Melatonin in the Regulation of Annual Testicular Events in Carp Catla catla: Evidence from the Studies on the Effects of Exogenous Melatonin,Continuous Light,and Continuous Darkness

The physiological significance of melatonin in the regulation of annual testicular events in a major carp Catla catla was evaluated through studies on the effects of graded dose (25, 50, or 100 µg/100 g body wt.) of melatonin exogenously administered for different durations (1, 15, or 30 days) and manipulation of the endogenous melatonin system by exposing the fish to constant darkness (DD) or constant light (LL) for 30 days. An identical experimental schedule was followed during the preparatory (February–March), pre‐spawning (April–May), spawning (July–August), and post‐spawning (September–October) phases of the annual cycle. Irrespective of the reproductive status of the carp, LL suppressed while DD increased the mid‐day and mid‐night values of melatonin compared to respective controls. Influences of exogenous melatonin varied in relation to the dose and duration of treatment and the reproductive status of the carp. However, testicular response to exogenous melatonin (at 100 µg, for 30 days) and DD in each reproductive phase was almost identical. Notably, precocious testicular maturation occurred in both DD and melatonin‐injected fish during the preparatory phase and in LL carps during the pre‐spawning phase. In contrast, testicular functions in both the melatonin‐treated and DD fish were inhibited during the pre–spawning and spawning phases, while the testes did not respond to any treatment during the post‐spawning phase. In conclusion, this study provided the first experimental evidence that melatonin plays a significant role in the regulation of annual testicular events in a sub‐tropical surface‐dwelling carp Catla catla, but the influence of this pineal hormone on the seasonal activity of testis varies in relation to the reproductive status of the concerned fish.     

Environmental correlate of the testicular events in a major carp Catla catla in an annual reproductive cycle

Current communication describes annual testicular events in free-living Indian major carp Catla catla and their probable environmental synchronizer(s). The study was initiated with month-wise evaluation of gametogenic and steroidogenic status of the testis, and thus dividing the annual testicular cycle into the preparatory spawning (November to March), the pre-spawning (April to June), the spawning (July to August) and the post-spawning (September to October) phases. An exhaustive statistical analysis of the data on the studied variables of testicular functions and various components of the environment indicated seasonal fluctuations of photoperiod as the major environmental factor associated with the seasonal reproductive activity of this carp. Ambient temperature appeared as a dependent variable of photoperiod, and thereby, may have substantial influences on the development of testis in Catla catla. Rainfall, on the other hand, showed significant correlation only with the peak reproductive activity, i.e. the act of spawning. Collectively, it appears logical to surmise that photo-thermal conditions may act as proximate and rainfall may play a role of ultimate environmental factor in the regulation of annual testicular events in Indian major carp Catla catla.     

Diurnal and seasonal profiles of melatonin in the liver and their correlates of ovarian functions under natural photo-thermal conditions in adult carp Catla catla

Present study aimed to demonstrate daily rhythm features of hepatic melatonin concentrations in relation to ovarian functions during four reproductive phases of an annual cycle by measuring the levels of melatonin, 17-β estradiol (E2), vitellogenin (Vg) and maturation inducing hormone (MIH) in the liver and/or serum of adult carp Catla catla. Melatonin titres in liver, irrespective of reproductive phase, underwent daily variations with a peak in early dark phase and nadir at midday. However, the acrophase (Ø) of serum melatonin varied from late night in preparatory phase to midnight in the remaining parts of annual cycle. Their amplitude was highest during post-spawning phase and lowest during spawning phase. Hepatic E2 levels showed daily peak at midday and seasonal peak during pre-spawning phase. Though levels of serum Vg proteins and MIH did not exhibit daily variations, underwent seasonal changes with the highest and lowest values during spawning and post-spawning phases respectively. Hepatic melatonin titres always displayed significant negative correlation with the levels of both E2 and Vg. In essence, our study presented the first data on the daily and seasonal rhythm features of hepatic melatonin in carp and underlined their temporal relationship with the functions of ovary in any fish species.     

Temporal Pattern of Ovarian Activity in a Major Carp Catla catla and its Possible Environmental Correlate in an Annual Cycle

Histophysiological studies of the ovary in a hitherto unconsidered major carp Catla catla, included in this investigative treatise, have allowed the present investigators to demonstrate the pattern of annual events in ovarian functions, as well as the inter-relationship between changes in the ovary and environmental conditions. Month-wise studies of cytological features along with various biochemical features of ovarian functions in free-living Catla revealed the existence of precise annual cyclic changes. On the basis of the studies made on gonads, the annual breeding cycle of this fish has been divided into four different phases, namely (a) the preparatory phase (January-March), (b) the pre-spawning phase (April-June), (c) the spawning phase (July-August), and (d) the post-spawning phase (September-December). A detailed analysis of the data employing various exploratory data-analytic techniques was aimed at drawing a possible relationship between the indices of different reproductive events (oogenesis and oocyte maturation, vitellogenesis, steroidogenesis) in mature free-living female Catla and the different components of the environment in an annual cycle. The results of the study indicate that annual variations in photoperiod, alone or in association with water temperature, may be the major environmental component which plays a significant role in the regulation of seasonal maturation of ovary in Catla. These observations provided a basis for further experimental studies on the specific role of photoperiods in the regulation of ovarian activities in the currently considered most economically important species of Indian major carp.     

Localization and dynamics of Mel1a melatonin receptor in the ovary of carp Catla catla in relation to serum melatonin levels

We studied the localization, sub-cellular distribution and daily rhythms of a 37 kDa melatonin receptor (Mel_1aR) in the ovary to assess its temporal relationship with the serum melatonin levels in four different reproductive phases in carp Catla catla. Our immunocytochemical study accompanied by Western blot analysis of Mel_1aR in the ovary revealed that the expression of this 37-kDa protein was greater in the membrane fraction than in the cytosol. Ovarian Mel_1aR protein peaked at midnight and fell at midday in each reproductive phase. Conversely, serum melatonin levels in the same fish demonstrated a minimum diurnal value at midday in all seasons, but a peak at midnight (during pre-spawning, spawning, and post-spawning phases) or at late dark phase (during preparatory phase). In an annual cycle, band intensity of Mel_1aR protein showed a maximum at night in the spawning phase and a minimum in the post-spawning phase, demonstrating an inverse relationship with the levels of serum melatonin. Our data provide first evidence of the presence of Mel_1a melatonin receptor in carp ovary and offer interesting perspectives especially for the study of the mechanisms of the control of its rhythmicity and its response to external factors.     

Circadian Variation in Circulating Levels of Melatonin, Testosterone and Estradiol-17β in a Tropical Freshwater Catfish, Clarias batrachus, during Different Phases of the Annual Reproductive Cycle

In the present investigation, daily variations in plasma levels of melatonin, testosterone and estradiol-17β were analysed in the tropical freshwater catfish, Clarias batrachus during early and late-preparatory, pre-spawning, late-spawning and post-spawning phases of its annual reproductive cycle. Plasma melatonin levels exhibited a significant circadian rhythm during all the reproductive phases but peaks were invariably at night. The amplitude of melatonin was high during the early-preparatory phase and low in the pre-spawning phase. Testosterone also displayed significant rhythm during all the studied phases except post spawning phase. Estradiol-17β revealed diurnal rhythm only during preparatory and pre-spawning phases. The peak in the levels of plasma testosterone and estradiol-17β were recorded during the photophase. During the late-preparatory phase, the rhythm of testosterone coincided with the rhythm of estradiol-17β.     

Circadian Variation in Circulating Levels of Melatonin,Testosterone and Estradiol-17β in a Tropical Freshwater Catfish,Clarias batrachus,during Different Phases of the Annual Reproductive Cycle

In the present investigation, daily variations in plasma levels of melatonin, testosterone and estradiol-17β were analysed in the tropical freshwater catfish, Clarias batrachus during early and late-preparatory, pre-spawning, late-spawning and post-spawning phases of its annual reproductive cycle. Plasma melatonin levels exhibited a significant circadian rhythm during all the reproductive phases but peaks were invariably at night. The amplitude of melatonin was high during the early-preparatory phase and low in the pre-spawning phase. Testosterone also displayed significant rhythm during all the studied phases except post spawning phase. Estradiol-17β revealed diurnal rhythm only during preparatory and pre-spawning phases. The peak in the levels of plasma testosterone and estradiol-17β were recorded during the photophase. During the late-preparatory phase, the rhythm of testosterone coincided with the rhythm of estradiol-17β.     

Photoperiodic regulation of annual testicular events in the Indian major carp Catla catla

The importance of photoperiods in the regulation of annual testicular events in the carp Catla catla was evaluated by subjecting them to either long (16 h light : 8 h dark) or short (8 h light : 16 h dark) photoperiods for 30 days during the preparatory, prespawning, spawning and postspawning phases of an annual gonadal cycle. In each reproductive phase, testicular responsiveness to subjected photoperiods was determined by comparing the gonadal status in corresponding groups of control or natural photoperiodic fish. The values of testicular weight, gametogenic index, as well as testicular activity of two steroidogenic enzymes (Δ⁵3β‐, and 17β‐hydroxysteroid dehydrogenase), and the serum titre of testosterone were considered as the indices of functional status of the testis in the fish concerned. During the prespawning phase, exposure of fish to a daily long photoperiod schedule resulted in precocious maturation of testis, while retardation of testicular growth was noted under the influences of short photoperiod. However, none of the employed photo‐schedules could influence the gametogenic and steroidogenic functions of the testis in the remaining part of the gonadal cycle. Collectively, the present study provides evidence for the first time that in the case of a commercially important carp, Catla catla, artificial, long photoperiods may be used for advanced testicular maturation, while reductions in maturation‐associated growth and deterioration in flesh quality may be avoided by submitting the fish to shorter day lengths during the prespawning phase of the reproductive cycle.     

Influence of altered photoperiods on serum melatonin and its receptors (MT1 and MT2) in the brain, retina, and ovary in carp Catla catla

Daily variation in melatonin receptor (MT1 and MT2) density in three specific tissues-brain, retina, and ovary-and its temporal relationship with serum melatonin were evaluated for the first time in a freshwater teleost, the carp Catla catla, under natural as well as altered photoperiods in different reproductive phases of the annual cycle. Cosinor analysis was used to determine rhythmic features of the serum melatonin and receptors (MT1 and MT2) in different tissues. In each photoperiodic group, irrespective of season, the daily minimum serum melatonin level was noted at midday. However, the daily peak value of melatonin varied in relation to both photo-schedules and reproductive phases. Under natural photoperiods (NPs; duration varied with seasons) and short photoperiods (SPs; light [L]:dark [D] 8:16), it occurred in the late dark phase during the preparatory phase, and at midnight in the remaining parts of the annual cycle. On the other hand, in each reproductive phase, compared to corresponding NP carp, the daily melatonin peak under long photoperiods (LPs; L:D 16:8) exhibited a phase delay of ～2-3?h (occurring during the late dark phase). The melatonin levels at each sampling point were highest during the postspawning phase and lowest during the spawning phase, irrespective of the photoperiodic history of the fish. In each tissue, Western blot analysis revealed a band at ～37?kDa and a band at ～36?kDa corresponding to the molecular weights of native MT1 and MT2 receptor proteins, respectively, with the band intensity of MT1 always being higher than that of a 36-kDa protein. The content of both melatonin receptor proteins varied significantly according to the studied tissue (being highest in the retina, intermediate in the brain, and lowest in the ovary), time in the daily cycle (peak at midnight and fall at midday), and reproductive phase in the annual cycle (highest in the spawning phase and lowest in the postspawning phase). Remarkably, no significant effects of altered photoperiod were detected on any rhythm parameters of either MT1 or MT2 in any of the studied tissues. Collectively, the results of the present study suggest a role of photoperiod in determining daily and seasonal profiles of serum melatonin, but not its receptor proteins, on the ovary or on any nongonad tissues in carp.     

Melatonin concentrations in relation to oxidative status and oocyte dynamics in the ovary during different reproductive phases of an annual cycle in carp Catla catla

The present study on carp Catla catla is the first attempt to search for a relationship between the concentrations of melatonin, oxidative status, and oocyte dynamics in the ovary of any fish. We measured the levels of melatonin, different antioxidative agents, and the marker of intracellular stress along with the profiles of different developmental stages of oocyte in the ovary of adult carp during four distinct phases in an annual reproductive cycle. Ovarian melatonin titers displayed significant seasonal variations with a peak during spawning and nadir during post-spawning, and thereby underlined its proximity to the course of ovarian development. A significant positive correlation was found between the ovarian levels of melatonin and the activity of superoxide dismutase (SOD), catalase (CAT), and glutathione transferase (GST), although each of them showed a negative correlation with the level of malondialdehyde (MDA)—a faithful indicator of intracellular stress. However, ovarian melatonin titers did not exhibit any correlation with the levels of reduced glutathione (GSH) and the activity of glutathione peroxidase (GPx). Collectively, our findings suggest that melatonin measured in carp ovary may be associated with an enhanced activity/level of selective antioxidative agents for reduction in oxidative stress to augment ovarian functions during the spawning.     

Influence of Altered Photoperiods on Serum Melatonin and Its Receptors (MT1 and MT2) in the Brain,Retina, and Ovary in Carp Catla catla

Daily variation in melatonin receptor (MT1 and MT2) density in three specific tissues—brain, retina, and ovary—and its temporal relationship with serum melatonin were evaluated for the first time in a freshwater teleost, the carp Catla catla, under natural as well as altered photoperiods in different reproductive phases of the annual cycle. Cosinor analysis was used to determine rhythmic features of the serum melatonin and receptors (MT1 and MT2) in different tissues. In each photoperiodic group, irrespective of season, the daily minimum serum melatonin level was noted at midday. However, the daily peak value of melatonin varied in relation to both photo-schedules and reproductive phases. Under natural photoperiods (NPs; duration varied with seasons) and short photoperiods (SPs; light [L]:dark [D] 8:16), it occurred in the late dark phase during the preparatory phase, and at midnight in the remaining parts of the annual cycle. On the other hand, in each reproductive phase, compared to corresponding NP carp, the daily melatonin peak under long photoperiods (LPs; L:D 16:8) exhibited a phase delay of ～2–3?h (occurring during the late dark phase). The melatonin levels at each sampling point were highest during the postspawning phase and lowest during the spawning phase, irrespective of the photoperiodic history of the fish. In each tissue, Western blot analysis revealed a band at ～37?kDa and a band at ～36?kDa corresponding to the molecular weights of native MT1 and MT2 receptor proteins, respectively, with the band intensity of MT1 always being higher than that of a 36-kDa protein. The content of both melatonin receptor proteins varied significantly according to the studied tissue (being highest in the retina, intermediate in the brain, and lowest in the ovary), time in the daily cycle (peak at midnight and fall at midday), and reproductive phase in the annual cycle (highest in the spawning phase and lowest in the postspawning phase). Remarkably, no significant effects of altered photoperiod were detected on any rhythm parameters of either MT1 or MT2 in any of the studied tissues. Collectively, the results of the present study suggest a role of photoperiod in determining daily and seasonal profiles of serum melatonin, but not its receptor proteins, on the ovary or on any nongonad tissues in carp. (Author correspondence: dgp_skmaitra@yahoo.co.in)     

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.     

The Pineal Gland,but Not Melatonin,Is Associated with the Termination of Seasonal Testicular Activity in an Annual Reproductive Cycle in Roseringed Parakeet Psittacula krameri

The role of the pineal gland and its hormone melatonin in the regulation of annual testicular events was investigated for the first time in a psittacine bird, the roseringed parakeet (Psittacula krameri). Accordingly, the testicular responsiveness of the birds was evaluated following surgical pinealectomy with or without the exogenous administration of melatonin and the experimental manipulations of the endogenous levels of melatonin through exposing the birds to continuous illumination. An identical schedule was followed during the four reproductive phases, each characterizing a distinct testicular status in the annual cycle, namely, the phases of gametogenic quiescence (preparatory phase), seasonal recovery of gametogenesis (progressive phase), seasonal initiation of sperm formation (pre‐breeding phase), and peak gametogenic activity (breeding phase). In each reproductive phase, the birds were subjected to various experimental conditions, and the effects were studied comparing the testicular conditions in the respective control birds. The study included germ cell profiles of the seminiferous tubules, the activities of steroidogenic enzymes 17β‐hydroxysteroid dehydrogenase (17β‐HSD), and Δ⁵3β‐hydroxysteroid dehydrogenase (Δ⁵3β‐ HSD) in the testis, and the serum levels of testosterone and melatonin. An analysis of the data reveals that the pineal gland and its hormone melatonin may play an inhibitory role in the development of the testis until the attainment of the seasonal peak in the annual reproductive cycle. However, in all probability, the termination of the seasonal activity of the testis or the initiation of testicular regression in the annual reproductive cycle appears to be the function of the pineal gland, but not of melatonin.     

Changes in melatonin binding sites under artificial light-dark, constant light and constant dark conditions in the masu salmon brain

To test whether the affinity (Kd) and total binding capacity (Bmax) of melatonin receptors exhibit daily and circadian changes in teleost fish whose melatonin secretion is not regulated by intra-pineal clocks, we examined the changes in melatonin binding sites in the brains of underyearling masu salmon Oncorhynchus masou under artificial light-dark (LD), constant light (LL) and constant dark (DD) conditions. In Experiment 1, fish were reared under a long (LD 16:8) or short (LD 8:16) photoperiod for 69 days. Blood and brains were sampled eight times at 3 h intervals. Plasma melatonin levels were high during the dark phase and low during the light phase in both photoperiodic groups. The Bmax exhibited no daily variations. Although the Kd slightly, but significantly, changed under LD 8:16, this may be of little physiological significance. In Experiment 2, fish reared under LD 12:12 for 27 days were exposed to LL or DD from the onset of the dark phase under LD 12:12. Blood and brains were sampled 13 times at 4 h intervals for two complete 24 h cycles. Plasma melatonin levels were constantly high in the DD group and low in the LL group. No significant differences were observed in the Kd and the Bmax between the two groups, and the Kd and the Bmax exhibited no circadian variation either in the LL or DD groups. These results indicate that light conditions have little effect on melatonin binding sites in the masu salmon brain.     

The pineal gland, but not melatonin, is associated with the termination of seasonal testicular activity in an annual reproductive cycle in roseringed parakeet Psittacula krameri

The role of the pineal gland and its hormone melatonin in the regulation of annual testicular events was investigated for the first time in a psittacine bird, the roseringed parakeet (Psittacula krameri). Accordingly, the testicular responsiveness of the birds was evaluated following surgical pinealectomy with or without the exogenous administration of melatonin and the experimental manipulations of the endogenous levels of melatonin through exposing the birds to continuous illumination. An identical schedule was followed during the four reproductive phases, each characterizing a distinct testicular status in the annual cycle, namely, the phases of gametogenic quiescence (preparatory phase), seasonal recovery of gametogenesis (progressive phase), seasonal initiation of sperm formation (pre-breeding phase), and peak gametogenic activity (breeding phase). In each reproductive phase, the birds were subjected to various experimental conditions, and the effects were studied comparing the testicular conditions in the respective control birds. The study included germ cell profiles of the seminiferous tubules, the activities of steroidogenic enzymes 17β-hydroxysteroid dehydrogenase (17β-HSD), and Δ⁵3β-hydroxysteroid dehydrogenase (Δ⁵3β- HSD) in the testis, and the serum levels of testosterone and melatonin. An analysis of the data reveals that the pineal gland and its hormone melatonin may play an inhibitory role in the development of the testis until the attainment of the seasonal peak in the annual reproductive cycle. However, in all probability, the termination of the seasonal activity of the testis or the initiation of testicular regression in the annual reproductive cycle appears to be the function of the pineal gland, but not of melatonin.     

Do thyroid and testis modulate the effects of pineal and melatonin on haemopoietic variables inClarias batrachus?

Involvement of pineal and its major hormone, melatonin, in the process of erythropoiesis in a freshwater catfish,Clarias batrachus has been investigated. The study was conducted during four phases, namely preparatory phase, spawning phase, postspawning phase and late postspawning phase of its annual reproductive cycle. During each phase a fish received either melatonin injections or subjected to pinealectomy. In addition, each fish in all the groups, received either iopanoic acid or cyproterone acetate or vehicle in the morning or late afternoon. Results clearly indicate that melatonin stimulates the rate of erythropoiesis inClarias batrachus. It appears that the extent of stimulation depends upon the phase of the annual reproductive cycle. However, in general, the pineal-or melatonin-induced modulation of blood variables is gonad dependent and thyroid seems to play a time of the day dependent subtle role     

Importance of photoperiods in the regulation of ovarian activities in indian major carp Catla catla in an annual cycle

The present study attempted for the first time to explore the importance of photoperiod in the regulation of seasonal ovarian functions in any subtropical major carp. Adult Indian major carp Catla catla were transferred to a long photoperiod (LP; LD 16:8) or a short photoperiod (SP; LD 8:16) for 30 days on 4 dates corresponding to the beginnings of 4 reproductive phases in an annual cycle, and responsiveness of the ovary was evaluated by comparison with the gonadal weight (I(G)), relative number of developing oocytes, serum levels of vitellogenin, and the activity of 2 important steroidogenic enzymes, that is, Delta(5)3beta-hydroxysteroid dehydrogenase and 17.beta-hydroxysteroid dehydrogenase, in the ovary of fish in a natural photoperiod. Exposure of fish to LP during the preparatory phase (February-March) resulted in a significant (p < 0.001) increase in the values of vitellogenin and in the activity of both the steroidogenic enzymes but not in the ovarian weight and in the relative number of different stages of oocytes. A more stimulatory influence of LP was noted during the prespawning phase (April-May), when precocious maturation of ovary was evident from a significant (p < 0.001) rise in the values of each studied features of ovarian functions. However, no ovarian response was found when the fish were transferred to LP during the spawning (July-August) and the postspawning (September-October) phases. On the other hand, the SP was found to have an inhibitory influence on ovarian growth and maturation during the prespawning and the spawning phases or to have no influences on ovarian functions during the preparatory and the postspawning phases of an annual cycle. The results of our study provide the first evidence that photoperiod per se plays an important role in the seasonal maturation of ovary in a subtropical freshwater major carp.     

Unusual testicular accessory organs, the testicular blind pouches of blennies (Teleostei, Blenniidae). Fine structure, (enzyme-) histochemistry and possible functions

The testicular blind pouches (TBPs) of Lipophrys canevae and Solaria pavo have an annual reproductive cycle with an active secretion phase during spawning, a regression phase correlated with necrotic processes during post-spawning, an inactive regeneration phase during inter-spawning and prespawning. As revealed by fine structure and enzyme-histochemistry the TBPs of L. canevae and S. pavo are involved in the production of steroid glucuronides, which possibly act as pheromones, Further functions are in the secretion of small amounts of sialomuciri and in lytic activity.     

Reproductive phase dependent circadian variations of plasma melatonin, testosterone, thyroxine and corticosterone in Indian palm squirrel, Funambulus pennanti

To explain the complex mechanism of environmental influence along with internal hormonal (factors) milieu on daily variations in the circulating levels of melatonin, testosterone, thyroxine and corticosterone were analyzed with the help of inferential statistics (Cosinor rhythmometry) in a seasonally breeding tropical rodent, F. pennanti during the reproductively active (RAP) and inactive phases (RIP). Plasma melatonin, thyroxine and corticosterone levels exhibited a significant circadian oscillation during both the active and inactive phases of the annual reproductive cycle. Melatonin showed higher amplitude during RIP in the circulating plasma. Testosterone presented a peak level during evening hours (16:00 - 18:00 h) during RAP only. The phase of thyroxine was noted ∼09:76 h and ∼10.35 h during active and inactive phases, respectively. Corticosterone showed a peak level at ∼12.00 h during both phases of the reproductive cycle. Further, in this tropical rodent, the minimum difference in photoperiod (∼3 - 4 hours) and maximum variation in temperature (max. 18°C - min. 10°C during RIP and max. 45°C - min. 32°C during RAP) and humidity (85% during RIP and 35% during RAP) regulated the diurnal rhythm of circulating melatonin circadian rhythm by ∼1 hour phase advance during RIP. In conclusion, the studied hormonal rhythms may be part of an integrative system to coordinate reproduction and physiological processes successfully with environmental factors.     

A preliminary report on the spawning behaviour and nature of sexual selection in the barbel, Barbus barbus (L.)

In the spring of 1974 the migration and spawning behaviour of barbel, Barbus barbus (L.) were observed. The migration and behaviour of males prior to the spawning attempts may be strategies designed to increase the quota of receptive females experienced by each male. Spawning attempts were divided into two categories: successful and failed. Successful attempts comprised six sequential sub-events: approach period, pre-spawning phase, spawning, post-spawning phase, dispersal period and inter-spawning period. The spawning and post-spawning phases did not occur in a failed attempt. The success or failure of attempts was dependent upon the number of males present. The advantage gained by females abandoning attempts when the male densities were high is discussed. In both the pre-spawning phase of failed attempts and the post-spawning phase of successful attempts, males attempted to drive away other male competitors by the use of male 'chase away' interactions.