Behavioural responses and biochemical correlates in Solea solea to gradualhypoxic exposure

The common sole, Solea solea, a benthic flatfish. is frequently exposed to environmental hypoxic conditions. A succession of behavioural responses to oxygen depletion can be observed. Under moderate hypoxia (80-20% air satura tion) the fish reduce spontaneous activity and the scope for activity is lo wered. At the onset of anaerobic metabolism (at 20% air saturation) the fis h lie still on the bottom, but with increasing severity of hypoxia they sta rt bending the body so as to lift the mouth above the bottom and to reach w ater layers with higher oxygen concentrations. Between 20 and 6% air satura tion, the sole applies two different metabolic strategies: (i) it activates anaerobic metabolism and (ii) it depresses its metabolism below standard m etabolic rate. At 5% air saturation or lower the fish swim up with burst- a nd panic-like movements and lose balance and swim in an uncoordinated manne r (below 3%), finally remaining paralysed at the bottom. The correlation of behavioural and metabolic responses clearly shows that under unfavourable oxygen conditions, escape behaviour and burst activity are induced as a las t response when other energy-saving alternatives (anaerobic metabolism and metabolic depression) seem to become insufficient.