Geotaxis in the Freshwater Dinoflagellate Peridinium bipes f. occultatum

Abstract

The characteristics of taxis in Peridinium bipes were examined using an experimental tank and a coupled microscope-CCD camera system. The vegetative cells exhibited negative geotaxis, since they swam upward irrespective of upward or downward irradiance. This behavior appeared to be the main factor in controlling the vertical migration of this species. The negative geotaxis was inferred to be caused by the distance between the center of gravity and the geometric center of the cell. We also estimated the geotactic and phototactic vertical swimming velocity of P. bipes and simulated the vertical migration of a P. bipes community with a numerical model. The model accurately reproduced the observed vertical migration pattern of the P. bipes community at irradiances over the range of 0 to 80.5μmol quanta m^-2 sec^-1. This model, in combination with a hydrodynamic model and a water quality model, will play a useful role in elucidating the appearance of freshwater red tides caused by P. bipes.