Abstract
In European sea bass (Dicentrarchus labrax), as in many other fish species, temperature is known to influence the sex of individuals, with more males produced at relatively high temperatures. It is however unclear to what extent growth or stress are involved in such a process, since temperature is known to influence both growth rate and cortisol production. Here, we designed an experiment aiming at reducing stress and affecting early growth rate. We exposed larvae and juveniles originating from both captive and wild parents to three different treatments: low stocking density, food supplemented with tryptophan and a control. Low stocking density and tryptophan treatment respectively increased and decreased early growth rate. Each treatment influenced the stress response depending on the developmental stage, although no clear pattern regarding the whole-body cortisol concentration was found. During sex differentiation, fish in the low-density treatment exhibited lower expression of gr1, gr2, mr, and crf in the hypothalamus when compared to the control group. Fish fed tryptophan displayed lower crf in the hypothalamus and higher level of serotonin in the telencephalon compared to controls. Overall, fish kept at low density produced significantly more females than both control and fish fed tryptophan. Parents that have been selected for growth for three generations also produced significantly more females than parents of wild origin. Our findings did not allow to detect a clear effect of stress at the group level and rather point out a key role of early sexually dimorphic growth rate in sex determination.
Highlights
In European sea bass (Dicentrarchus labrax), as in many other fish species, temperature is known to influence the sex of individuals, with more males produced at relatively high temperatures
The stress response in fish begins when the hypothalamic-pituitary-interrenal (HPI) axis is stimulated to release corticotropin-releasing factor (CRF) by the hypothalamus, which is carried by the portal system to the pituitary where it stimulates the synthesis and secretion of pro-opiomelanocortin (POMC)-derived peptides involved in the mediation and regulation of the stress response, including the secretion of the adrenocorticotropic hormone (ACTH)
Fish kept at low density exhibited a high early growth rate and produced less cortisol following a temperature increase when compared to the control larvae
Summary
In European sea bass (Dicentrarchus labrax), as in many other fish species, temperature is known to influence the sex of individuals, with more males produced at relatively high temperatures. Somehow surprisingly, the various studies conducted on phylogenetically distant fish species led to the conclusion that, when stress is involved in sex determination, it always triggers m aleness[8,17] This raises fascinating questions regarding the adaptive potential of being male in stressful conditions[17]. The fact that early exposure to elevated temperature triggers maleness in European sea bass could be viewed as a paradox since relatively high temperature generally promote fish growth in aquaculture conditions (i.e. food is not limited)[30]. This is why testing the stress hypothesis is relevant in this species
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