Changes in the Specific Gravity of Small Yellow Croaker (Larimichthys polyactis) During Early Life History

초록

This study investigated the vertical distribution of Larimichthys polyactis larvae by measuring their specific gravity during different developmental stages and under external stimuli. Specific gravity was assessed in larvae from post-hatching to the yolk-sac and flexion larval stages using salt solutions with varying salinities. Specific gravity decreased by the 4th day post-hatching and increased on the 6th day. The results suggest that the reduction in specific gravity is linked to the expansion of subcutaneous space and yolk depletion. At the same time, the increase corresponds to air bladder expansion and organ development. Specific gravity also fluctuated according to diurnal rhythms, with notable changes occurring between day and night. During the early stages, no significant changes in specific gravity were observed during hatching and yolk-sac absorption. However, as the air bladder expanded and the tail differentiated, the larvae exhibited alternating trends in specific gravity, showing higher values during the day and lower values at night. Experiments further demonstrated that light intensity had a more significant impact on specific gravity than changes in water temperature. Specific gravity declined in darker conditions and shifted according to light intensity, while temperature changes primarily delayed the timing of these shifts. These findings provide crucial baseline data on the vertical distribution of L. polyactis larvae and their survival strategies in response to environmental factors.

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