The voluminal changes of swim bladder of larval red sea bream Pagrus major.

Abstract

Rearing experiments were made to determine the mechanism of diel swim bladder inflation and deflation in the red sea bream Pagrus major larvae. Swim bladder volume index (V/L3•10-6) was found to increase remarkably with growth during the larval stage, and remain nearly constant in the juvenile stage.The volume index was higher at night than in the day time under natural light conditions. However, under constant 24-h illumination such differences could not be observed. By changing the light conditions from dazk to light, the volume index decreased to half within 2h. Furthermore, with the reverse changing from light to dark, it increased to maximum level about lh after the onset of dark. Sealing the water surface of the tank with a layer of liquid paraffin, did not interfere with swim bladder inflation under dark condition and larvae clearly displayed the diel rhythm of night-inflation and day-deflation under natural light conditions.These results suggest that increase in the swim bladder volume in the physoclist species of red sea bream, is not caused by swallowing air at the water surface such as takes place in the physostome species of anchovy, but by the internal gaseous exchange mechanism of the swim bladder according to environmental light conditions.