Pyrodinium bahamense var. compressum is the main organism causing toxic red tides in the Philippine waters. From 1987 when it first occurred in Manila Bay, the Pyrodinium recurred almost yearly up to 1998 except in 1997 which is an El Nino year.

Grazing is one of the factors considered to contribute in the decline of phytoplankton blooms. However, the role of grazers in the termination of Pyrodinium bahamense var. compressum bloom is completely unknown.

Preliminary in vitro feeding experiment showed that starved N. scintillans could feed on Pyrodinium cells. Filaments of mucus spread out from the cells of Noctiluca and entangled together to form a mucus web. Noctiluca fed on Pyrodinium cells trapped in the mucus and the aggregation caused the downward movement of Noctiluca which then discharged themselves from the web and float individually back to the surface.

The present study is a pioneering work to investigate the interactions of N. scintillans with Pyrodinium and to determine the impact of Noctiluca on Pyrodinium blooms.