OBSERVATIONS ON THE PECULIAR DIURNAL MIGRATION OF A RED TIDE DINOPHYCEAE IN TROPICAL SHALLOW WATERS1

ABSTRACT The dinoflagellate Peridinium cf. quinquecorne Abé forms red tide‐like blooms in eutrophic shallow waters in the Philippines. The organism moves into a distinct near‐surface layer when intensive solar radiation occurs, but only during the incoming tide. Shortly before high tide, regardless of light levels, the dinoflagellates seem to disappear. Simple experiments show that once intensive radiation has been reduced Peridinium quinquecorne moves out of the water column and attaches itself to solid objects away from the light. The morphology of the organism, especially as related to attachment, was studied through SEM. Its high swimming velocity and the reaction to radiation and tidal changes are described. The possibility that, superimposed on its reaction to light, this dinoflagellate may follow intrinsic tide‐dependent oscillations is discussed.