The vertical migration of the noxious raphidophyte Heterosigma akashiwo was studied in Tanigawa Fishing Port, the Seto Inland Sea. This species migrated toward the surface in the morning with velocities of 1.02-1.3 m/h. Cells were aggregated in the bottom layer at night, and a dense bloom was observed in surface water during daytime. Laboratory experiments clearly revealed that photo-period gave crucial influences on the pattern of diel vertical migration. H. akashiwo showed a clear phototaxis. During the later light period to later dark period, specific gravities of H. akashiwo revealed high values, and low values during the rest of the period. Swimming directions were observed and H. akashiwo represented upward direction during light period and downward direction in dark period. Chloroplasts were also synchronously replicated from the later light period to middle dark period, and vegetative cells also divided synchronously from the mid dark period to the end of dark period. Diel vertical migration probably contribute to form and maintain the red tides of H. akashiwo in coastal seas by virtue of enabling them to uptake macronutrients and micronutrients such as iron etc from bottom sediments at night.
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