Developmental Changes in Specific Gravity in the Early Stages of the Japanese Flounder Paralichthys olivaceus

The specific gravities of larvae and juveniles of the Japanese flounder Paralichthys olivaceus were measured in a series of saline solutions of different specific gravities. The specific gravity increased gradually from 1.022 to 1.028 in the egg and prelarval stages and markedly from 1.028 to 1.060 in the postlarval period. It remained nearly constant after the completion of metamorphosis. In the early postlarval period, a diet change in specific gravity was detected temporarily, viz, day-increase and night-decrease, which was induced by a photocycle. The specific gravity of larvae reduced with decreasing specific gravity of the ambient water. From these results, it is suggested that ontogenetic changes in the buoyancy of larvae and juveniles of the flounder control their habitat transition. However, it seems to be necessary for the larvae having negative buoyancy, even at the pelagic phase, to swim actively upwards to maintain position in the water column and migrate vertically.