Résumé
The European seabass is an active euryhaline teleost that migrates and forages in waters of widely differing salinities. Oxygen uptake (MO2) was measured in seabass (average mass and forklength 510 g and 34 cm, respectively) during exercise at incremental swimming speeds in a tunnel respirometer in seawater (SW) at a salinity of 30‰ and temperature of 14°C, and their maximal sustainable (critical) swimming speed (Ucrit) determined. Cardiac output (Q) was measured via an ultrasound flow probe on their ventral aorta. The fish were then exposed to acute reductions in water salinity, to either SW (control), 10‰, 5‰, or freshwater (FW, 0‰), and their exercise and cardiac performance measured again, 18 h later. Seabass were also acclimated to FW for 3 weeks, and then their exercise performance measured before and at 18 h after acute exposure to SW at 30‰. In SW, seabass exhibited an exponential increase in MO2 and Q with increasing swimming speed, to a maximum MO2 of 339±17 mg kg−1 h−1 and maximum Q of 52.0±1.9 ml min−1 kg−1 (mean±1 SEM; n=19). Both MO2 and Q exhibited signs of a plateau as the fish approached a Ucrit of 2.25±0.08 bodylengths s−1. Increases in Q during exercise were almost exclusively due to increased heart rate rather than ventricular stroke volume. There were no significant effects of the changes in salinity upon MO2 during exercise, Ucrit or cardiac performance. This was linked to an exceptional capacity to maintain plasma osmolality and tissue water content unchanged following all salinity challenges. This extraordinary adaptation would allow the seabass to maintain skeletal and cardiac muscle function while migrating through waters of widely differing salinities.