Ontogeny of osmoregulation in the European sea bass Dicentrarchus labrax L

Under natural conditions, sea bass eggs hatch in the open sea and young-of-the-year sea bass are found close to estuaries and enter brackish and even freshwater lagoons in the Mediterranean and the eastern Atlantic. The ontogeny of osmoregulation in the European sea bass, Dicentrarchus labrax, was studied in different developmental stages, from hatching to large juveniles, exposed to a range of salinities at 17 degree C. The experiments were carried out from March to August 1998 in southern France. The type of hyper-hypoosmotic regulation did not change during development. All stages hyper-regulated at low salinity (under 10- 11 ppt) and hypo-regulated at higher salinities. The acquisition of the full ability to hypo-regulate occurred in four steps. Osmoregulatory capacity was size- and age-dependent and reached its maximum for fish 17-26 mm long, 63-86 days after hatching. The iso-osmotic salinity was 10.2-11.6 ppt. Our results suggest that early development of osmoregulatory ability, and thus of salinity tolerance in sea bass, may provide an advantageous flexibility for the timing of migration towards low-salinity habitats.