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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 on heatwaves have focused on sessile organisms, but these extreme events can also impact mobile species. Here we simulated a three-week marine heatwave 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 (≈ 13 °C) produced significantly more sperm than those exposed to a relatively warm thermal treatment (≈ 16 °C). However, 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, our results suggest only moderated effects of a potential winter heatwave on the reproductive performance of male European sea bass.

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 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 so, 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, influence of habitat, life stage, sex and weight on basal plasma cortisol levels are 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.

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.

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.

In recent years, observations of distinct organisms have linked the quality of the environment experienced by a given individual and the sex it will develop. In most described cases, facing relatively harsh conditions resulted in masculinization, while thriving in favorable conditions promoted the development of an ovary. This was shown indistinctively in some species presenting a genetic sex determination (GSD), which were able to sex-reverse, and in species with an environmental sex determination (ESD) system. However, this pattern strongly depends on evolutionary constrains and is detected only when females need more energy for reproduction. Here, I describe the mechanisms involved in this environmentally driven sex allocation (EDSA), which involves two main energy pathways, lipid and carbohydrate metabolism. These pathways act through various enzymes and are not necessarily independent of the previously known transducers of environmental signals in species with ESD: calcium–redox, epigenetic, and stress regulation pathways. Overall, there is evidence of a link between energy level and the sexual fate of individuals of various species, including reptiles, fish, amphibians, insects, and nematodes. As energy pathways are evolutionarily conserved, this knowledge opens new avenues to advance our understanding of the mechanisms that allow animals to adapt their sex according to the local environment.

Introduction: The sea bass, Dicentrarchus labrax, is an important species for European mariculture, mainly in Mediterranean countries. In this species, females grow faster and reach a higher weight than males. However, most farmed fish batches show a highly male-skewed sex-ratio. The sea bass has a complex sex determination system, combining both genetic (polygenic) and environmental (temperature) influences. Low initial temperatures (<17°C) favor female sex determination. In contrast, temperature in the late post-larval phase has the opposite effect, with cold temperature favoring male differentiation and high temperature (up to 23°C) favoring female differentiation (Clota et al., 2021). However, data on the combined effects of genotype and temperature are lacking. We need to know whether there is genotype-by-environment (GxE) interaction or not, to deploy a strategy towards producing monosex female individuals for production while keeping a balanced sex-ratio for hatchery and selective breeding. Results: Starting at 1 day and using incremental rearing periods at 16°C (31 to 244 days), followed by an increase to 21°C, we first showed that, while 31 days at 16°C led to 26% of females, increased cold exposure promoted the proportion of females, up to 46% after 74 days at 16°C. Conversely, exposure to 16°C for a period longer than 74 days progressively reduced the proportion of females, reaching a minimum of ≈ 10% after 230 days at 16°C. In a second experiment, we demonstrated that after an initial rearing of fish for 90 days at 16°C, a secondary exposure to four different temperatures (19, 21, 23, 25°C) resulted in different sex-ratios, with more females at higher temperatures (from 30% at 19°C to 49% at 25°C). Then, we examined the genotype-by-environment interaction in both periods, by genotyping fish with a 57K SNP chip, comparing 1) two groups reared at 16 or 21°C during the first 60 days and 2) four groups of fish reared at 19, 21, 23 or 25°C after 90 days at 16

Fish sex determination is often considered as governed by either genetic or environmental factors, but the European sea bass (Dicentrarchus labrax) defies this theory. In this species, a polygenic threshold sex determination system was demonstrated (Vandeputte et al. 2007), where the genetic sex tendency is influenced by larval rearing temperature to determine the phenotypic sex (Piferrer et al, 2005). In this study, we applied two thermal treatments during early larval stage, a low temperature protocol (16°C, LT), known to favour balanced sex-ratios and a high temperature masculinizing protocol (21°C, HT). We combined various “-omics” approaches to characterize this temperature-dependent polygenic sex determination of European sea bass. We produced 8 families by mating 8 males with the same female. The progenies were reared in common garden under two thermal treatments (LT, HT) in triplicate. Fish at four different key developmental stages encompassing the temperature sensitive period were sampled. We predicted the genetic sex tendency (eGST) of the animals using a genomic relationship matrix derived from 57K SNPs from the DLabCHIP array (Griot et al., 2021) with a threshold animal model. This was completed by a transcriptomic approach, whole-body energy measurements. The proportion of females was 53.4% at LT and 25.3% at HT, showing a marked masculinization at high temperature. We found that the eGST accurately predicted the future phenotypic sex. We provided 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. Overall, we describe for the first time a sex determination system resulting from continuous genetic and environmental influences in an animal, which provides significant progress in our understanding of the mechanisms underlying temperature-induced masculinization in fish

Cortisol is recognized as a physiological indicator of stress in fish. However, this hormone is typically measured in plasma samples. In this study, cortisol content was evaluated for the first time in the scales of tropical tuna. The sample included 20 skipjack tuna (Katsuwonus pelamis) and 25 yellowfin tuna (Thunnus albacares) collected in the Atlantic (the Gulf of Guinea off the Ivory Coast) and the Indian Ocean (off Reunion Island), and their scales were analyzed using an ELISA test. The results indicated that on average, cortisol concentration (1) did not show any geographical pattern, (2) was independent of fish size, and (3) was significantly higher in skipjack (mean 4.75 ± 6.56 ng g−1) than in yellowfin (mean 1.65 ± 1.85 ng g−1), although the difference was mainly due to four skipjack individuals. Larger datasets would be needed to confirm any species difference in cortisol concentration. The particularly high cortisol concentration observed in four individuals may be due to fish from different schools, suggesting the need for future sampling from free-swimming schools to investigate intra- and inter-school variability in cortisol concentration. This study opens the door for future research, including the collection of scales on tagged individuals, to investigate the links between chronic stress and behavior in these species.