To study the ecological significance of gas bladder, an experimental study was made on the inflation and deflation of gas bladder of the Japanese anchovy larvae (21.5–32.7 mm TL) which were caught in July 1995 in Suruga Bay, Japan. With the decrease of indoor illuminance, the
percentage of larvae with inflated gas bladders increased, and at the illuminance of 0.07 Lx, all larvae inflated the gas bladders. Air inhalation to gas bladders was performed in three different modes. Once the gas bladders inflated, they did not deflate at all, even when the illuminance
reached over 200 Lx. Gas bladders only began to deflate after the larvae ingested Artemia nauplii. The deflation of gas bladders was slow and incomplete, compared to gas bladder deflation in the sea. The nighttime behavior differed markedly between the larvae with inflated gas bladders
and those with deflated ones; the frequency of rising and sinking per minute was 3.1 on average in the former, while 17.7 in the latter. When the larvae were starved, it took 9 d for the deflated gas bladder group to die and 13 d for the inflated gas bladder group. These facts indicate that
the inflation of gas bladders decrease energy consumption at night when feeding does not take place, and is one of the important adaptations for survival of anchovy larvae.
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