SCIENTIA MARINA 78(4) December 2014, 493-503, Barcelona (Spain) ISSN-L: 0214-8358 doi: http://dx.doi.org/10.3989/scimar.03992.02C Fitness difference between cryptic salinity-related phenotypes of sea bass (Dicentrarchus labrax) Bruno Guinand 1,2, Nolwenn Quéré 1,2, Frédérique Cerqueira 1,3, Erick Desmarais 1,3, François Bonhomme 1,2 1 Institut des Sciences de l’Evolution de Montpellier, CNRS-UMR 5554 (Université Montpellier 2), cc65, 34095 Montpellier cedex 5, France. E-mail:
[email protected] 2 Station Méditerranéenne de l’Environnement Littoral, 2 Avenue des Chantiers, 34200 Sète, France. 3 LabEx CeMEB (Centre Méditerranéen Environnement Biodiversité), Université Montpellier II, place E. Bataillon, cc63, 34095 Montpellier Cedex 5, France. Summary: The existence of cryptic salinity-related phenotypes has been hypothesized in the “euryhaline” sea bass (Dicen- trarchus labrax). How differential osmoregulation costs between freshwater and saltwater environments affect fitness and phenotypic variation is misunderstood in this species. During an experiment lasting around five months, we investigated changes in the whole body mass and in the expression of growth-related genes (insulin-like growth factor 1 [IGF-1]; growth hormone receptor [GHR]) in the intestine and the liver of sea bass thriving in sea water (SSW), successfully acclimated to freshwater (SFW), and unsuccessfully acclimated to freshwater (UFW). Albeit non-significant, a trend toward change in body mass was demonstrated among SSW, UFW and SFW fish, suggesting that SSW fish were a mixture of the other phe- notypes. Several mortality peaks were observed during the experiment, with batches of UFW fish showing higher expression in the osmoregulatory intestine due to down-regulation of genes in the liver and significant up-regulation of GHR in the intestine compared with SFW fish.