Spawning Rhythm Research Articles

A progestin regulates the prostaglandin pathway in the neuroendocrine system in female mudskipper Boleophthalmus pectinirostris

Sex hormones regulate the reproductive cycle through brain-pituitary axis, but the molecular mechanism is still enigmatic. In the reproductive season, the mudskipper Boleophthalmus pectinirostris possesses a semilunar periodicity spawning rhythm, which coincides with the semilunar periodicity variations in 17α-hydroxyprogesterone, the precursor of 17α,20β-dihydroxy-4-pregnen-3-one (DHP), a sexual progestin in teleosts. In the present study, we investigated in vitro the brain transcriptional differences between DHP-treated tissues and control groups using RNA-seq. Differential expression analysis revealed that 2700 genes significantly differentially expressed, including 1532 up-regulated and 1168 down-regulated genes. The majority of prostaglandin pathway-related genes were dramatically up-regulated, especially the prostaglandin receptor 6 (ptger6). Tissue distribution analysis revealed that ptger6 gene was ubiquitously expressed. In situ hybridization results showed that ptger6, nuclear progestin receptor (pgr), and DHP-induced c-fos mRNA were co-expressed in the ventral telencephalic area, the ventral nucleus of ventral telencephalic area, the anterior part of parvocellular preoptic nucleus, the magnocellular part of magnocellular preoptic nucleus, the ventral zone of periventricular hypothalamus, the anterior tubercular nucleus, the periventricular nucleus of posterior tuberculum, and the torus longitudinalis. DHP significantly enhanced promoter activities of ptger6 via Pgr. Together, this study suggested that DHP regulates the prostaglandin pathway in the neuroendocrine system of teleost fish.

Spawning time of black sea bream Acanthopagrus schlegelii, related to underwater photoperiodism in oyster farms.

The embryonic development times, spawning timing and hatching rates of the black sea bream Acanthopagrus schlegelii were examined to investigate the potential effect of seawater temperature, tides and photoperiod on the reproductive rhythm of this species in Hiroshima Bay, Japan. Low temperatures decreased hatching rates and extended the hatching time, and the minimum temperature threshold for hatching was 15°C. Back-calculated spawning times indicated that the peak of spawning occurred just before sunset and the reduction in diurnal light intensity around the oyster rafts acted as a trigger for spawning. In contrast, no correlation was found between spawning rhythms and tidal cycle. The results highlight the important role of oyster farms in the reproductive cycle and population dynamics of A. schlegelii in Hiroshima Bay, the main spawning ground for this species in Japan. The study findings provide insights for the sustainable management of this important sparid species.

Characterization of spawning rhythms of a sparidae aquaculture species, the white seabream ( Diplodus sargus ), in the south of Portugal

One of the most visible features of fish reproduction is seasonality; however, daily rhythmicity is present in most species and should not be disregarded. This study investigated the seasonal and daily spawning rhythms of the white seabream (Diplodus sargus) in captivity, and a possible endogenous control of the daily rhythm. Spawns were monitored daily, during an entire reproductive season by using an automatic and programmable egg collector. The spawning season lasted 5 months, from January until June, when temperatures oscillated between 11.5 and 24.4°C. Female fecundity was higher between 17.5 and 20.4°C, suggesting a temperature influence in spawning. This species showed a very clear daily spawning rhythm (Cosinor analysis, p < 0.001), strongly related with photoperiod. Eggs started to be released at sunset and peaked around 21:00 h. Egg quality parameters also changed along the spawning period, with viability and hatching rates, increasing towards the end of the night period. The spawning rhythm persisted (Cosinor analysis, p < 0.001) for 5 days under constant light conditions, suggesting the presence of an endogenous clock mechanism which controlled this circadian pattern. The present results represent an advantage for the optimization of egg production protocols in this species, and an added value to the current knowledge of marine fish physiology.

Expressions of melanopsins in telencephalon imply their function in synchronizing semilunar spawning rhythm in the mudskipper Boleophthalmus pectinirostris

The mudskipper Boleophthalmus pectinirostris inhabits intertidal mudflats, exhibiting semilunar reproductive rhythms. To investigate whether melanopsin is possibly involved in the synchronization of the semilunar spawning rhythm in the female mudskipper, we first cloned all four melanopsin subtypes (opn4m1, opn4m3, opn4x1, opn4x2) in B. pectinirostris. Results from RTq-PCR showed that significantly higher transcription levels of all four melanopsin subtypes were observed in the eyes rather than other tissues. In brain, all four melanopsin subtypes were also detectable in different regions, including the telencephalon, in which the expression of melanopsin has not been reported in other teleosts. The transcription levels of opn4m3 and opn4x1 in the telencephalon exhibited a daily fluctuation pattern. When females entered the spawning season, opn4m1 and opn4x1 transcript levels increased significantly in the telencephalon. During the spawning season, the transcript levels of opn4m3 and opn4x1 in the telencephalon appeared to have a cyclic pattern associated with semilunar periodicity, exhibiting two cycles with a peak around the first or the last lunar quarters. Results from ISH showed that, opn4x1 mRNA was localized in the medial of dorsal telencephalic area, dorsal nucleus of ventral telencephalic area (Vd), ventral nucleus of ventral telencephalic area (Vv), anterior part of parvocellular preoptic nucleus, magnocellular part of the magnocellular preoptic nucleus (PMmc), habenular and ventral zone of hypothalamus. Intriguingly, gnrh3 mRNA was also located in Vd, Vv and PMmc. Taken together, our results suggested that melanopsins, e.g. opn4x1, expressed in the telencephalon might mediate semilunar spawning activity in the female mudskipper.

Coastal road mortality of land crab during spawning migration

Land crabs are threatened by ocean sprawl even though they act as keystone consumers in coastal forest. Female land crabs must migrate to the sea annually to release larvae. However, they face the risk of road mortality which reduces ecological connectivity. We investigated the spawning migration rhythm and the roadkill of land crab. Migrating crabs and roadkilled crabs were recorded on coastal roads in South Korea from July 28 to August 27 in 2018. Female land crabs mainly released zoeae during spring tide. The number of roadkilled crabs also synchronized with migration peak. A majority (95%) of 739 roadkilled carcasses were female crabs. As a result, the female crabs accounted only 29.6% of the population which can lead to a population decline. The roadkill density was the highest in a residential area without cement guardrails. These results suggest the mitigation actions for land crab roadkill. Among them, prohibiting vehicular traffic between sunset and midnight during spring tides in the breeding season should increase the viability of the population.

Light conditions for commercial hatching success in Nile tilapia (Oreochromis niloticus)

Light plays an important role in the development of biological rhythms in fish. Previous studies on tilapia has revealed that spawning rhythms and juvenile growth are significantly influenced by photoperiod. However, the effect of light conditions on the hatching rhythms are unexamined thus far. Therefore, the aim of this study was to investigate the impact of the light conditions on the hatching in Nile tilapia. To this end, the central composite experimental design and response surface methodology were utilized to study the combined effects of photoperiod and light intensity on the hatching rate as well as the growth of early fry. Based upon the results of trial tests, regimes of photoperiod and light intensity ranged from 2 to 24 h and from 100 to 2000 lx (1.26 to 25.20 μmol m−2 s−1), respectively. The results demonstrated that within the factorial space developmental rhythms at the egg stage were significantly influenced by light conditions, signaling that the timing of hatching in Nile tilapia could be regulated by ambient light conditions. The more unsuitable the light conditions, the longer the eggs took to hatching (or the stronger inhibitory effect of light conditions on hatching), underlining the importance and necessity of affording optimized photoenvironment during commercial incubation as well as early fry development of tilapia. The effect of light intensity was greater than that of photoperiod during early ontogeny. The interaction between photoperiod and light intensity was found to be present, implying that the effect of only photoperiod upon reproduction and growth of Nile tilapia in all previous studies might be modified. Relationship of hatching to light conditions also could be reliably modelled. The optimal photoperiod/intensity combinations, 13.45 h/1020 lx (12.85 μmol m−2 s−1), were derived via model optimization, the optimality of which had been verified under commercial hatching conditions. These findings emphasize that the light conditions are essential and should be taken into account in optimizing the protocols of the commercial incubation in hatcheries of Nile tilapia.

Daily rhythms in the reproductive axis of Nile tilapia (Oreochromis niloticus): Plasma steroids and gene expression in brain, pituitary, gonad and egg

Control of reproduction is one of the key issues to extend tilapia aquaculture. In most fish species, reproduction is a rhythmic process which ensures that reproduction timing matches the most favorable environmental conditions to maximize the survival of offspring. Although the existence of daily spawning rhythms has been reported for a number of fish species, the daily rhythms in neuroendocrine factors along the BPG (brain-pituitary-gonadal) axis remains mostly unknown. The objective of this paper was to investigate the existence of daily rhythms in BPG gene expression and plasma sex steroids in tilapia (Oreochromis niloticus). To this end, 36 males and 36 females were kept in separate tanks at 29 ± 0.5 °C and in a 12 h:12 h light/dark cycle. After 4 weeks of acclimation, male and female tilapia were mixed into six groups at a female:male ratio of 6:6. Two weeks later, fish were anesthetized and sampled every 4 h during a 24-hour cycle, and blood, brain, pituitary, gonad and eggs were collected for later analyses. The expression levels of the genes from the brain (gnrh1, gnrh2, gnrh3, kiss2, gnih), pituitary (fshβ and lhβ), gonad (star, cyp11a, cyp17, cyp19a1a and amh) and eggs (zp3b and fyna) were analyzed by qPCR. ELISA was used to analyze testosterone and estradiol in male and female plasma, respectively, and maturation-inducing hormone (MIH) in plasma in both sexes. The results revealed timely-coordinated daily rhythms along the BPG axis of tilapia. The expression of the brain genes showed that their acrophase was distributed throughout the LD cycle. However, a coordinated and harmonious pattern of daily rhythms appeared on the following levels of the BPG axis. The peaks in expression of the pituitary genes were reached at night, while the gonadal genes and plasma sex steroids peaked in the light phase. These findings can be useful for improving breeding protocols for tilapia aquaculture and for defining optimum timing for reproduction in captivity.

Seasonal changes in broodstock spawning performance and egg quality in ballan wrasse (Labrus bergylta)

Sea lice continue to be one of the largest issues for the salmon farming industry and the use of ballan wrasse (Labrus bergylta) as a biological control is considered to be one of the most sustainable solutions in development. Broodstock management has proved challenging in the initial phases due to the significant lack of understanding of basic reproductive physiology and behaviour in the species. The aim of the study was to monitor captive breeding populations throughout a spawning season to examine timing and duration of spawning, quantify egg production, and look at seasonal changes in egg quality parameters as well as investigate the parental contribution to spawning events. A clear spawning rhythm was shown with 3–5 spawning periods inclusive of spawning windows lasting 1–9days followed by interspawning intervals of 8–12days. Fertilization rate remained consistently high (>87.5%) over the spawning season and did not differ significantly between spawning populations. Hatch rate was variable (0–97.5%), but peaked in the middle of the spawning season. Mean oocyte diameter and gum layer thickness decreased slightly over the spawning season with no significant differences between spawning populations. Fatty acid (FA) profile of eggs remained consistent throughout the season and with the exception of high levels of ARA (3.8±0.5% of total FA) the FA profile was similar to that observed in other marine fish species. Parental contribution analysis showed 3 out of 6 spawning events to be single paired mating while the remaining 3 had contributions from multiple parents. Furthermore, the proposed multiple batch spawning nature of this species was confirmed with proof of a single female contributing to two separate spawning events. Overall this work represents the first comprehensive dataset of spawning activity of captive ballan wrasse, and as such and will be helpful in formulating sustainable broodstock management plans for the species. Statement of relevanceFirst dataset of spawning activity in captive ballan wrasse.

Diurnal and Circadian Oscillations in Expression of Kisspeptin, Kisspeptin Receptor and Gonadotrophin‐Releasing Hormone 2 Genes in the Grass Puffer, A Semilunar‐Synchronised Spawner

In seasonally breeding animals, the circadian and photoperiodic regulation of neuroendocrine system is important for precisely-timed reproduction. Kisspeptin, encoded by the Kiss1 gene, acts as a principal positive regulator of the reproductive axis by stimulating gonadotrophin-releasing hormone (GnRH) neurone activity in vertebrates. However, the precise mechanisms underlying the cyclic regulation of the kisspeptin neuroendocrine system remain largely unknown. The grass puffer, Takifugu niphobles, exhibits a unique spawning rhythm: spawning occurs 1.5-2 h before high tide on the day of spring tide every 2 weeks, and the spawning rhythm is connected to circadian and lunar-/tide-related clock mechanisms. The grass puffer has only one kisspeptin gene (kiss2), which is expressed in a single neural population in the preoptic area (POA), and has one kisspeptin receptor gene (kiss2r), which is expressed in the POA and the nucleus dorsomedialis thalami. Both kiss2 and kiss2r show diurnal variations in expression levels, with a peak at Zeitgeber time (ZT) 6 (middle of day time) under the light/dark conditions. They also show circadian expression with a peak at circadian time 15 (beginning of subjective night-time) under constant darkness. The synchronous and diurnal oscillations of kiss2 and kiss2r expression suggest that the action of Kiss2 in the diencephalon is highly dependent on time. Moreover, midbrain GnRH2 gene (gnrh2) but not GnRH1 or GnRH3 genes show a unique semidiurnal oscillation with two peaks at ZT6 and ZT18 within a day. The cyclic expression of kiss2, kiss2r and gnrh2 may be important in the control of the precisely-timed diurnal and semilunar spawning rhythm of the grass puffer, possibly through the circadian clock and melatonin, which may transmit the photoperiodic information of daylight and moonlight to the reproductive neuroendocrine centre in the hypothalamus.

Cloning and expression of melatonin receptors in the mudskipper Boleophthalmus pectinirostris: Their role in synchronizing its semilunar spawning rhythm

The mudskipper Boleophthalmus pectinirostris, a burrow-dwelling fish inhabiting intertidal mudflats, spawns only once during the spawning season around either the first or last lunar quarters. To understand the molecular mechanisms regulating this semilunar spawning rhythm, we cloned all melatonin receptor subtypes (mtnr1a1.4, mtnr1a1.7, mtnr1b, and mtnr1c). Expression of three melatonin receptor subtypes (except mtnr1c) was found in the ovaries. In contrast, the expression of all receptor subtypes was found in the diencephalon and the pituitary. In the fully-grown follicles, only mtnr1a1.7 mRNA was detected in both the isolated follicle layers and denuded oocytes. Interestingly, the transcript levels of both mtnr1a1.4 in the diencephalon and mtnr1a1.7 in the ovary displayed two cycles within one lunar month, and peaked around the first and last lunar quarters. We used 17α,20β-dihydroxy-4-pregnen-3-one (DHP), a maturation-inducing hormone, as a biomarker to examine the involvement of melatonin receptors in the control of the spawning cycle. Melatonin significantly increased the plasma DHP level 1h post intraperitoneal injection. Melatonin also directly stimulated ovarian fragments in vitro to produce a significantly higher amount of DHP. Taken together, these results provided the first evidence that melatonin receptors were involved in the synchronization of the semilunar spawning rhythm in the female mudskipper by acting through the HPG axis and/or directly on ovarian tissues to stimulate the production of DHP.

Daily spawning and locomotor activity rhythms of European sea bass broodstock (Dicentrarchus labrax)

Fish have an internal biological clock that drives their daily rhythms, including those of feeding and reproduction. The European sea bass has been reported to exhibit a phase inversion of its diel feeding pattern: nocturnal in winter (coinciding with the spawning period) and diurnal during the remainder of the year. In this study, we investigated the locomotor activity of sea bass during the spawning and resting periods as well as the possible existence of a daily spawning rhythm by calculating the number of eggs (and their viability) released every hour. The results showed that sea bass locomotor activity was clearly nocturnal (68±4.3% of the daily activity displayed at night) during the spawning season and mostly diurnal (65.5±7.5% of the activity displayed during the day) out of the reproductive period. Spawning occurred during the dark phase with two peaks, 6 and 11h after lights off. Egg viability was highest in the batch released near the spawning peaks. These findings could be useful for optimizing egg production and to improve hatchery practices, which should take into account the reproduction rhythms of sea bass.

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 (LDW), blue light (LDB; λpeak = 463 nm), or red light (LDR; λpeak = 685 nm). Eggs exposed to LDB had the highest hatching rate (94.5% ± 1.9%), whereas LDR 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 LDB (zeitgeber time 8 [ZT8]; ZT0 representing the onset of darkness) in relation to LDW and LDR (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 LDB showed the best growth and quickest development (advanced eye pigmentation, mouth opening, and pectoral fins), whereas larvae under LDR 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. (Author correspondence: borja@um.es)

Molecular cloning of two gonadotropin receptors and their distinct mRNA expression profiles in daily oogenesis of the wrasse Pseudolabrus sieboldi

In fish, asynchronous development of ovarian follicles, the simultaneous advance of vitellogenesis and oocyte maturation in one ovary, is a rational reproductive strategy to spawn consecutively in one spawning season. In this study, to clarify the mode of action of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in asynchronous ovarian follicle development in daily egg production, we cloned cDNAs of the follicle-stimulating hormone receptor (FSHR) and luteinizing hormone receptor (LHR) in the bambooleaf wrasse ( Pseudolabrus sieboldi), which exhibits clear diurnal spawning rhythms over 1 month. In addition, different developmental stages of ovarian follicles were isolated from whole ovaries at various daily time points on 1 day in the spawning season, and mRNA expression levels of FSHR and LHR were analyzed. Sequence analysis showed distinct differences in the number of putative leucine-rich repeats at the extracellular domain between FSHR and LHR, suggesting a difference in ligand-specificity. Real-time PCR analyses revealed that FSHR mRNA was highly expressed in early yolk-stage follicles but decreased at the end of vitellogenesis. In contrast, the expression of LHR mRNA was maintained at low levels in vitellogenic stage follicles but markedly elevated at the end of the vitellogenic and early migratory nucleus stages, thereafter markedly dropping in the late migratory nucleus stage. The present results suggest that co-regulation of vitellogenesis and oocyte maturation in one ovary is controlled by the stage-distinctive expression levels of FSHR and LHR mRNA in ovarian follicles, and daily switching of sensitivity from FSH to LH is required for daily egg production.

Aggregating Behavior of the Grass Puffer,Takifugu niphobles, Observed in Aquarium During the Spawning Period

The grass puffer (Takifugu niphobles) exhibits unique spawning behavior. Large numbers of fish aggregate to coastal spawning beds several hours before high tide during the spring tide. In order to examine the environmental and physiological regulation of this semilunar-synchronized spawning rhythm, the aggregating and spawning behaviors of the grass puffer were observed in the field, and in an aquarium without tidal changes. The fish aggregated to a spawning bed in the rising tidal phases both in the morning and evening during the spring tide, and several days after the spring tide. Spawning occurred on several days when large numbers of fish (200-1000) aggregated to the spawning bed. The timing of aggregation and spawning was tightly connected to the tidal changes; aggregation occurred 2-3 h before high tide, and spawning occurred 2 h before high tide. In the aquarium, in which a slope was constructed with pebbles, small groups of mature fish aggregated on the slope only in the rising tidal phases during and after the spring tide, when the fish aggregated in the field. However, there was no spawning in the aquarium. The aggregating behavior observed in the aquarium without tidal changes suggests that the semilunar reproductive rhythm is endogenously maintained with surprising precision during the spawning period in grass puffer.

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.

Synchronisation to light and feeding time of circadian rhythms of spawning and locomotor activity in zebrafish

Reproduction in most fish is a seasonal phenomenon, since spawning occurs at a precise moment of the year to ensure maximal survival of the offspring. Nevertheless, fish reproduction cannot be considered an exclusively annual phenomenon, since spawning might also show daily rhythmicity. In this study, we used an automatic programmable egg collector to investigate the existence of circadian spawning and activity rhythms in zebrafish ( Danio rerio L.), and their synchronization to different light and feeding cycles. Under 14L:10D, the results showed a diurnal spawning rhythm with an acrophase at ZT3 (lights went on at ZT0). Activity rhythms were also diurnal (74.4% of the total daily activity occurring during daytime), peaking immediately after lights on, in anticipation of spawning. Feeding at night did not change the diurnal spawning rhythm, but altered the daily pattern of activity, whose diurnal percentage dropped to 49.6%. When applying 1 h of darkness at ZT3, fish shifted the time of spawning to ZT7, while 1 h of darkness applied at ZT7 resumed spawning to ZT3. Under continuous light, locomotor activity rhythms persisted with τ = 22.3 h and the spawning rhythm maintained its phase relationship with them, with an acrophase at CT3. In short, these findings revealed the existence of circadian spawning and locomotor rhythms in zebrafish. The two rhythms are in phase with each other and both are synchronized by light, though only locomotion is influenced by feeding time.

Tropical monsoon environments and the reproductive cycle of the orange-spotted spinefoot Siganus guttatus

Negligible fluctuation in water temperature and photoperiod may cause fish species in tropical environments to utilize additional environmental factors to initiate reproductive activities during suitable periods of the year. We examined periodic changes in ovarian development of orange-spotted spinefoot (Siganus guttatus) in coral reefs off Karimunjawa Archipelago, Indonesia, and compared them with changes in regional environmental factors. The gonadosomatic index of this species increased initially during March–May and again during September–November. Concomitant with the increase, yolk-laden oocytes were observed (histologically) in ovaries, suggesting that the ovary develops twice a year. Initiation of reproductive activities was consistent with transition periods between rainy and dry seasons, suggesting that environmental cues linked to tropical monsoons influence reproductive activity. This suggestion is inconsistent with previous findings for the same species, but in higher latitudes, that the initiation of reproductive activity was influenced by increases in photoperiod and water temperature. It was observed that spinefoot exhibit plasticity in the use of environmental factors within habitats. Synchronized spawning occurred twice during the reproductive season, and was repeated between the new and first quarter of the lunar cycle. This periodicity was consistent with the spawning rhythm in other regions, suggesting that this species may perceive cues from the moon to synchronize spawning.

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)

Diel spawning periodicity of red snapper Lutjanus campechanus in the northern Gulf of Mexico

Ovaries of red snapper Lutjanus campechanus were examined histologically to determine rates of oocyte maturation, diel spawning periodicity and whether lunar cycle influenced spawning rhythm. Hydration of red snapper oocytes began during the mid‐morning hours; c. 5 h was necessary for oocytes to become fully hydrated and ovulation occurred no more than 5 h after oocytes attained full hydration. Appearance of fresh postovulatory follicles after 1330 hours and the absence of hydrated oocytes after 1830 hours signified that red snapper spawning occurred during this 5 h period. In addition, evidence of a peak in spawning was seen near 1600 hours. Postovulatory follicles degenerated within a 24 h time period. A lunar spawning cycle was not evident.

How do multiple environmental cycles in combination determine reproductive timing in marine organisms? A model and test

Summary Each of the multiple environmental cycles – the light–dark, tidal, semilunar, lunar and seasonal cycles – may have an independent, specific effect on reproductive timings of marine organisms and can alone or in combination produce a variety of temporal reproductive patterns. Assuming that each ecological interaction with these fluctuating environmental cycles generates a unique functional relationship between a component of fitness and the timing of reproduction, a model is proposed that predicts that all fitness components are multiplicative and therefore the reproductive timing is entrained or driven by a cycle(s) having a sharp functional curve of the relevant fitness component. According to this model, many of the reproductive patterns, ranging from diel to lunar, commonly seen in marine organisms were explained by changing relative kurtosis among functional curves of fitness components. Each curve could also differ in its optimum and skewness, making various intermediates and modifications possible. As a model system for the model to be applied, intraspecific variation in the timing of spawning by a puffer, Takifugu niphobles (Jordan &amp; Snyder) was examined. The patterns of daily variation in a day's spawning time within a population and geographical variation in spawning days among populations indicated that (1) the semilunar cycle (i.e. spring–neap tide cycle) has no direct influence on spawning days, (2) a combination between the functional curves of the fitness components associated with the light–dark and tidal cycles produce an apparent biweekly, semilunar spawning rhythm, (3) the curves may be skewed toward earlier and later times, respectively, and (4) their form may differ among populations. This model approach argues that to understand the ecology and evolution of temporal patterns of reproduction, one must properly decouple and evaluate the form of the functional curve of a fitness component associated with each of the multiple environmental cycles.