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Rapid environmental changes will be the major challenge that most biota will have to deal with in the near future. Extreme events, such as marine heatwaves, are becoming more frequent and could be spatially uniform at a regional scale for a relatively long period of time. To date, most research studies on heatwaves have focused on sessile organisms, but these extreme events can also impact mobile species. Here, a 3‐week marine heatwave was simulated to investigate its effects on the male reproductive performance of a Mediterranean Sea emblematic species, the European sea bass Dicentrarchus labrax. Males from the control condition (c. 13°C) produced significantly more sperm than those exposed to a relatively warm thermal treatment (c. 16°C). Nonetheless, neither the percentage of motile spermatozoa nor most of the other sperm motility parameters were significantly affected by the rearing temperature over the whole period. Overall, the results of this study suggest only moderated effects of a potential winter heatwave on the reproductive performance of male European sea bass. [ABSTRACT FROM AUTHOR]

Within the thermal tolerance range of fish, metabolism is known to escalate with warming. Rapid thermic changes also trigger a series of physiological responses, including activation of the stress axis, producing cortisol. Fish have adapted to their environment by producing a low level of plasmatic cortisol when unstressed (basal), so that thriving in their natural temperature should not impact their basal cortisol levels. Yet, surprisingly, little is known on how temperature affects cortisol within and between fish species. Here, we conducted a phylogenetic meta‐analysis to (1) test whether temperature can explain the differences in basal cortisol between species and (2) evaluate the role of temperature on differences in cortisol levels between individuals of a same species. To do this, we retrieved basal plasma cortisol data from 126 studies, investigating 33 marine and freshwater fish species, and correlated it to water temperature. Intra‐species variability in basal plasma cortisol levels was further investigated in two species: the European sea bass Dicentrarchus labrax and the Nile tilapia Oreochromis niloticus. Factors such as life stage, sex and weight were also considered in the analyses. Overall, our phylogenetic analysis revealed a clear positive correlation between basal cortisol level and the temperature at which the fish live. The role of temperature has also been confirmed within D. labrax, while it failed to be significant in O. niloticus. In this paper, the influence of habitat, life stage, sex and weight on basal plasma cortisol levels is also discussed. Since some abiotic parameters were not included in the analysis, our study is a call to encourage scientists to systematically report other key factors such as dissolved oxygen or salinity to fully depict the temperature‐cortisol relationship in fishes. [ABSTRACT FROM AUTHOR]

Bolder individuals have greater access to food sources and reproductive partners but are also at increased risk of predation. Boldness is believed to be consistent across time and contexts, but few studies have investigated the stability of this trait across variable environments, such as varying stress loads or long periods of time. Moreover, the underlying molecular components of boldness are poorly studied. Here, we report that boldness of 1154 European sea bass, evaluated using group risk-taking tests, is consistent over seven months and for individuals subjected to multiple environments, including a chronically stressful environment. Differences in risk-taking behaviour were further supported by differences observed in the responses to a novel environment test: shy individuals displayed more group dispersion, more thigmotaxic behaviour and lower activity levels. Transcriptomic analyses performed on extreme phenotypes revealed that bold individuals display greater expression for genes involved in social and exploration behaviours, and memory in the pituitary, and genes involved in immunity and responses to stimuli in the head kidney. This study demonstrates that personality traits come with an underpinning molecular signature, especially in organs involved in the endocrine and immune systems. As such, our results help to depict state–behaviour feedback mechanisms, previously proposed as key in shaping animal personality. [ABSTRACT FROM AUTHOR]

In most animals, sex determination occurs at conception, when sex chromosomes are segregated following Mendelian laws. However, in multiple reptiles and fishes, this genetic sex can be overridden by external factors after fertilization or birth. In some species, the genetic sex may also be governed by multiple genes, further limiting our understanding of sex determination in such species. We used the European sea bass (Dicentrarchus labrax) as a model and combined genomic (using a single nucleotide polymorphism chip) and transcriptomic (RNA-Sequencing) approaches to thoroughly depict this polygenic sex determination system and its interaction with temperature. We estimated genetic sex tendency (eGST), defined as the estimated genetic liability to become a given sex under a liability threshold model for sex determination, which accurately predicts the future phenotypic sex. We found evidence that energetic pathways, concerning the regulation of lipids and glucose, are involved in sex determination and could explain why females tend to exhibit higher energy levels and improved growth compared to males. Besides, early exposure to high temperature up-regulated sox3, followed by sox9a in individuals with intermediate eGST, but not in individuals showing highly female-biased eGST, providing the most parsimonious explanation for temperature-induced masculinization. This gonadal state was maintained likely by DNA methylation and the up-regulation of several genes involved in histone modifications, including jmjd1c. Overall, we describe a sex determination system resulting from continuous genetic and environmental influences in an animal. Our results provide significant progress in our understanding of the mechanisms underlying temperature-induced masculinization in fish. [ABSTRACT FROM AUTHOR]

Temperature‐dependent sex determination (TSD) can be observed in multiple reptile and fish species. It is adaptive when varying environmental conditions advantage either males or females. A good knowledge of the thermosensitive period is key to understand how environmental changes may lead to changes in population sex ratio. Here, by manipulating temperature during development, we confirm that cold temperature (16°C) increases the proportion of fish that develop as females in European sea bass (Dicentrarchus labrax) until 56 days posthatching, but show that it has an opposite effect at later stages, with the proportion of males reaching ~90% after 230 days at 16°C. This is the first observation of opposite effects of temperature at different time periods on the sex ratio of a vertebrate. Our results highlight the potential complexity of environmental effects on sex determination. [ABSTRACT FROM AUTHOR]

Hyperallometric reproduction, whereby large females contribute relatively more to the renewal of the population than small females, is purported to be widespread in wild populations, especially in fish species. Bioenergetic models derived from a sufficiently general metabolic theory should be able to capture such a relationship but it was recently stated that no existing models adequately capture hyperallometric reproduction. If this were true it would seriously challenge our capacity to develop robust predictions of the life history and population dynamics in changing environments for many species. Here, using the European sea bass (Dicentrarchus labrax) as a test case, we demonstrate multiple ways that hyperallometric reproduction in a population may emerge from the Dynamic Energy Budget (DEB) theory, some inherently related to the metabolism and life history and others related to plastic or genetically based intraspecific variation. In addition, we demonstrate an empirical and modelled hypoallometric scaling of reproduction in this species when environment is controlled. This work shows how complex metabolic responses may underlie apparently simple relationships between weight and reproduction in the wild and provides new and testable hypotheses regarding the factors driving reproductive scaling relationships found in the wild. [ABSTRACT FROM AUTHOR]

• Energy allocation genes are involved in determining phenotypic sex of some animals. • Temperature differentially modulates ampk1, ghrelin, mtor, and nucb1. • Temperature and sex interaction occurs for the expression of spry1 and pank1a. • Ampk1 , mtor , pank1 , and spry1 are involved in sex differentiation. • Spry1 and sox3 are early markers of masculinization in the European sea bass. Water temperature governs physiological functions such as growth, energy allocation, and sex determination in ectothermic species. The European sea bass (Dicentrarchus labrax) is a major species in European aquaculture, exhibiting early dimorphic growth favoring females. The species has a polygenic sex determination system that interacts with water temperature to determine an individual's sex, with two periods during development that are sensitive to temperature. The current study investigated the influence of water temperature on energy allocation and sex-biased genes during sex determination and differentiation periods. RNA-Sequencing and qPCR analyses were conducted in two separate experiments, of either constant water temperatures typical of aquaculture conditions or natural seasonal thermal regimes, respectively. We focused on eight key genes associated with energy allocation, growth regulation, and sex determination and differentiation. In Experiment 1, cold and warm temperature treatments favored female and male proportions, respectively. The RNA-seq analysis highlighted sex-dependent energy allocation transcripts, with higher levels of nucb1 and pomc1 in future females, and increased levels of egfra and spry1 in future males. In Experiment 2, a warm thermal regime favored females, while a cold regime favored males. qPCR analysis in Experiment 2 revealed that ghrelin and nucb1 were down-regulated by warm temperatures. A significant sex-temperature interaction was observed for pank1a with higher and lower expression for males in the cold and warm regimes respectively, compared to females. Notably, spry1 displayed increased expression in future males at the all-fins stage and in males undergoing molecular sex differentiation in both experimental conditions, indicating that it provides a novel, robust, and consistent marker for masculinization. Overall, our findings emphasize the complex interplay of genes involved in feeding, energy allocation, growth, and sex determination in response to temperature variations in the European sea bass. [ABSTRACT FROM AUTHOR]

Mass reproduction is widely used in fish farms because it is relatively easy and inexpensive. It also enables the development of mass selection programmes that provide gains for traits with high heritability. The main weakness of this approach is that it is currently impossible to know which females participated in reproductive events, unless by conducting expensive genomic experiments. We tested oviduct-inserted floating passive integrated transponder tags as a new method to detect (i) the identity of spawners and (ii) the timing of spawning. We first conducted a preliminary experiment using four small tanks (1.5 m3) each containing one male and one female tracked by infrared video cameras to test the experimental device. We then tested it in "real" aquaculture conditions, using a bigger tank (10 m3) containing 89 adult fish. Results showed that this tracking system accurately identified the timing of spawning of individual fish. We confirm that European sea bass preferentially spawn at sunrise or sunset. This proof-of-concept developed for one commercially important fish species could also be used for novel species of interest for aquaculture , for example, to determine the exact timing of spawning after hormonal treatment of novel species. • There is a need to track individual spawning events to improve aquaculture practices. • The oviduct-inserted floating Pit-tag accurately indicates the timing of spawning and identity of spawner. • Associated to video-recording, the device allows to provide cues on reproductive behaviour of fishes [ABSTRACT FROM AUTHOR]

One of the primary challenges in European seabass farming is optimizing the number of females, which grow faster and mature later than males. However, typical rearing practices often result in populations with a high proportion of males. To test whether photoperiod, temperature and fasting impact the sex ratio, two distinct experiments were conducted. In the first one, we tested the effects of two photoperiods (12L:12D vs 10L:14D), applied until 90 dph at 16 °C. Then, on the same fish, photoperiod was set to 12D:12L and four different temperature treatments (19 °C, 21 °C, 23 °C, and 25 °C) were applied from 90 to 160–192 dph. In a second experiment, we tested the impact of fasting at four different times during the sex determination window: from 60 to 65 dph; 81 to 87 dph; 102 to 110 dph and 123 to 133 dph, at a temperature of 21 °C. Neither photoperiod (in the first experiment) nor starvation during larval development (in the second experiment) altered sex ratio. Conversely, we revealed (in the first experiment) that the proportion of females increased with post-larval rearing temperature: 30.4% at 19 °C, 36.3% at 21 °C, 44.1% at 23 °C, and 48.7% at 25 °C. Thus, late warm thermal treatments, following an initial cold treatment at 16 °C until 90 dph, can be useful to compensate the deficit of females in farmed sea bass production. • Late high temperature can yield females in seabass. • This can be applied to improve seabass production • We found no effect of photoperiod or starvation on sex ratio. [ABSTRACT FROM AUTHOR]

Coping styles consist of a coherent set of individual physiological and behavioral differences in stress responses that are consistent across time and context. Such consistent inter-individual differences in behavior have already been shown in European sea bass (Dicentrarchus labrax), but the associated mechanisms are still poorly understood. Here, we combine physiological measurements with individual behavioral responses in order to characterize coping styles in fish. Fish were tagged and placed in a tank for group risk-taking tests (GRT) at 8 months of age to evaluate boldness using the proxy latency of leaving a sheltered area towards an open area. A subsample of these fish were individually challenged 16 months later using an open field test (OFT), in which the boldness was assessed after being placed in a shelter within an open arena. Latency to exit the shelter, time spent in the shelter, and distance travelled were recorded for this purpose. The blood and brain were then collected to evaluate plasma cortisol concentration and neurotransmitter levels (dopamine, norepinephrine, serotonin, and related metabolites), as well as brain transcription of key genes involved in stress axis regulation (gr1, gr2 , mr, crf), neurogenesis (neurod1, neurod2, pcna), and neuronal development (egr1). Fish acting bolder in the GRT were not necessarily those acting bolder in the OFT, highlighting the relatively low consistency across different types of tests performed with a 16-months interval. There was, however, a significant correlation between stress markers and boldness. Indeed, mRNA levels of mr, crf, gr2, egr1 , and neurod2 , as well as norepinephrine levels were higher in shy than bold fish, whereas brain serotonergic activity was lower in shy fish. Overall, our study highlights the fact that boldness was not consistent over time when testing context differed (group vs. alone). This is in agreement with previous literature suggesting that social context play a key role in boldness measurement and that the particular life history of each individual may account in shaping the personality fate of a fish. • Boldness is not consistent when characterized using different types of test over an interval period of 16 months (group vs. individual tests). • After open field test, plasma cortisol concentration is similar between bold and shy fish. • Shy fish, however, display higher level of transcription of mr , gr2 , crf , egr1 and neurod2 (trend) in the whole brain. • Shy fish display higher level of norepinephrine and lower turnover ratio between serotonin and s hydroxyindoleacetic acid in the whole brain. [ABSTRACT FROM AUTHOR]

• Small within-day increases in temperature are physiologically stressful. • Whole-fish transcriptomic analysis revealed a signature of warming. • Fish kept at high temperature accumulate more cortisol in scales than those kept at low temperature. • We propose fish scales as a relevant indicator of global warming in wild populations. Changes in ocean water temperature associated with global climate change are bound to enormously affect fish populations, with potential major economic consequences in the aquaculture and fisheries industries. A link between temperature fluctuations and changes in fish stress response is well established. In this study, we aimed to assess the effects of a short- (4 days) or a long-term (4 months) exposure to warm temperature in the stress physiology of European sea bass (Dicentrarchus labrax) larvae and juveniles. First, cortisol (i.e. the main stress hormone in fishes) analysis was used to confirm that a steady and short-term elevation of temperature acts as a physiological stressful event in these fish, and cortisol release is indeed above a metabolic increase linked to temperature. Moreover, our results verified that measurement of cortisol released into the water can be reliably employed as a non-invasive indicator of acute thermal stress in experimental conditions. Secondly, the different effects on the genetic cascade underlying the stress response between long-term low or high thermal treatments were evaluated at two larval development stages via candidate-gene and whole-transcriptome approaches. Interestingly, opposite expression for some key stress genes (nr3c1, nr3c2 and hsd11b2) were observed between developmental stages, highlighting the distinct adaptive mechanisms controlling the primary and secondary responses to a stressor. Surprising expression patterns for some understudied genes involved in the stress axis were also revealed, including crhr1, mc2r, mc5r, trh or trhr, which should be further explored. Finally, evaluation of cortisol content in scales was successfully used as a biomarker of chronic thermal stress, with 10x more cortisol in fish kept at 21 °C vs 16 °C after 4 months, supporting the gene expression results observed. The use of such a method as a proxy of long-term stress, unprecedented in the literature, holds a vast array of applications in further research, in particular, in the investigation of the impact of global warming on wild fish populations. [ABSTRACT FROM AUTHOR]