Coping strategies in a strongly schooling fish, the common carp Cyprinus carpio

Evolutionary relationships between metabolism and behaviour require genetic correlations

As selection acts on multivariate phenotypes, the evolution of traits within populations not only depends on the genetic basis of each trait, but also on the genetic relationships among traits. As metabolic rate is often related to vital traits such as growth, physiology and behaviour, its variation and evolution is expected to have important repercussions on individual fitness. However, the majority of the correlations between metabolic rate and other traits has been based on phenotypic correlations, while genetic correlations, basis for indirect selection and evolution, have been overlooked. Using a case study, we explore the importance of properly estimating genetic correlations to understand and predict evolution of multivariate phenotypes. We show that selection on metabolic traits could result in indirect selection mainly on growth-related traits, owing to strong genetic correlations, but not on swimming or risk-taking and sociability behaviour even if they covary phenotypically. While phenotypic correlation can inform about genetic correlation direction, caution is needed in predicting the magnitude of genetic correlation. Therefore, even though phenotypic correlations among physiological and behavioural traits could be useful, deriving evolutionary conclusions based purely on them is not robust. In short, proper estimation of genetic correlations is needed when predicting evolutionary consequences. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Personality Affects Zebrafish Response to Sertraline

Sertraline is widely prescribed to treat anxiety and depression. Sertraline acts by blocking serotonin, norepinephrine, and dopamine transporters systems and has been detected in surface waters globally, where it may impact fish behavior. We classified zebrafish personality on three behavioral axes, boldness, anxiety, and sociability, assigning fish as either high or low in each category. The fish were exposed to nominal concentrations of 0, 5, 50, 500, or 5000 ng/L sertraline (measured concentrations: <10, 21.3, 370, and 2200 ng/L, respectively) to assess changes in boldness, anxiety, and sociability after 7 and 28 days. We also measured shoaling behavior and response to an alarm cue, and determined the gut microbiome of a subset of fish. After 7 days there was no overall effect of sertraline on boldness, but there was an interaction between initial personality and sex, with a stronger impact on females classified as low-boldness personality. Sertraline reduced sociability in all treatments compared with the control, but there was again an interaction between sertraline and initial personality. Fish that were classified as low-sociability responded more strongly to sertraline. After 7 days, fish exposed to a nominal concentration of 5000 ng/L (2200 ng/L measured) showed higher anxiety than controls, with the overall pattern of initial behavior retained. After 28 days, similar patterns were observed, but with higher variation. There was only a weak association between the gut microbiome and personality. Overall, the study highlights the importance of considering initial behavior, which can affect response to pollutants. Our results may also be applicable to human studies and provide a mechanism to explain why different individuals respond differently to the drug. Environ Toxicol Chem 2024;43:132-146. © 2023 SETAC.

Are bold-shy personalities of European perch (Perca fluviatilis) linked to stress tolerance and immunity? A scope of harnessing fish behavior in aquaculture

The sensitivity to stress and its impact on immunity are supposedly related to a fish's personality. In the present study, European perch (Perca fluviatilis) were exposed to an open-field and a novel-object test to identify distinctive shy and bold individuals. This series of cognitive tests revealed clear differences between proactive individuals with pronounced exploration behavior (bold personality) and reactive individuals that took a freeze-hide position (shy personality). A cohort of shy and bold perch was then exposed to elevated stocking density. Frozen activity and lower explorative behavior were related to higher basal and stocking-induced cortisol levels compared to proactive individuals. Since cortisol is a well-known modulator of immune-gene expression, we used multiplex real-time PCR to profile the differential immune responses to the intraperitoneal injection of Aeromonas hydrophila in the head kidney and peritoneal cells of bold and shy perch individuals. These expression differences between stimulated bold and shy perch were generally modest, except for the genes encoding the complement component c3 and the matrix metallopeptidase mmp9. The strong differential expression of these two bactericidal and inflammatory genes in the context of the modestly regulated features suggests that a fish's personality is linked to a particular immune-defense strategy. In conclusion, our approach, based on behavioral video observations, phagocytosis and enzyme assays, immunogene-expression profiling, and quantification of stress-relevant metabolites, revealed indications for divergent coping styles in cohorts of bold or shy European perch. This divergence could be exploited in future selective breeding programs.

Individual differences in response to alcohol and nicotine in zebrafish: Gene expression and behavior

Alcohol and nicotine are psychoactive substances responsible for serious health consequences. Although the biological mechanisms of alcohol and nicotine have been studied extensively, individual differences in the response to these drugs have received little attention. Here we evaluated gene expression and behavior of bold and shy individuals after acute exposure to alcohol and nicotine. For this, zebrafish were classified as bold and shy individuals based on emergence tests, and then fish were exposed to 0.00, 0.10, and 0.50% alcohol or 0.00, 1.00, and 5.00 mg/L nicotine and their anxiety-like and locomotor behavior was observed. After behavioral assessment, brain mRNA expression (ache, bdnf, gaba1, gad1b, th1, and tph1) was evaluated. Locomotion patterns differed between profiles depending on alcohol and nicotine concentration. Anxiety increased in shy fish and decreased in bold fish after exposure to both drugs. Alcohol exposure induced an increase in tph1 mRNA expression in bold fish, while bdnf mRNA expression was increased in shy fish. Nicotine increased ache, bdnf, and tph1 mRNA levels in both profiles, but at higher levels in bold fish. Based on our research, we found that alcohol induces anxiogenic effects in both bold and shy zebrafish. Additionally, shy individuals exposed to a low concentration of nicotine exhibited stronger anxiety-like responses than their bold counterparts. These findings further support the validity of using zebrafish as a dependable tool for studying the effects of drugs and uncovering the underlying mechanisms associated with individual variations.

Metabolic rate does not explain performance on a short-term memory task or personality traits in juvenile chickens (Gallus gallus domesticus)

Metabolic rate determines life processes and the physiological requirements of an individual, and has recently been implicated as a driver of inter-individual variation in behaviour, with positive correlations associated with boldness, exploration and aggressive behaviours being recorded. While the link between metabolism and personality has been explored, little is known about the influence of metabolism on cognitive abilities. Here we used juvenile female chickens (Gallus gallus domesticus) to investigate the relationships between metabolic rate at rest, short-term memory, personality, and dominance. Resting metabolic rates of the chicks were measured over a three-week period, concurrently with measures of short-term memory using an analogue of the radial arm maze. We also measured latency to leave the shelter (boldness), neophobia (fear of novel objects) and dominance within a group, both before and after short-term memory trials. We found that metabolic rate did not explain inter-individual differences in short-term memory, personality traits or dominance, suggesting that energy allocated to these traits is independent of individual metabolic rate, and providing evidence for the independent energy-management hypothesis. Differences in short-term memory were also not explained by boldness or neophobia. Variation in behaviour in chicks, therefore, appears to be driven by separate, currently unknown variables.

Boldness, activity, and aggression: Insights from a large-scale study in Baltic salmon (Salmo salar L)

Atlantic salmon (Salmo salar) display high levels of agonistic behavior in aquaculture farms, resulting in fin damage and chronic stress. Aggression affects fish growth and performance negatively, and presents a serious welfare problem. Indeed, it would be beneficial to identify, separate or exclude overly aggressive individuals. Research on behavioral syndromes suggests that aggressive behavior may correlate with other behavioral traits, such as boldness and locomotory activity. We aimed to develop a high-throughput method to quantify and predict aggressive behavior of individual parr in hatchery-reared Baltic salmon (Salmo salar L.). We screened approximately 2000 parr in open field (OF) and mirror image stimulation (MIS) tests. We extracted seven variables from video tracking software for each minute of the tests; distance moved and duration moving (activity), the duration in and number of entries to the center of the arena (boldness), the distance moved in and duration spent in the area adjacent to the mirror during the MIS test (aggressiveness) and head direction (lateralization). To investigate the relationship between activity, boldness and aggression we first correlated the first six variables to one another. Second, we assigned individuals to high, medium, low or zero aggression groups based on the MIS test and quantified activity and boldness in each group. Third, we analyzed whether the fish viewed the mirror with the left or right eye. Our results show that medium and low aggressive fish were the most active, while highly aggressive fish showed average activity. Aggressive groups did not differ in boldness. Activity and boldness were positively correlated. Finally, we detected a preference for fish to view the mirror with the left eye. We conclude that aggressiveness cannot be predicted from the results of the OF test alone but that the MIS test can be used for large-scale individual aggression profiling of juvenile salmon.

The Role of Metabolic Phenotype in the Capacity to Balance Competing Energetic Demands

AbstractGiven the critical role of metabolism in the life history of all organisms, there is particular interest in understanding the relationship between individual metabolic phenotypes and the capacity to partition energy into competing life history traits. Such relationships could be predictive of individual phenotypic performances throughout life. Here, we were specifically interested in whether an individual fish's metabolic phenotype can shape its propensity to feed following a significant stressor (2-min exhaustive exercise challenge). Such a relationship would provide insight into previous intraspecific observations linking high metabolism with faster growth. Using a teleost fish, the barramundi (Lates calcarifer), we predicted that individuals with high standard metabolic rates (SMRs) and maximal metabolic rates (MMRs) would be faster to recover and resume feeding after exercise. Contrary to our prediction, neither SMR nor MMR was correlated with latency to feed after exercise (food was offered at 0.5, 1.5, 3, and 18 h after exercise). Only time after exercise and individual fish ID were significant predictors of latency to feed. Measurements of MMR from the same individuals (three measurements spaced 8-12 d apart) revealed a moderate degree of repeatability (R=0.319). We propose that interindividual differences in biochemical and endocrine processes may be more influential than whole-organism metabolic phenotype in mediating feeding latency after exercise.

Exposure of zebrafish to an environmental mixture of persistent organic pollutants triggers an increase in anxiety-like syndrome but does not affect boldness in unexposed offspring

Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are persistent organic pollutants (POPs) that are present as complex mixtures in all environmental compartments, including aquatic ecosystems. However, little is known about the effects of such complex mixtures on teleost behaviour. In this study, zebrafish (Danio rerio) were chronically exposed to an environmentally relevant mixture (MIX) containing 22 PCB and 7 PBDE congeners through diet from 5 days post fertilization onwards. MIX-exposed F0 fish produced offspring (F1 and F2 generations) that were fed using plain food and grown until adulthood. In each generation, five behavioural traits (i.e. boldness, activity, sociality, exploration and anxiety) were evaluated by the mean of different experimental set-ups. Two distinct behavioural syndromes were identified: boldness, positively correlated to activity and exploration; and anxiety, associated with low sociality. F0 fish did not display any behavioural disruption resulting from POP exposure whereas F1 MIX fish were bolder than fish from other generations but did not differ significantly from F1 controls. F2 MIX fish displayed a higher anxiety syndrome than F2 controls. This is of particular importance since such behavioural changes in offspring generations may have persistent ecological consequences, may affect fitness and hence cause detrimental effects on wild fish populations exposed to POP mixtures.

Effectiveness of physical barriers and enhanced fertilization in controlling predation on tilapia and catfish aquaculture systems by four piscivorous water bird families

Waterbirds cause substantial fish-stock losses in open aquaculture systems, particularly in developing countries where fish-ponds are smaller and predator control methods largely manual or under-resourced. This study: (1) used three fish-pond treatment meassures to assess their efficiencies in deterring predation pressure by four piscivorous waterbird families in small tilapia and catfish farms in western Kenya; and (2) distinguished bird group(s) most effectively deterred by these measures. The treatment measures were: coarse-grid wire mesh barriers; finer-grid wire barriers; and enhanced pond fertilization. Twelve fish-ponds were randomly sampled to assess birds' pond-neigborhood assemblages and their predation deterrence responses to pond treatment effects. Bird species richness was not affected by pond cover status, enhanced pond fertilization or type of pond cover barrier. However, pond-cover status, singularly and interactively with enhanced fertilization, reduced bird encounter rates while cover barrier type did not. Conversely, cover status, cover barrier type and fertilization each separately but not interactively contributed to improved deterrence to bird predation rates overall. However, while predation by families of larger birds was effectively reduced by enhancing pond fertilization or cover barriers, predation by families of smaller birds was prevented only by fine-grid chicken-mesh barriers. These results demonstrate that using enhanced fertilization and physical barriers can significantly contribute to reduction in predation pressure on open-culture pond-fish by most piscivorous birds, but may not always be effective if used separately. Effectiveness of combination of measures chosen will depend on types of target bird species and their feeding habits. The results constitute additional knowledge on field techniques useful in diversifying solution options for minimizing impacts of vertebrate predation on pond-fish stocks toward promoting sustainable aquaculture production and improving rural human nutrition.

What Forms, Maintains, and Changes the Boldness of Swimming Crabs (Portunus trituberculatus)?

Boldness of personality is an important theme in animal behavior and has significant ecological and evolutionary consequences. Studies on boldness in crustaceans typically focus on their behavior, while relatively few studies have focused on the formation and maintenance of and change in boldness, such as energy metabolism and neurotransmission. In this study, we measured the boldness of swimming crabs (Portunus trituberculatus) and analyzed the relationship between boldness and oxygen consumption rate, energy concentration, and the relative expression of energy-metabolism-related and 5-HT genes in mRNA. The results showed that boldness remained stable across repeated tests but changed under dangerous conditions. Swimming crabs could be divided into bold and shy individuals. Bold individuals consumed oxygen at a significantly higher rate than shy individuals. Lactate and glucose concentrations in hemolymph were significantly lower in bold individuals than in shy individuals, and mRNA relative expression of Na+/K+-ATPase and 5-HT genes was significantly higher in bold than in shy individuals. Preliminary results indicate that energy metabolism and neurotransmitters may underlie the formation and maintenance of personality characteristics of swimming crabs. Swimming crabs also exhibit behavioral flexibility in order to cope with risks. This may be an adaptation to their complex environments.

Suspended Structures Reduce Variability of Group Risk-Taking Responses of Dicentrarchus labrax Juvenile Reared in Tanks

Structural enrichment is considered a useful tool to improve the welfare conditions of captive fish by deliberately increasing the physical heterogeneity and complexity of captivity environments. However, the potential effects of structural enrichment on the stress response at the group level and on social interactions have not been well studied yet. In this study, we demonstrate that suspended vertical structures (U-shaped ropes) can reduce behavioural variability among fish groups (tank level) of European seabass (Dicentrarchus labrax) juveniles. Differences in behavioural responses during group risk-taking tests (e.g., number of passes per fish) between treatments were detected, and these responses in seabass in enriched captive conditions were more homogeneous among tanks compared to fish from non-enriched tanks. These results suggest a positive effect of the structural enrichment on social stabilisation and response to stressful events at the tank level in seabass. However, further research is still needed to improve the knowledge of the potential effects of structural enrichment on fish welfare and aquaculture management, considering different enrichment designs, intensities, and strategies according to farming conditions, biological needs, and preferences of the fish species and life-stage reared in captivity.

The response of common carp (Cyprinus carpio) to insonified bubble curtains

Acoustic bubble curtains have been marketed as relatively low cost and easily maintained behavioural deterrents for fisheries management. Their energy efficiency can be improved by reducing air flow and exploiting bubble resonance. In a series of three flume experiments, we: (1) investigated the reactions of carp to a low air flow bubble curtain, (2) compared the effectiveness of resonant versus non-resonant insonified bubble curtains (for the same volume flux of gas injected through the nozzles) to deter passage, and determined the stimuli responsible for eliciting deterrence, and (3) included the effect of visual cues generated by the bubble curtain. This study showed that bubble curtains with a higher proportion of resonant bubbles deterred carp relatively better. Passage rejection was likely influenced by multiple cues at distances within a body length of the fish- specifically the rate of change in both particle motion and flow velocity caused by rising bubbles. All acoustic bubble curtains were less effective in the presence of daylight, suggesting that vision plays an important role at mediating carp reactions. We discuss the importance of ascertaining the bubble size distribution, in addition to the gas flow rate and aperture size, when characterising acoustically active bubble curtains.

Network analysis of intra- and interspecific freshwater fish interactions using year-around tracking

A long-term, yet detailed view into the social patterns of aquatic animals has been elusive. With advances in reality mining tracking technologies, a proximity-based social network (PBSN) can capture detailed spatio-temporal underwater interactions. We collected and analysed a large dataset of 108 freshwater fish from four species, tracked every few seconds over 1 year in their natural environment. We calculated the clustering coefficient of minute-by-minute PBSNs to measure social interactions, which can happen among fish sharing resources or habitat preferences (positive/neutral interactions) or in predator and prey during foraging interactions (agonistic interactions). A statistically significant coefficient compared to an equivalent random network suggests interactions, while a significant aggregated clustering across PBSNs indicates prolonged, purposeful social behaviour. Carp (Cyprinus carpio) displayed within- and among-species interactions, especially during the day and in the winter, while tench (Tinca tinca) and catfish (Silurus glanis) were solitary. Perch (Perca fluviatilis) did not exhibit significant social behaviour (except in autumn) despite being usually described as a predator using social facilitation to increase prey intake. Our work illustrates how methods for building a PBSN can affect the network's structure and highlights challenges (e.g. missing signals, different burst frequencies) in deriving a PBSN from reality mining technologies.

Hydropower-induced selection of behavioural traits in Atlantic salmon (Salmo salar)

Renewable energy projects such as hydropower facilities contribute towards meeting the world`s growing energy demands and urgent need for mitigating climate change. However, such infrastructure has the potential to substantially alter the environment which, in turn, can induce new challenges related to for instance fish migration conditions. As a consequence, local adaptations related to pre-development migration conditions may be affected for influenced populations. To explore selection regimes operating at a river hydropower plant, we monitored Atlantic salmon smolt individuals during their seaward migration. When passing the hydropower plant, the smolts chose between a surface fish passage or a submerged turbine intake. Smolts were scored for behavioural type (basal locomotor activity, net restrain (a measure of escape responses) and willingness to leave a familiar environment) prior to their migration choice, and we found that smolts with high basal activity had higher probability of using the fish passage than the turbine intake. In addition, migration route choice was a partly consistent trait in that fish that had previously passed a hydroelectric facility by using a fish passage rather than the turbine intake were significantly more likely to use it again when faced with the same choice. Higher mortality among turbine migrants could potentially reduce or eliminate particular behaviour types within populations- and the corresponding population genetic diversity that is essential to cope with future environmental challenges.

Genomes of Anguillid Herpesvirus 1 Strains Reveal Evolutionary Disparities and Low Genetic Diversity in the Genus Cyprinivirus

Anguillid herpesvirus 1 (AngHV-1) is a pathogen of eels and a member of the genus Cyprinivirus in the family Alloherpesviridae. We have compared the biological and genomic features of different AngHV-1 strains, focusing on their growth kinetics in vitro and genetic content, diversity, and recombination. Comparisons based on three core genes conserved among alloherpesviruses revealed that AngHV-1 exhibits a slower rate of change and less positive selection than other cypriniviruses. We propose that this may be linked to major differences in host species and corresponding epidemiological circumstances. Efforts to derive evolutionary rate estimates for cypriniviruses under various theoretical models were ultimately unrewarding. We highlight the potential value of future collaborative efforts towards generating short-term evolutionary rate estimates based on known sequence sampling dates. Finally, we revealed that there is significantly less genetic diversity in core gene sequences within cyprinivirus species clades compared to species in the family Herpesviridae. This suggests that cyprinivirus species may have undergone much more vigorous purifying selection post species clade divergence. We discuss whether this may be linked to biological and anthropogenic factors or to sampling bias, and we propose that the comparison of short-term evolutionary rates between species may provide further insights into these differences.

Comparative transcriptome and methylome analysis of the hindbrain in olive flounder (Paralichthys olivaceus) considering individual behavior-type and oxygen metabolism

In previous studies we employed multiple behavior assays, including propensity to feed, simulated trawl capture and escape response, to prove the presence of bold and shy personality (BP,SP) in olive flounder (Paralichthys olivaceus). However, the molecular mechanism of the different personality has not been elucidated. In this study, firstly, we found that the SP flounder had lower red blood cell count (RBC) and haemoglobin concentration (HBG) than BP flounder. Secondly, the transcriptomic profiles of the hindbrain in flounder with distinct personality were compared. A total of 144 differently expressed genes (DEGs) were identified, including 70 up-regulated and 74 down-regulated genes in SP flounder compared with BP flounder. Genes involved in hypoxia stress were detected in SP flounder, accompanied with down-regulation of ribosomal RNA synthesis. In addition, genes related with calcium signaling pathway, including endothelin, b-Fos, c-Fos and c-Jun were up-regulated in SP flounder. Furthermore, personality-related genes including UI, CCK, c-Fos showed significantly higher level in SP flounder than in BP flounder. GO enrichment analysis indicated that the GO categories "the tight junction pathway" and "lipid transport or localization pathway" were enriched in SP flounder, suggesting that the central nervous system homeostasis would be compromised. Thirdly, using a simple and scalable DNA methylation profiling method (MethylRAD), which allows for methylation analysis for DEGs in RNA-seq, we found that only part of gene expression was negatively associated with promoter methylation. Altogether, our study will not only lay a foundation for further studies on animal personality but also facilitate the selective breeding of olive flounder in aquaculture.

Analysis across diverse fish species highlights no conserved transcriptome signature for proactive behaviour

Background

Consistent individual differences in behaviour, known as animal personalities, have been demonstrated within and across species. In fish, studies applying an animal personality approach have been used to resolve variation in physiological and molecular data suggesting a linkage, genotype-phenotype, between behaviour and transcriptome regulation. In this study, using three fish species (zebrafish; Danio rerio, Atlantic salmon; Salmo salar and European sea bass; Dicentrarchus labrax), we firstly address whether personality-specific mRNA transcript abundances are transferrable across distantly-related fish species and secondly whether a proactive transcriptome signature is conserved across all three species.

Results

Previous zebrafish transcriptome data was used as a foundation to produce a curated list of mRNA transcripts related to animal personality across all three species. mRNA transcript copy numbers for selected gene targets show that differential mRNA transcript abundance in the brain appears to be partially conserved across species relative to personality type. Secondly, we performed RNA-Seq using whole brains from S. salar and D. labrax scoring positively for both behavioural and molecular assays for proactive behaviour. We further enriched this dataset by incorporating a zebrafish brain transcriptome dataset specific to the proactive phenotype. Our results indicate that cross-species molecular signatures related to proactive behaviour are functionally conserved where shared functional pathways suggest that evolutionary convergence may be more important than individual mRNAs.

Conclusions

Our data supports the proposition that highly polygenic clusters of genes, with small additive effects, likely support the underpinning molecular variation related to the animal personalities in the fish used in this study. The polygenic nature of the proactive brain transcriptome across all three species questions the existence of specific molecular signatures for proactive behaviour, at least at the granularity of specific regulatory gene modules, level of genes, gene networks and molecular functions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07317-z.

The importance of individual variation for the interpretation of behavioural studies: ethanol effects vary with basal activity level in zebrafish larvae

Standardization and reduction of variation is key to behavioural screening of animal models in toxicological and pharmacological studies. However, individual variation in behavioural and physiological phenotypes remains in each laboratory population and can undermine the understanding of toxicological and pharmaceutical effects and their underlying mechanisms. Here, we used zebrafish (ABTL-strain) larvae to explore individual consistency in activity level and emergence time, across subsequent days of early development (6-8 dpf). We also explored the correlation between these two behavioural parameters. We found inter-individual consistency over time in activity level and emergence time, but we did not find a consistent correlation between these parameters. Subsequently, we investigated the impact of variation in activity level on the effect of a 1% ethanol treatment, suitable for our proof-of-concept case study about whether impact from pharmacological treatments might be affected by inter-individual variation in basal locomotion. The inter-individual consistency over time in activity level did not persist in this test. This was due to the velocity change from before to after exposure, which turned out to be a dynamic individual trait related to basal activity level: low-activity individuals raised their swimming velocity, while high-activity individuals slowed down, yielding diametrically opposite response patterns to ethanol exposure. We therefore argue that inter-individual consistency in basal activity level, already from 6 dpf, is an important factor to take into account and provides a practical measure to improve the power of statistical analyses and the scope for data interpretation from behavioural screening studies.

A World for Reactive Phenotypes

Humans currently occupy all continents and by doing so, modify the environment and create novel threats to many species; a phenomenon known as human-induced rapid environmental changes (HIREC). These growing anthropogenic disturbances represent major and relatively new environmental challenges for many animals, and invariably alter selection on traits adapted to previous environments. Those species that survive often have moved from their original habitat or modified their phenotype through plasticity or genetic evolution. Based on the most recent advances in this research area, we predict that wild individuals with highly plastic capacities, relatively high basal stress level, and that are generally shy—in other words, individuals displaying a reactive phenotype—should better cope with sudden and widespread HIREC than their counterparts' proactive phenotypes. If true, this selective response would have profound ecological and evolutionary consequences and can therefore impact conservation strategies, specifically with respect to managing the distribution and abundance of individuals and maintaining evolutionary potential. These insights may help design adaptive management strategies to maintain genetic variation in the context of HIREC.

The fast and the furious-An experimental investigation of the pace of life and risky speed choice in traffic

Despite discernible improvements in the last decades, speeding is still a pertinent problem for road safety, fuel efficiency, and greenhouse gas mitigation. In order to understand individual speeding decisions, we need a better understanding of who speeds. In our paper, we test whether individuals' general pace of life is associated with speeding decisions. We use a novel speed-choice experiment that confronts participants with a scenario in which they repeatedly decide between driving fast or slow. This decision is associated with different accident risks. Before the experiment, each participant's pace of life was measured. Our results show that individuals with a slower pace of life are more likely to choose slow in the experiment and are also more likely to switch to slow, even when they had success by driving fast in the preliminary round. Therefore, individuals' pace of life may contribute to our understanding of speeding.

Examining the effects of chronic, lake-wide elevated temperatures on behavioural expression in largemouth bass, Micropterus salmoides

Many behaviours have differential fitness consequences across thermal and ecological contexts, indicating that both ecological shifts and warming temperatures induced by climatic change may alter how organisms behave. However, empirical evidence of temperature-driven behavioural selection in natural systems is lacking. We compared behaviours and behavioural syndromes related to activity, exploration, boldness and aggression in populations of largemouth bass (Micropterus salmoides) from ambient lakes to the those from artificially warmed, power plant cooling lakes to investigate changes in behaviours associated with warmer environments. Activity, exploration, boldness and aggression of juvenile largemouth bass were assessed in laboratory conditions using a novel environment assay and a risky situation assay. We found that activity and exploratory behaviours were higher and decreased through first year ontogeny in populations from heated lakes, whereas these behaviours were lower and showed no relationship through ontogeny in populations from ambient lakes. We attribute these differences to the changes in food source availability in heated lakes associated with temperature-driven ecological effects. Bold and aggressive behaviours tended to differ between populations, as did correlations between behaviours, but did not differ between ambient and heated lakes. The findings of this work identify that large ecological changes associated with warming environments, such as food availability, may drive changes in some aspects of behavioural expression in largemouth bass but that other aspects of behavioural expression may be driven by lake-specific factors not related to warming.

Zebrafish Danio rerio shows behavioural cross-context consistency at larval and juvenile stages but no consistency between stages

Coping style is defined as a set of individual physiological and behavioural characteristics that are consistent across time and context. In the zebrafish Danio rerio, as well as in many other animals, several covariations have been established among behavioural, physiological and molecular responses. Nonetheless, not many studies have addressed the consistency in behavioural responses over time starting at the larval stage. Therefore, this study aimed to improve the understanding of behavioural consistency across contexts and over time in zebrafish from the larval to juvenile stages. Two distinct experiments were conducted: a larval stage experiment (from 8 to 21 days post fertilization, dpf) and a juvenile stage experiment (from 21 to 60 dpf). On one hand, the larval experiment allows to focus on the transition between 8 and 21 dpf, marked by significant morphological changes related to the end of larval stage and initiation of metamorphosis. On the other hand, the juvenile experiment allows to properly cover the period extending from the end of larval stage to the juvenile stage (60 dpf), including metamorphosis which is itself completed around 45 dpf. Within each experiment, boldness was determined using a group risk-taking test to identify bold and shy individuals. A novel environment test was then performed at the same age to evaluate consistency across contexts. Groups of fish (either bold or shy) were bathed in an alizarin red S solution for later identification of their initially determined coping style to evaluate behavioural consistency over time. Fish were then reared under common garden conditions and challenged again with the same behavioural tests at a later age (21 and 60 dpf in the larval and juvenile experiments, respectively). Behavioural consistency was observed across contexts, with bold fish being more active and expressing higher thigmotaxis regardless of age. There was, however, little behavioural consistency over age, suggesting behavioural plasticity during development. Moreover, the use of alizarin red S to conduct this experiment provides new perspectives for the further study of the longitudinal evolution of various traits, including behaviour, over life stages in fish.

The presence of a conspecific induces risk-taking behaviour and enlargement of somata size of dopaminergic neurons in the brain of male medaka fish

Boldness and risk-taking behaviours in animals are important traits to obtain advantages such as habitation, food resources, reproductive success and social dominance. Risk-taking behaviour is influenced by physiological and environmental conditions; however, whether individual fish become bolder by the presence of conspecifics remains unknown. In this study, a light-dark preference test was conducted using medaka fish (Oryzias latipes) with or without a neighbouring conspecific. It was found that individual medaka male fish preferred a light environment and avoided a dark environment, whereas the display of a neighbouring conspecific enhanced the time the male spent in the dark environment (i.e., this condition encouraged risk-taking). The blood glucose level increased in fish confined to the dark condition but did not increase in light-preferring fish and risk-taking fish. Large somata expressing tyrosine hydroxylase, which is the rate-limiting enzyme in dopamine synthesis, were detected in the telencephalic and diencephalic brain regions in risk-taking medaka, whereas large somata were detected in the diencephalic region in medaka confined to the dark condition. These findings indicated that medaka is a good fish model to explore the central roles of dopaminergic neurons in the telencephalon and the diencephalon, which regulate risk-taking behaviour.

Contrasting neurochemical and behavioral profiles reflects stress coping styles but not stress responsiveness in farmed gilthead seabream (Sparus aurata)

In fish, as well as in other vertebrates, contrasting suites of physiological and behavioral traits, or coping styles, are often shown in response to stressors. However, the magnitude of the response (i.e. stress responsiveness) has been suggested to be independent of stress coping style. One central neurotransmitter that has been associated with both stress responsiveness and differences in stress coping styles is serotonin (5-hydroxytryptamine, 5-HT). In this study, we investigated to what extent stress responsiveness reflects differences in stress coping, and the potential involvement of the 5-HT system in mediating such differences in farmed Gilthead seabream. Initially, fish were classified as proactive or reactive based on their behavioural response to net restraint. Following 1.5 months, fish classified as proactive still showed a higher number of escape attempts and spent longer time escaping than those classified as reactive. These differences were reflected in a generally higher brain stem 5-HT concentration and a lower telencephalic 5-HT activity, i.e. the ratio of 5-hydroxyindoleacetic acid (5-HIAA) to 5-HT, in proactive fish. Independent of stress coping styles, stress responsiveness was reflected in elevated 5-HIAA concentrations and 5-HIAA/5-HT ratios in telencephalon and brain stem together with increased plasma cortisol concentrations at 0.5 and 2 h following the last net restraint. The current results show that 5-HT signaling can reflect different behavioural output to a challenge which are independent of neuroendocrine responses to stress and lend support to the hypothesis that stress coping styles can be independent of stress responsiveness.

Effects of early rearing enrichments on modulation of brain monoamines and hypothalamic-pituitary-interrenal axis (HPI axis) of fish mahseer (Tor putitora)

Enriching rearing environment is the strategy suggested for improving the post release survivorship of captive-reared animals. Here, an attempt has been made to evaluate the impact of early rearing enrichment on the hypothalamic-pituitary-interrenal axis (HPI axis), blood glucose, and brain dopaminergic and serotonergic systems of Tor putitora. Fifteen-day-old hatchlings of T. putitora were reared up to advanced fry stage in barren, semi-natural, and physically enriched environments and compared them with regard to pre-stress and post-stress levels of whole-body cortisol, blood glucose, brain serotonergic activity (5HIAA/5HT ratio), dopaminergic activity (DOPAC/DA and HVA/DA ratios) and norepinephrine (NE) levels. Significantly low basal whole-body cortisol, glucose and brain NE levels were observed in a physically enriched group of fish as compared to the other two groups. However, after acute stress, all rearing groups showed elevated levels of cortisol, blood glucose, brain 5HIAA/5HT, DOPAC/DA and HVA/DA ratios and NE levels but the magnitude of response was different among different rearing groups. The barren reared group showed a higher magnitude of response as compared to semi-natural and physically enriched groups. Similarly, the recovery rate of whole-body cortisol, blood glucose, and whole-brain monoamines were long-lasting in barren-reared mahseer. We illustrate that increased structural complexity (physical enrichment) during the early rearing significantly modulates various physiological and stress-coping mechanisms of mahseer.

Linking stress coping styles with brain mRNA abundance of selected transcripts for Senegalese sole (Solea senegalensis) juveniles

In fish, proactive and reactive individual stress copying styles (SCS) have been used to resolve variation in molecular expression data. Stress coping styles have been previously described in several stages of Solea senegalensis by validating for the species the use of standard behavioural screening tests. The present study aimed to link behavioural SCS tests with brain transcript abundance in early Senegalese sole juveniles in order to observe the natural variation in a molecular pathway in this species. A total of 50 juveniles were subjected to three individual behavioural (Restraining, New environment and Confinement) and one group (Risk-taking) screening tests. The fish were classified in SCS categories by applying a hierarchical cluster to the variable "Total activity" (the total activity time that the fish was moving in each individual test). Three categories were defined, proactive, intermediate and reactive sole. Six transcripts were chosen and tested, one related to basic metabolism (gapdh-2), three to feeding behaviour (per1, igf-Ia, pparß) and two to the stress response (crh-BP and hsp90aa) in 30 juveniles (10 individuals per SCS category) using rt-qPCR to observe differences in the abundance of those transcripts among SCS. Four transcripts were differentially expressed (DETs) among them. The transcript gapdh-2 showed up-regulation for proactive and intermediate SCS sole while reactive individuals showed down-regulation. Target mRNAs per1, igf-Ia and pparß, showed different levels of up-regulation for proactive and reactive fish while intermediates were highly down-regulated. Surprisingly no differences in stress related transcripts were observed. Correlations were found between variation in coping styles and variation in the abundance of mRNAs involved in important biological functions in Senegalese sole. These results are the first evidence of the relationship between the behavioural individual variation and the fluctuation in brain transcripts abundance in Senegalese sole.

Linking stocking densities and feeding strategies with social and individual stress responses on gilthead seabream (Sparus aurata)

Intensive aquaculture and poor management practices can cause stress and compromise welfare of farmed fish. This study aimed to assess the potential links between stocking densities and feeding methods with social and individual stress responses on juvenile seabream (Sparus aurata) through risk-taking and hypoxia tests. Seabream was first experimentally reared under two different densities: high (HD: 11-65 kg m^-3) and low (LD: 3-15 kg m^-3). After 120 days under these conditions, increment in fish weight was not affected by different stocking densities. HD seemed to induce a stronger schooling behavior on seabream juveniles seeking for the group safety during the risk test; while LD increased the mean number of movements per fish recorded and the time of first response. Additionally, HD conditions delayed the time of first response of proactive fish during hypoxia tests. Glucose levels were higher in reactive fish compared to proactive ones, being highly significant in fish reared at HD. In parallel, juvenile seabream was also experimentally reared for 106 days under two different feeding strategies: hand-feeding (HF) and self-demanding feeding (DF), which influenced fish growth and foraging behavior at group and individual level. HF method induced a positive effect on fish weight compared to DF systems. Time of first response during both hypoxia and risk-taking tests was shorter in HF fish than DF fish, and the mean number of movements per fish during risk-taking behavior tests was lower for DF fish compared to HF fish. No differences were found in glucose and cortisol concentrations between behavioral traits (proactive/reactive) and feeding strategies. Triggering actions of seabream in DF systems were also assessed, which seemed to be highly dependent on particular individuals and not related to proactive individuals. DF systems however reinforce the social hierarchy within the fish group, which might lead to a higher competitiveness for resources among fishes, increasing the social hierarchy, and therefore, the stress. The findings of this study provide valuable information to the industry for the management of fish stress and welfare under production conditions at social and individual level.

Behavioral and physiological responses to stocking density in sea bream (Sparus aurata): Do coping styles matter?

Stocking density is considered a stress factor for fish and is therefore one of the numerous concerns about fish welfare in an aquaculture context. Stress coping styles (SCS) are defined as a coherent set of individual physiological and behavioral differences in stress responses that are consistent across time and context and appear to be promising for improving fish welfare in aquaculture. The aim of the present study was to describe the physiological and zootechnical performances of gilthead sea bream (Sparus aurata) at different stocking densities (low density, LD: 15 kg/m³ and high density, HD: 30 kg/m³), depending on individual SCS. To do so, the fish SCS were first screened by measuring boldness (prior to the experiment). Three consecutive samplings were performed over the experiment to measure several blood parameters, including hematocrit (Hct), red blood cell count (RBCC), hemoglobin (Hb), cortisol, adrenalin, noradrenalin, glucose, lactate, and lysozyme, to infer the consequence of the SCS profile on the welfare condition in response to stocking density. Finally, swimming activity was recorded in a subsample of individuals (9 BOLD and 9 SHY individuals per density), and BOLD individuals displayed higher swimming activity than SHY ones at HD, while the opposite pattern was observed at LD. According to principal component analysis, physiological parameters are linked to the SCS profile, mostly at the beginning of the experiment, while density effects on physiology remain during the entire experiment duration. In conclusion, regarding all the variables observed, fish SCS appeared to be promising criteria to select the most adaptive individuals relating to rearing conditions and therefore improve welfare.

Consistent individual competitive ability in rainbow trout as a proxy for coping style and its lack of correlation with cortisol responsiveness upon acute stress

For a given fish species, individuals are different in their ability to cope with stressors; each individual has its own set of physiological and behavioral responses to stress (stress-coping style). This individual diversity is of importance when considering the welfare of fish reared in aquaculture facilities. In this study with rainbow trout (Oncorhynchus mykiss) we investigated the link between the ability to compete for food of each individual (used as a proxy of dominance behavior/proactive stress-coping style) and its ability to cope with stress; we hypothesized that fish that are better competitors would be more robust against common aquaculture stressors. We screened 680 rainbow trout individuals for competition ability. This was done by submitting groups of 20 individuals to a 1-week competition trial where they were kept at low stocking density and were provided a restricted amount of food. A 15% of the screened fish were selected as "winners" and another 15% were selected as "losers", based on growth rates during the competition trials. Fish were re-tested in a second competition trial after several weeks, to assess for consistency of competitive ability. Winner and loser fish were individually exposed to confinement and their neuroendocrine stress response was evaluated (serotonergic activity in telencephalon and brain stem, plasma levels of cortisol, glucose and lactate). Furthermore, behavioral responses to confinement and net restraining tests were also investigated. The results showed good temporal consistency of competitive ability in the lapse of time of the experiments. Besides, competitive ability showed a positive association to fish activity during the net restraining tests. However, plasma stress marker data showed a lack of relevant differences between the acute stress responses of winner and loser fish, adding up to the body of evidence suggesting that stress responsiveness might not be consistently linked to SCS in vertebrates. This, together with the inability of winner fish to outperform loser fish in usual stocking density conditions, suggests that there is no clear welfare or performance benefits in selecting fish of a specific coping style for fish farming, at least in the domesticated trout population used in the current study.

Coping styles in European sea bass: The link between boldness, stress response and neurogenesis

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.

Reproductive success of a marine teleost was correlated with proactive and reactive stress-coping styles

The present study investigated the relationship between reproductive success and stress-coping styles in gilthead seabream Sparus aurata in captivity. To characterise stress-coping styles, a total of 22 breeders were submitted to three different individual-based tests, one group-based test and post-handling glucocorticoid quantification. To assess spawning participation, a microsatellite analysis was performed on a total of 2698 larvae, which allowed each offspring to be assigned unambiguously to a single parental couple. Overall, S. aurata showed defined proactive and reactive behavioural traits. Proactive breeders exhibited higher levels of activity and risk taking and lower glucocorticoid blood levels than reactive breeders. The stress-coping style traits were consistent over time and context (different tests). Breeders that contributed to a higher number of progeny exhibited proactive behaviours, while those showing low progeny contribution exhibited reactive behaviour. Therefore, breeders with a high proportion of progeny (> 20%) had significantly higher activity and risk taking and lower cortisol than breeders with low progeny contribution (< 20%). In addition, males were more proactive than females and males exhibited significantly higher activity, risk taking and lower cortisol than females. This study is the first to establish in S. aurata breeders: (a) a relationship between stress-coping styles and spawning success; (b) a relationship between stress-coping styles and gender; and (c) the existence of proactive and reactive traits at the adult stage.

Energy metabolism and personality in wild-caught fall field crickets

Standard metabolic rate (SMR) is known to be highly variable across levels of biological organisation (e.g., species, populations, among individuals, within individuals). Some of the variation in SMR can be attributed to factors such as diet, temperature, and body mass, yet much of the residual variation in SMR remains unexplained. Intuitively, we can expect SMR to co-vary with "personality", but the rapidly accumulating empirical evidence on this topic remains equivocal. The goal of this study was to test for a link between SMR and a behavioural syndrome at the among-individual level in wild-caught fall field crickets (Gryllus pennsylvanicus). Paired measurements of SMR and two behavioural traits were repeatedly taken over a two-month period, thus allowing to estimate the among-individual correlations (r_ind) separately from the residual (within-individual) correlations. The two behavioural traits (latency to exit a refuge in a novel environment and "freezing" time following a stressful stimulus) were significantly and moderately repeatable and were found to be part of a syndrome, as indicated by a strong and positive among-individual correlation (r_ind = 0.82 ± 0.27). Yet, only latency to exit was significantly and positively correlated with SMR (r_ind = 0.45 ± 0.21), suggesting that the link between boldness and SMR may be driven by individual differences in responses to novelty and not to simulated predatory cues. Since we found that bold individuals (short latency to exit) consistently had a lower SMR than shy individuals (long latency to exit), our results go against the pace-of-life syndrome hypothesis. Instead, our results suggest the presence of constrains in the energy budgets of crickets, which generated an allocation trade-off between energy spent on physical activity vs. maintenance costs (SMR).

Agonistic behaviour and energy metabolism of bold and shy swimming crabs Portunus trituberculatus

The metabolism and behaviour of crustaceans are highly flexible, and the inter-individual variation in these traits is evolutionarily and ecologically significant. We analysed the relationships among personality traits (boldness, activity and hesitancy), agonistic behaviour and energy status (glycogen, glucose and lactate) in the swimming crab Portunus trituberculatus The main results were as follows. (1) Boldness was significantly correlated with activity and hesitancy. Bold crabs were more likely to initiate and win a fight. In bold individuals, the frequencies of 'move to', 'cheliped display', 'grasp' and 'contact' were significantly higher than those of shy individuals, whereas the frequency of 'move away' was significantly lower than that of shy individuals. (2) Before fighting, the glucose concentrations in the haemolymph of bold individuals were significantly lower than those of shy individuals, whereas the concentrations of lactate showed the opposite trend. There were no significant differences in glycogen and lactate concentrations in the claw muscle between bold and shy individuals. (3) After fighting, the glucose and lactate concentrations in the haemolymph of both bold and shy individuals were significantly higher than those before fighting. The glucose concentrations in the haemolymph were significantly higher in bold individuals than shy individuals. In addition, bold individuals showed a larger increase in glucose in the haemolymph but a smaller increase in lactate compared with shy individuals. (4) After fighting, the glycogen concentrations in the claw muscle were significantly lower than those before fighting; however, there were no significant differences in the concentrations of lactate in the claw muscle. These results indicated that the agonistic behaviour of the swimming crab is related to its behavioural type. Energy reserves may be one of the factors affecting the personality traits and agonistic behaviour in crabs. These results should lay a foundation for in-depth understanding of the relationships among crustacean personality, agonistic behaviour and metabolic physiology.

Behavioral differences within and among populations of an African cichlid found in divergent and extreme environments

Animals are increasingly faced with human-induced stressors that vary in space and time, thus we can expect population-level divergence in behaviors that help animals to cope with environmental change. However, empirical evidence of behavioral trait divergence across environmental extremes is lacking. We tested for variation in behavioral traits among 2 populations of an African cichlid fish (Pseudocrenilabrus multicolor victoriae Seegers, 1990) that experience extremes of dissolved oxygen (DO) and turbidity and are known to vary in a number of physiological and life history traits associated with these stressors. Using a common garden rearing experiment, F1 progeny from wild-caught parents originating from a swamp (low DO, clear) and a river (high DO, turbid) were reared in high DO, clear water. Predator simulation assays were conducted to test for (1) variation in boldness, general activity, and foraging activity between populations, (2) differences in correlations between behaviors within and across populations, and (3) repeatability of behaviors. There was strong evidence for divergence between populations, with swamp fish being more bold (i.e., leaving refuge sooner after a simulated predator attack) and active (i.e., spent more time out of refuge) than river fish. Across populations there were positive correlations between foraging activity and both boldness and general activity; however, within populations, there was only a strong positive relationship between foraging activity and boldness in the river population. Here, we have demonstrated that populations that originate from drastically different environments can produce progeny that exhibit measurable differences in behaviors and their correlated relationships even when reared under common conditions.

Do Metabolic Traits, Vulnerability to Angling, or Capture Method Explain Boldness Variation in Eurasian Perch?

The pace-of-life syndrome (POLS) concept predicts that individuals with high baseline metabolic rates demonstrate high boldness, aggressiveness, and activity, especially in food acquisition, with associated relatively greater energy requirements. In fishes, these behaviors may increase individual vulnerability to angling. To test the predictions of the POLS concept, we quantified individual standard metabolic rate (SMR) and boldness in both wild-caught and hatchery-reared Eurasian perch (Perca fluviatilis). We found both SMR and boldness to be repeatable traits but detected no correlation between them. Individual vulnerability to angling was assessed in the hatchery-reared perch, but we found no difference in boldness or SMR between vulnerable and nonvulnerable perch. Wild-caught perch were ice fished using either natural or artificial bait, and we observed no differences in boldness or SMR with respect to bait type or capture order. Our findings do not support the predictions of the POLS concept and, consistent with earlier studies in perch, suggest that angling may not drive selection against boldness in this species.

Food deprivation reduces social interest in the European sea bass Dicentrarchus labrax

Periods of food deprivation up to several months are common features for fishes and in such conditions, fitness will be determined by their capacity to maximize food encounter while minimizing predation risk. In this context, the propensity to take risk and the willingness to associate with conspecifics are particularly important as they contribute to alleviating the trade-off between predation avoidance and foraging efficiency. This study examined to what extent food deprivation modulates fish risk-taking and social behaviours, as well as the relationship between them. To address these issues juvenile European sea bass were either fed daily with a maintenance ration or food-deprived during 3 weeks. Risk-taking and sociability were assessed through measurements of fish willingness to explore a novel environment, to interact with a novel object or a conspecific. Multivariate analysis allowed the identification of three behaviours, risk-taking, exploratory activity and solitariness. Food-deprived fish interacted less with conspecifics than control fish. After food-deprivation, no difference in terms of risk-taking and exploratory patterns was observed. Finally, the relationship between risky-taking and solitariness was influenced by the feeding status. When food-deprived fish with higher propensity to take risk displayed increased solitariness while, when fed normally they interacted more with conspecifics.

Biological clock function is linked to proactive and reactive personality types

Background

Many physiological processes in our body are controlled by the biological clock and show circadian rhythmicity. It is generally accepted that a robust rhythm is a prerequisite for optimal functioning and that a lack of rhythmicity can contribute to the pathogenesis of various diseases. Here, we tested in a heterogeneous laboratory zebrafish population whether and how variation in the rhythmicity of the biological clock is associated with the coping styles of individual animals, as assessed in a behavioural assay to reliably measure this along a continuum between proactive and reactive extremes.

Results

Using RNA sequencing on brain samples, we demonstrated a prominent difference in the expression level of genes involved in the biological clock between proactive and reactive individuals. Subsequently, we tested whether this correlation between gene expression and coping style was due to a consistent change in the level of clock gene expression or to a phase shift or to altered amplitude of the circadian rhythm of gene expression. Our data show a remarkable individual variation in amplitude of the clock gene expression rhythms, which was also reflected in the fluctuating concentrations of melatonin and cortisol, and locomotor activity. This variation in rhythmicity showed a strong correlation with the coping style of the individual, ranging from robust rhythms with large amplitudes in proactive fish to a complete absence of rhythmicity in reactive fish. The rhythmicity of the proactive fish decreased when challenged with constant light conditions whereas the rhythmicity of reactive individuals was not altered.

Conclusion

These results shed new light on the role of the biological clock by demonstrating that large variation in circadian rhythmicity of individuals may occur within populations. The observed correlation between coping style and circadian rhythmicity suggests that the level of rhythmicity forms an integral part of proactive or reactive coping styles.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0618-0) contains supplementary material, which is available to authorized users.