around 17-20 degrees C) for one year. Although no significant day/night differences were detected in monthly samplings, the existence of an annual rhythm of E(2) and T (p<0.01) with an acrophase in February was revealed by COSINOR analysis. Maximum values were reached in March for both steroids (6.1+/-1.7 ng mL(-1) at mid-dark, MD and 4.0+/-0.6 ng mL(-1) at mid-light, ML for E2 and 1.4+/-0.4 ng mL(-1) at MD and 0.8+/-0.1 ng mL(-1) at ML for T) in anticipation of the spawning season (May-June). As regards seasonal daily rhythms, the presence of daily oscillations was revealed. At the spring solstice (21st March) a daily rhythm was observed for both steroids (COSINOR, p<0.01), with an acrophase at 20:00 h (E(2)) and at 21:08 h (T). In summer, autumn and winter no daily rhythms were observed due to the low steroid levels at those seasons. When Senegal sole females were submitted to an attenuated annual thermal cycle, the steroid rhythm disappeared (there was no surge in spring, as in the control group) and these fish did not spawn, despite being subjected to natural photoperiod conditions. This result underlined the importance of the natural annual fluctuation of water temperature and photoperiod on the synchronization of the spawning season and on the onset of steroidogenesis.     

Synchronizing Effect of Photoperiod on the Dual Phasing of Demand-Feeding Rhythms in Sea Bass

Under natural environmental conditions, sea bass feeding rhythms are nocturnal in winter and diurnal the rest of the year. In this paper we describe the effect of contracting and expanding photoperiods and two skeleton photoperiods (SP) on the dual feeding rhythms of sea bass ( Dicentrarchus labrax L. ). To this end, twelve animals were held individually with access to self-feeders. First, the lights on and lights off were progressively delayed and advanced respectively by one hour in group 1 (G1), and conversely in group 2 (G2), so that the fish were exposed from a light/dark (LD) 12L:12D cycle to 2:22 LD (G1) and DL (G2) cycles and finally 0.25:23.75 LD (G1) and DL (G2). In the second experiment two SP's were used involving two light pulses separated by 12 hours, each pulse lasting 0.25 hours during the first two weeks and one hour during the succeeding two weeks. The results showed that diurnal and nocturnal sea bass tended to confine their feeding phase following the contraction of the LD cycle. Both SP's failed to simulate a complete photoperiod. In conclusion, the LD cycle appeared to drive the daily feeding rhythms but, the photoperiod length did not itself control the inversions of nocturnal and diurnal fish, so that other factors, in addition to photoperiod, may be involved in the control of the annual rhythms of phase inversions in sea bass.     

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

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

Synchronizing Effect of Photoperiod on the Dual Phasing of Demand-Feeding Rhythms in Sea Bass

Under natural environmental conditions, sea bass feeding rhythms are nocturnal in winter and diurnal the rest of the year. In this paper we describe the effect of contracting and expanding photoperiods and two skeleton photoperiods (SP) on the dual feeding rhythms of sea bass (Dicentrarchus labrax L.). To this end, twelve animals were held individually with access to self-feeders. First, the lights on and lights off were progressively delayed and advanced respectively by one hour in group 1 (G1), and conversely in group 2 (G2), so that the fish were exposed from a light/dark (LD) 12L:12D cycle to 2:22 LD (G1) and DL (G2) cycles and finally 0.25:23.75 LD (G1) and DL (G2). In the second experiment two SP's were used involving two light pulses separated by 12 hours, each pulse lasting 0.25 hours during the first two weeks and one hour during the succeeding two weeks. The results showed that diurnal and nocturnal sea bass tended to confine their feeding phase following the contraction of the LD cycle. Both SP's failed to simulate a complete photoperiod. In conclusion, the LD cycle appeared to drive the daily feeding rhythms but, the photoperiod length did not itself control the inversions of nocturnal and diurnal fish, so that other factors, in addition to photoperiod, may be involved in the control of the annual rhythms of phase inversions in sea bass.     

Support for a rare pattern of temperature-dependent sex determination in archaic reptiles: evidence from two species of tuatara (Sphenodon)

Background

In many birds, day length (=photoperiod) regulates reproductive cycle. The photoperiodic environment varies between different seasons and latitudes. As a consequence, species at different latitudes may have evolved separate photoperiodic strategies or modified them as per their adaptive need. We studied this using house sparrow as a model since it is found worldwide and is widely investigated. In particular, we examined whether photoperiodism in house sparrows (Passer domesticus) at 27°N, 81°E shared features with those exhibited by its conspecifics at high latitudes.

Results

Initial experiment described in the wild and captive conditions the gonad development and molt (only in captives) cycles over a 12-month period. Both male and female sparrows had similar seasonal cycles, linked with annual variations in day length; this suggested that seasonal reproduction in house sparrows was under the photoperiodic control. However, a slower testis and attenuated follicular growth among captives indicated that other (supplementary) factors are also involved in controlling the reproductive cycle. Next experiment examined if sparrows underwent seasonal variations in their response to stimulatory effects of long day lengths. When birds were transferred every month over a period of 1 year to 16 hours light:8 hours darkness (16L:8D) for 17–26 weeks, there was indeed a time-of-year effect on the growth-regression cycle of gonads. The final experiment investigated response of house sparrows to a variety of light-dark (LD) cycles. In the first set, sparrows were exposed for 31 weeks to photoperiods that were close to what they receive in between the period from sunrise to sunset at this latitude: 9L:15D (close to shortest day length in December), 12L:12D (equinox, in March and September) 15L:9D (close to longest day length in June). They underwent testicular growth and regression and molt in 12L and 15L photoperiods, but not in 9L photoperiod. In the second set, sparrows were exposed for 17 weeks to photoperiods with light periods extending to different duration of the daily photosensitivity rhythm (e.g. 2L:22D, 6L:18D, 10L:14D, 14L:10D, 18L:6D and 22L:2D). Interestingly, a slow and small testicular response occurred under 2L and 10L photoperiods; 6L:18D was non-inductive. On the other hand, 14L, 18L and 22L photoperiods produced testicular growth and subsequent regression response as is typical of a long day photostimulation.

Conclusion

Subtropical house sparrows exhibit photoperiodic responses similar to that is reported for its population living at high latitudes. This may suggest the conservation of the photoperiodic control mechanisms in birds evolved over a long period of time, as a physiological strategy in a temporally changing environment ensuring reproduction at the best suited time of the year.     

Biological rhythm of leukocyte concentration in calves under natural conditions

The objective of this study was to describe daily rhythms in leukocyte concentration in calves. The work was carried out during June - July 2004. Six animals were exposed to a natural photoperiod of about 10:14 LD. 5 ml blood samples were drawn from the jugular vein every two hours during the 1st, 4th, 14th, 20th and 30th days of life. Concentration of leukocytes was measured with a hematological counter and analyzed with repeated measures ANOVA and Cosinor analysis. Leukocyte concentration did not show a circadian rhythm at the moment of birth, but exhibited ultradian components that adjusted to daily patterns by the 30th day of life, with an acrophase at 0:35 h and R = 0.97%.     

Photoperiodic responses during larval development and diapause of two geographic ecotypes of the rice stem maggot, Chlorops oryzae

Chlorops oryzae is bivoltine in northern Japan but trivoltine in the southern part of the country. In the bivoltine strain, both the egg and larval stages were found to be sensitive to photoperiod. When the egg stage was exposed to a long-day photoperiod (16L:8D), larval development showed a short-day type response, and mature third-instar larvae entered a summer diapause under a long-day photoperiod (15L:9D). When eggs experienced short days, the first-instar larvae entered a winter diapause under short-day conditions, and the critical photoperiod in the larval stage ranged from about 14L:10D to about 12L:12D as the photoperiod experienced by the eggs increased from 12L:12D to 14L:10D. However, the development of the larvae after overwintering was not influenced by the photoperiod. In the trivoltine strain, larval development was retarded under a 14L:10D photoperiod but not under either shorter or longer photoperiods, when larvae had spent the egg stage under a 16L:8D photoperiod. The critical photoperiod of the larval stage for the induction of a winter diapause in the first instar was about 12L:12D, though it varied to some extent with the photoperiod during the egg stage. Thus, Chlorops oryzae was able to adapt itself to the local climatic conditions by the development of variable and complicated photoperiodic responses.     

Effects of photoperiod and temperature on the regulation of the onset of maturation in the estuarine fish Menidia beryllina (Cope) (Atherinidae)

Menidia beryllina (Cope) is an annual species that inhabits coastal estuaries along the east coast of the United States from Cape Cod to the Gulf of Mexico. In Rhode Island, USA, its peak spawning time and the duration of the spawning vary among years and estuaries. However, the onset of gonadal maturation is more consistent (early May), suggesting that it may be regulated by more consistent cues than those regulating spawning. To determine the effects of photoperiod and temperature on the regulation of the onset of maturation two laboratory experiments were conducted and the results compared to field observations. In the first experiment, fish collected from the field in February were exposed to each of four treatments: increasing photoperiod/increasing temperature; increasing photoperiod/low temperature; low photoperiod/increasing temperature; and low photoperiod/low temperature. Only fish exposed to both increasing photoperiod and increasing temperature were able to complete maturation. Fish exposed to low photoperiod and increasing temperature responded by enlarging their livers, a response that was also observed in field fish collected in the fall. Fish exposed to the remaining two treatments neither matured their gonads nor enlarged their livers. In the second experiment fish collected from the field in early March were exposed to three treatments with different photoperiod regimes (daylight constant at 9.5 h, increasing up to 12 h, or increasing up to 15 h) and one increasing temperature regime. Fish in the 9.5-h treatment initiated maturation but were not able to complete the process, those exposed to the 12-h photoperiod matured and spawned for a short period of time before the gonads began to regress, and those exposed to the 15-h photoperiod matured and spawned large numbers of eggs throughout the remainder of the experiment. The 9.5- and 12-h photoperiod exposures also resulted in accumulation of reserves in the liver in both females and males. The 15-h photoperiod treatment resulted in liver enlargement in females, which were undergoing vitellogenesis, but not in males. Males exposed to the 9.5- and 12-h photoperiod accumulated significantly more visceral fat than those exposed to the 15-h photoperiod. In females, the amount of visceral fat accumulated was inversely proportional to the hours of light. These findings suggest that this species has evolved mechanisms that enable it to anticipate the coming of winter as well as the coming of suitable breeding conditions and ensure that it exhibits the appropriate response at the appropriate time (reserve accumulation for the winter or gonad maturation in the spring).