THE ECOLOGICAL UNDERSTANDINGS FOR VERTICAL MIGRATION OF HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE)

Abstract

To understand the ecological advantages for vertical migration of Heterosigma akashiwo, we focused on the internal cycle which tightly linked with vertical migration behavior. We investigated this phenomenon in the laboratory and natural environment. In laboratory experiments, cells accumulated at the surface during the light period, and migrated to the bottom during the dark period. This phenomenon occurred with cell division; it began approximately 3 h before the beginning of the light period. Epifluorescence microscopy revealed that cells collected from the surface were in mitosis phase at 3 h after the beginning of the light period. In the field experiments, cells divided from 2 h before sunrise to 4 h after sunrise and cells began to migrate to the surface before sunset when these cells started to divide. Both experiments clearly demonstrated that vertical migration to the surface accompanies cell division. To clarity the relationship between cell division and vertical migration, we arrested the cell cycle. However, the vertical migration of cells kept. To clarity the relationship between internal clock and vertical migration, we observed vertical migration pattern under continuous light and dark conditions. It was kept both conditions and upward migration started when the light-on signal started 3 hours earlier than in the control. We confirmed that the vertical migration of H. a