Lunar and daily spawning rhythms of Senegal sole Solea senegalensis

Exploring Photoreceptor Gene Expression and Seasonal Physiology in Mediterranean Swordfish (Xiphias gladius)

Mature and immature female swordfish show different gonadal expression patterns of melatonin synthesis-related and opsin genes, indicating that seasonality affects gonadal circadian genes, especially through opsin modulation. The high energy demands of reproduction may cause mature female swordfish to become more susceptible to environmental stressors/changes, potentially affecting their health post-reproduction. The modulation of melatonin and opsin expression may also influence swordfish biology, impacting basal metabolism and stress responses. This study aimed to investigate the hepatic expression patterns of genes involved in melatonin and opsins activity after the reproductive period, comparing mature and immature females. Liver samples of swordfish females, collected from the Central Adriatic Sea, were analyzed to evaluate the expression of a melatonin receptor (mel1b), acetylserotonin O-methyltransferase (asmt), opsins (sws, VA opsin and opsin4), and stress response (sod1, sod2 and hsp4b) genes, while histological analysis focused on hepatic melanomacrophages, melanomacrophage centers, and lipid content. The expression of all genes analyzed did not differ between mature and immature females except for sws, which was significantly downregulated in mature females. The overall health conditions of mature and immature females appeared comparable, as suggested by the histological analysis and the absence of significant differences in the expression of stress response genes. Although the study used a limited number of samples and lacked a comparison between the reproductive and non-reproductive periods, the preliminary results suggest that opsins may play a role in modulating physiological processes beyond reproduction.

Sex-specific vertical movements of spawning atlantic cod in coastal habitats inferred from acoustic telemetry

Fish spawning location and behaviour can be challenging to detect, especially in deep water. Here we utilise two large acoustic telemetry datasets from western and mid Norway to study the vertical movement dimension of individual Atlantic cod Gadus morhua spawning behaviour in their natural habitats. The datasets comprise ~ 6 million depth detections from 644 sexed, mature cod, collected at seven different Norwegian coastal cod spawning grounds during 2017-2021. During the spawning period, females were typically found at shallower depths compared to males, while this pattern was much less pronounced or absent outside this period. Furthermore, we identify a hitherto undescribed sex-dependent vertical movement behaviour: periodic descents, where females made deep dives, putatively associated with egg release and spawning, approximately every third day. Females not showing this behaviour tended to occupy deeper positions in the water column than the "periodic descent" females and hence may approach and retreat from spawning males more horizontally. These observed sex-specific behaviours correspond well to the lek-like description of the cod mating system and with cod reproductive physiology, where females are known to mature and release batches of eggs at intervals of several days as part of a bet-hedging strategy.

Lunar Age-Dependent Oscillations in Expression of the Genes for Kisspeptin, GnIH, and Their Receptors in the Grass Puffer during the Spawning Season

Grass puffer is a semilunar-synchronized spawner: spawning occurs on beaches only for several days of spring tide around new moon (lunar age 0) and full moon (lunar age 15) every 2 weeks from spring to early summer. To investigate the role of kisspeptin and gonadotropin-inhibitory hormone (GnIH) in the semilunar-synchronized spawning, lunar age-dependent expression of the genes encoding kisspeptin (kiss2), kisspeptin receptor (kissr2), GnIH (gnih), GnIH receptor (gnihr), gonadotropin-releasing hormone 1 (GnRH1) (gnrh1), and three gonadotropin (GTH) subunits (gpa, fshb, lhb) was examined in the male grass puffer, which was kept in an aquarium under natural light condition in a lunar month during the spawning period. In the brain, both kiss2 and kissr2 showed lunar variations with a peak at lunar age 10, while both gnih and gnihr showed semilunar variations with two peaks at lunar age 0 and 20. On the other hand, gnrh1 showed semilunar variation with two peaks at lunar age 0 and 15. In the pituitary, kiss2, kissr2, gnih, and gnihr showed similar variations to those shown in the brain. The fshb and lhb mRNA levels showed semilunar variations with two peaks at lunar age 0 and 15. The present study shows lunar and semilunar oscillations of kiss2/kissr2 and gnih/gnihr expressions, respectively, with their peaks around spring tide in the brain and pituitary along with the semilunar expressions of gnrh1 and the pituitary GTH subunit genes. These results suggest that the lunar age-dependent expressions of the kisspeptin, GnIH, and their receptor genes may be primarily important in the control of the precisely timed semilunar spawning of the grass puffer.

Does exposure to moonlight affect day/night changes in melatonin and metabolic parameters in Amazonian fish?

Lunar cycle modulates the rhythmic activity patterns of many animals, including fish. The effect of the moonlight cycle on daily melatonin and metabolic parameters was evaluated in matrinxã (Brycon amazonicus) subjected to external natural lighting. Eighty juvenile were distributed in 4 tanks of 1m3 (20 fish/tank) and divided into two groups. One group was exposed to the full moon and the other group to the new moon for 30 days, which corresponds to the duration of the lunar period. At the end of the lunar phase, 6 fish from each group were anesthetized to collect blood, tissue and eye samples at midday and midnight. The comparison between the light and dark periods revealed a significant increase in plasma and ocular melatonin in the last period. However, there was no significant difference for plasma melatonin between moons. Ocular melatonin presented higher concentrations during the new moon. Glucose, total proteins, cortisol, liver glutathione and gill lipid peroxidation were higher in the full moon compared to in the new moon. Plasma triglyceride was higher during the night for the full moon, and the opposite was found for the new moon. Total cholesterol values were higher at night regardless the moon phase. Glutathione in the gills and lipid peroxidation in the liver showed no significant differences. These results highlight the importance of considering both the day and lunar cycles for melatonin and metabolic parameters in species of commercial interest and susceptible to stressful situations in rearing conditions.

Involvement of melatonin in transducing moon-related signals into the reproductive network of the female honeycomb grouper Epinephelus merra

Most groupers (genus Epinephelus) inhabiting tropical and subtropical waters exhibit lunar-related reproductive cycles. Their gametes develop synchronously toward and are released around the species-selected moon phase. Periodical changes in cues from the moon are likely used as zeitgeber, and the hypothalamic-pituitary-gonadal (HPG) axis may be activated after cues are perceived by the sensory organ and transduced as internal signals. The objective of this study was to examine weekly changes in mRNA expression profiles of gonadotropin-releasing hormones (gnrh1 and gnrh2) and the β-subunit of gonadotropins (fshβ and lhβ) during the spawning season (May to June) of the female honeycomb grouper Epinephelus merra, which spawns around the full moon period. When mature females were collected based on the lunar cycle, the gonadosomatic index peaked around the full moon. Ovarian histology revealed that oocytes laden with yolk developed toward the full moon and, subsequently, ovulatory follicles appeared around the last quarter moon, confirming lunar-related spawning with a full moon preference. Real-time quantitative polymerase chain reaction analyses revealed high abundances of fshβ and lhβ toward the first quarter moon, whereas concentrations of gnrh1 and gnrh2 increased around the last quarter moon and the first quarter moon, respectively, suggesting that transcription levels of these hormones fluctuate with the lunar cycle. The measurement of melatonin in the eye around the new moon and the full moon revealed that the ocular melatonin content was higher around the new moon than around the full moon, suggesting that the honeycomb grouper can perceive changes in moonlight. In addition, implantation of an osmotic pump containing melatonin into the body cavity of E. merra reduced the transcription levels of gonadotropins, suggesting that melatonin negatively affects hormonal synthesis at the HPG axis. We concluded that melatonin plays an essential role in transducing periodical changes in moonlight and that decreases in melatonin levels from the new moon to the full moon activate the HPG axis for entrainment of gonadal development and spawning.

Meagre's melatonin profiles under captivity: circadian rhythmicity and light sensitiveness

The present study reveals the first characterization of the plasma melatonin rhythms of the meagre (Argyrosomus regius) under aquaculture conditions. Melatonin levels were monitored during a 24 h cycle under a photoperiod of 16 L:8D and under constant darkness (DD), respectively to characterize the daily rhythm of this indoleamine and to test its endogenous origin. Besides, to identify which light intensities are perceived as night or day by this species, the degree of inhibition of nocturnal melatonin production caused by increasing intensities of light was tested (3.3, 5.3, 10.5, and 120 μW/cm²), applying 1 h light pulses at Mid-Dark. The result for melatonin daily rhythm in plasma showed a typical profile: concentration remained low during all daytime points, increasing greatly during dark points, with maximum values at 16:00 and 22:00 h, zeitgeber time. Under DD conditions, the plasma melatonin profile persisted, with a similar acrophase but with a lower amplitude between subjective day and night periods, indicating this rhythm as being endogenously driven. Moreover, meagre seemed to be very sensitive to dim levels of illumination during the night, since an intensity of just 3.3 μW/cm² inhibited melatonin production. However, only the pulse of 5.3 μW/cm² caused a melatonin drop till daytime concentrations. Thus, the threshold of light detection by the pineal organ was suggested as being located between 3.3 and 5.3 μW/cm². Such results are an added value for this species biology knowledge, and in consequence to its adaptation to aquaculture conditions, allowing the improvement of culture husbandry protocols.

Circadian expression of DNA methylation and demethylation genes in zebrafish gonads

This research aimed at investigating the light synchronization and endogenous origin of daily expression rhythms of eight key genes involved in epigenetic mechanisms (DNA methylation and demethylation) in zebrafish gonads. To this end, 84 zebrafish were distributed into six tanks, each one containing 14 fish (7 males and 7 females). Animals were subjected to 12 h light:12 h dark cycles (LD, lights on at ZT0 h) and fed randomly three times a day during the light phase. Locomotor activity rhythms were recorded in each tank for 20 days to test their synchronization to light. Then, zebrafish were fasted for one day and gonad samples were collected every 4 h during a 24 h cycle (ZT2, 6, 10, 14, 18, and 22 h). The results revealed that most of the epigenetic genes investigated exhibited a significant daily rhythm. DNA methylation genes (dnmt4, dnmt5, dnmt7) exhibited a daily rhythm of expression with a nocturnal acrophase (ZT14:01~ZT22:17 h), except for dnmt7 in males (ZT2:25 h). Similarly, all DNA demethylation genes (tet2, tdg, mb4, gadd45aa, and apobec2b) revealed the existence of statistically significant daily rhythms, except for gadd45aa in females. In females, tdg, mb4, and apobec2b presented a nocturnal peak (ZT14:20 ~ ZT22:04 h), whereas the tet2 acrophase was diurnal (ZT4:02 h). In males, tet2, tdg, and gadd45aa had nocturnal acrophases (ZT18:26~ZT21:31 h), whereas mb4 and apobec2b displayed diurnal acrophases (ZT5:28 and ZT4:02 h, respectively). To determine the endogenous nature of gene expression rhythms, another experiment was performed: 12 groups of 14 fish (7 males and 7 females) were kept in complete darkness (DD) and sampled every 4 h during a 48 h cycle (CT2, 6, 10, 14, 18, 22, 26, 30, 34, 38, 42, and 46 h). Under DD, most of the genes (7 out of 8) presented circadian rhythmicity with different endogenous periodicities (tau), suggesting that the epigenetic mechanisms of DNA methylation and demethylation in the gonads follow an internal control, functioning as part of the translation network linking the environment into somatic signals in fish reproduction.

Daily Rhythms of the Expression of Key Genes Involved in Steroidogenesis and Gonadal Function in Zebrafish

Fish present daily and seasonal rhythms in spawning and plasmatic levels of steroids that control reproduction. However, the existence of the rhythms of expression of the genes that underlie the endocrine mechanisms responsible for processes such as steroidogenesis and reproduction in fish have still been poorly explored to date. Here we investigated the daily pattern of the expression of key genes involved in sex steroid production that ultimately set the sex ratio in fish. Adult zebrafish were maintained under a 12:12 h light-dark cycle at a constant temperature of 27°C and were sampled every 4 h during a 24-hour cycle. The expression of key genes in the gonads and brains of female and male individuals were analyzed. In gonads, the expression of aromatase (cyp19a1a, ovarian aromatase) and the antimüllerian hormone (amh, testis) was rhythmic, with almost opposite acrophases: ZT 5:13 h (in the light phase) and ZT 15:39 h (at night), respectively. The expression of foxl2 (forkhead box L2) was also rhythmic in the ovary (acrophase located at ZT 5:02 h) and the expression of dmrt1 (doublesex and mab-3-related transcription factor 1) was rhythmic in testes (acrophase at ZT 18:36 h). In the brain, cyp19a1b (brain aromatase) and cyp11b (11beta-hydroxylase) presented daily differences, especially in males, where the expression peaked at night. These results provide the first evidence for marked time-of-the-day-dependent differences in the expression of the genes involved in sex ratio control, which should be considered when investigating processes such as reproduction, sex differentiation and steroidogenesis in fish.

Distinct expression profiles of three melatonin receptors during early development and metamorphosis in the flatfish Solea senegalensis

Melatonin actions are mediated through G protein-coupled transmembrane receptors. Recently, mt1, mt2, and mel1c melatonin receptors were cloned in the Senegalese sole. Here, their day-night and developmental expressions were analyzed by quantitative PCR. These results revealed distinct expression patterns of each receptor through development. mel1c transcripts were more abundant in unfertilized ovulated oocytes and declined during embryonic development. mt1 and mt2 expression was higher at the earliest stages (2–6 days post-fertilization), decreasing before (mt2) or during (mt1) metamorphosis. Only mt1 and mel1c expression exhibited day-night variations, with higher nocturnal mRNA levels. These results suggest different roles and transcriptional regulation of these melatonin receptors during flatfish development and metamorphosis.

Endocrine (plasma cortisol and glucose) and behavioral (locomotor and self-feeding activity) circadian rhythms in Senegalese sole (Solea senegalensis Kaup 1858) exposed to light/dark cycles or constant light

The existence of daily rhythms under light/dark (LD) cycles in plasma cortisol, blood glucose and locomotor and self-feeding activities, as well as their persistence (circadian nature) under constant light (LL), was investigated in Senegalese sole (Solea senegalensis). For the cortisol and glucose rhythms study, 48 soles were equally distributed in 8 tanks and exposed to a 12:12 LD cycle and natural water temperature (experiment 1). After an acclimation period, blood was sampled every 3 h until a 24-h cycle was completed. Blood glucose levels were measured immediately after sampling, while plasma cortisol was measured later by ELISA. In experiment 2, the fish were exposed to LL for 11 days, and after this period, the same sampling procedure was repeated. For the study of locomotor and self-feeding rhythms (experiment 3), two groups of sole were used: one exposed to LD and the other to LL. Each group was distributed within 3 tanks equipped with infrared photocells for the record of locomotor activity, and self-feeders for feeding behavior characterization. The results revealed a marked oscillation in cortisol concentrations during the daily cycle under LD, with a peak (35.65 ± 3.14 ng/ml) in the afternoon (15:00 h) and very low levels during the night (5.30 ± 1.09 ng/ml). This cortisol rhythm persisted under LL conditions, with lower values (mean cortisol concentration = 7.12 ± 1.11 ng/ml) and with the peak shifted by 3 h. Both rhythms were confirmed by COSINOR analysis (p < 0.05). The synchronizing role of temperature and feeding schedule, in addition to light, is also discussed. Diel rhythms of glucose were not evident in LD or LL. As to locomotor and self-feeding activity, a very marked rhythm was observed under LD, with higher activity observed during the night, with acrophases located at 2:14 and 3:37 h, respectively. The statistical significance of daily rhythms was confirmed by COSINOR analysis. Under LL, both feeding and locomotor rhythms persisted, with an endogenous period (τ) around 22.5 h. In short, our findings described for the first time the existence of circadian cortisol and behavioral circadian rhythms in flat fish. Such results revealed the importance of taking into account the time of day when assessing stress responses and evaluating physiological indicators of stress in fish.

The circadian clock machinery during early development of Senegalese sole (Solea senegalensis): effects of constant light and dark conditions

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

Sex-specific changes in the expression of kisspeptin, kisspeptin receptor, gonadotropins and gonadotropin receptors in the Senegalese sole (Solea senegalensis) during a full reproductive cycle

Kisspeptin is thought to have a major role in the control of the onset of puberty in vertebrates. However, our current understanding of its function in fish and how it integrates with other hormones is incomplete due to the high diversity of this group of animals and a still limited amount of available data. This study examined the temporal and spatial changes in expression of kisspeptin, gonadotropins and their respective receptors in the Senegalese sole during a full reproductive cycle. Kiss2 and kiss2r expression was determined by qRT-PCR in the forebrain and midbrain while expression of fshβ and lhβ was determined in the pituitary and fshr and lhr in the gonads. Plasma levels of testosterone (T), 11-ketotestosterone (11-KT) and estradiol-17β were measured by ELISA and gonadal maturation was assessed histologically. In males, kiss2 and kiss2r expression in the brain areas examined was highest towards the end of winter, just before the spawning season, which took place the following spring. This coincided with maximum levels of pituitary fshβ and lhβ, plasma T and 11-KT and the highest number of maturing fish. However, these associations were not evident in females, since the highest expression of kiss2, kiss2r and gonadotropins were observed in the fall, winter or spring, depending upon the variable and tissue considered. Taken together, these data show not only temporal and spatial, but also sex-specific differences in the expression of kisspeptin and its receptor. Thus, while expression of kiss2 in Senegalese sole males agrees with what one would expect according to its proposed role as a major regulator of the onset of reproduction, in females the situation was not so clear, since kiss2 and kiss2r expression was highest either before or during the spawning season.

Cloning, tissue expression pattern and daily rhythms of Period1, Period2, and Clock transcripts in the flatfish Senegalese sole, Solea senegalensis

An extensive network of endogenous oscillators governs vertebrate circadian rhythmicity. At the molecular level, they are composed of a set of clock genes that participate in transcriptional-translational feedback loops to control their own expression and that of downstream output genes. These clocks are synchronized with the environment, although entrainment by external periodic cues remains little explored in fish. In this work, partial cDNA sequences of clock genes representing both positive (Clock) and negative (Period1, Period2) elements of the molecular feedback loops were obtained from the nocturnal flatfish Senegalese sole, a relevant species for aquaculture and chronobiology. All of the above genes exhibited high identities with their respective teleost clock genes, and Per-Arnt-Sim or basic helix-loop-helix binding domains were recognized in their primary structure. They showed a widespread distribution through the animal body and some of them displayed daily mRNA rhythms in central (retina, optic tectum, diencephalon, and cerebellum) and peripheral (liver) tissues. These rhythms were most robust in retina and liver, exhibiting marked Period1 and Clock daily oscillations in transcript levels as revealed by ANOVA and cosinor analysis. Interestingly, expression profiles were inverted in retina and optic tectum compared to liver. Such differences suggest the existence of tissue-dependent zeitgebers for clock gene expression in this species (i.e., light for retina and optic tectum and feeding time for liver). This study provides novel insight into the location of the molecular clocks (central vs. peripheral) and their different phasing and synchronization pathways, which contributes to better understand the teleost circadian systems and its plasticity.

The clock gene Period3 in the nocturnal flatfish Solea senegalensis: Molecular cloning, tissue expression and daily rhythms in central areas

Clock genes are responsible for generating and sustaining most rhythmic daily functions in vertebrates. Their expression is endogenously driven, although they are entrained by external cues such as light, temperature and nutrient availability. In the present study, a full-length coding region of Solea senegalensis clock gene Period3 (Per3) has been isolated from sole brain as a first step in understanding the molecular basis underlying circadian rhythms in this nocturnal species. The complete cDNA is 4141 base pairs (bp) in length, including an ORF of 3804bp, a 5'UTR of 247bp and a 3'UTR of 90bp. It encodes a putative PERIOD3 protein (PER3) of 1267 amino acids which shares the main functional domains conserved between transcription factors regulating the circadian clock pathway. Sole PER3 displays high identity with PER3 proteins from teleost species (61-77%) and lower identity (39-46%) with other vertebrate PER3 sequences. This gene is expressed in all examined tissues, being mRNA expression particularly evident in retina, cerebellum, diencephalon, optic tectum, liver and ovary. Per3 exhibits a significant daily oscillation in retina and optic tectum but not in diencephalon and cerebellum. Our results suggest an important role of Per3 in the circadian clockwork machinery of visually-related areas of sole.

Melatonin receptors in a pleuronectiform species, Solea senegalensis: Cloning, tissue expression, day-night and seasonal variations

Melatonin receptors are expressed in neural and peripheral tissues and mediate melatonin actions on the synchronization of circadian and circannual rhythms. In this study we have cloned three melatonin receptor subtypes (MT1, MT2 and Mel1c) in the Senegalese sole and analyzed their central and peripheral tissue distribution. The full-length MT1 (1452 nt), MT2 (1728 nt) and Mel1c (1980 nt) cDNAs encode different proteins of 345, 373, 355 amino acids, respectively. They were mainly expressed in retina, brain and pituitary, but MT1 was also expressed in gill, liver, intestine, kidney, spleen, heart and skin. At peripheral level, MT2 expression was only evident in gill, kidney and skin whereas Mel1c expression was restricted to the muscle and skin. This pattern of expression was not markedly different between sexes or among the times of day analyzed. The real-time quantitative PCR analyses showed that MT1 displayed higher expression at night than during the day in the retina and optic tectum. Seasonal MT1 expression was characterized by higher mRNA levels in spring and autumn equinoxes for the retina, and in winter and summer solstices for the optic tectum. An almost similar expression profile was found for MT2, but differences were less conspicuous. No day-night differences in MT1 and MT2 expression were observed in the pituitary but a seasonal variation was detected, being mRNA levels higher in summer for both receptors. Mel1c expression did not exhibit significant day-night variation in retina and optic tectum but showed seasonal variations, with higher transcript levels in summer (optic tectum) and autumn (retina). Our results suggest that day-night and seasonal variations in melatonin receptor expression could also be mediating circadian and circannual rhythms in sole.