Abstract
Facing environmental change, organisms may have to flee or to acclimate. Acclimation is based on the ability of organisms tomodify their phenotype, i.e. some of their biological traits (metabolism, behaviour, physiology) in order to survive in a given environment.This is called phenotypic plasticity. However, individuals of the same species may differ within their genotype and phenotypes. This isreferred as inter-individual or intra-individual variability. This variability can be found in the plasticity of organisms to respond to rapid orlong-term changes in their environment. The European sea bass Dicentrarchus labrax is a marine fish whose range extends from the northernEuropean coasts to the African sides and the Mediterranean sea. It undertakes seasonal migrations from the juvenile stage in lagoons andestuaries and sometimes even in rivers. It is therefore a plastic species in terms of osmoregulatory physiology, capable of withstanding a widerange of salinity: from 0 to 90 ppt. On the other hand, in the laboratory, a high variability in the osmoregulatory response in fresh water wasfound. The objectives of this thesis were to characterise the phenotypic plasticity and intraspecific variability of the Mediterranean sea bass inrelation to desalination.To do this, fish were transferred to fresh water and studied by different approaches. The first one allowed me to understand theeffects of desalination on respiratory metabolism and hypoxia tolerance. In fresh water, the respiratory capacities of sea bass differ fromthose of seawater with a higher metabolic rate, as well as a higher tolerance to hypoxia. In addition, the study of the transcripts revealsdifferent responses after two weeks and two months in fresh water. Individuals unable to tolerate fresh water were characterised by lowerbehavioural traits (velocity, displacement) and biochemical traits (osmotic pressure, blood chloride and sodium) than those of freshwatertolerant. The study of transcripts (ionic transporters and hormone receptors) revealed that intolerance to fresh water is partly due to aninability at the renal level to reabsorb ions and thus maintain the hydromineral balance. The study of the phenotypic variability ofMediterranean sea bass in fresh water has been repeated at different ages. It shows that tolerance/intolerance to fresh water is a stablephenomenon in terms of proportions (30% intolerant), but labile over time (it is not necessarily the same individuals who are tolerant orintolerant to fresh water following successive transfers). This suggests random, genetic and/or epigenetic regulatory mechanisms. Apreliminary study of DNA cysteine methylation concluded that freshwater transfer influences overall DNA methylation. The link betweenintraspecific variability related to freshwater tolerance and DNA methylation remains to be investigated.The last approach used was to study telomere dynamics (length and expression of the telomerase gene) as potential markers ofhypo-osmotic stress in sea bass. The method, now in development, does not indicate any effect of hypo-osmotic stress on telomere dynamics.Nevertheless, the response of intolerant sea bass to fresh water has yet to be confirmed by increasing the number of individuals. Thesepreliminary results suggest that transitions to hypo-osmotic environments do not cause more oxidative damage at the branchial level. Itwould be interesting to use this approach in the context of other environmental stressors and to explore other organs telomere dynamics.