Artificial controls of feeding rhythm of the prawn Penaeus japonicus.

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Feeding experiments were undertaken to investigate the acclimation ability of the prawn Penaeus japonicus to different photoperiods. Activity patterns were observed under 24 h photoperiods as 12L12D(12 h, light;12 h, dark), 12D12L, 24dim, 24D and 12dim12D. Forthe individuals of 12D12L and 24D, histological samplings of the brain and optic ganglion were conducted to examine activities of the intercerebral and medulla terminalis cells. The cellular activity was estimated by measuring the relative size of the nucleus (nucleo-cytoplasm ratio).The natural feeding rhythm was disturbed or changed according to the feeding time under respective photoperiods. Achievements of the artificial control of the daytime feeding were shown in the cases of 12D12L, 24dim and 12dim12D, provided that feedings were conducted during morning hours. The cellular activity on the nucleo-cytoplasm ratio indicated that the medulla termnalis cell would continue to show higher values during the daytimc than duringthe night-time regardless of reversal and continuous dark conditions. For the activity of theintercerebral cell, its proper rhythm was disturbed under 24D condition. These results seemedto support the existence of the biological clock in the medulla terminalis.