Thirteen species of dinoflagellates, ranging in size from 16 to 48 pm, were tested for particle rejection behavior in the copepods Calanuspacificusand Paracalanusparvus. Five dinoflagellates were rejected as food: Gonyaulax tamarensis (429). G. tamarensis (Ipswich), Ptychodiscus brev~s, Scrippsiella trochoidea and Protoceratium reticulatum. The response of copepods to P. reticulatum was examined in detail. Starved copepods could not b e induced to feed on this species; C. pacificus maintained in bloom concentrations ceased reproduction and had high mortality. Both the cells themselves and filtrate from the cell culture suppressed feeding on normally edible dinoflagellates.Direct observations showed that P. reticulatum cells cause reverse peristalsis and regurgitation, and that Ptychodiscus brevis cells cause elevated heart rate and loss of motor control by C. pacificus We conclude the particle rejection behaviour we observed is chemically mediated, and that it may b e a n important factor in formation and maintenance of monospecific dinoflagellate blooms. We suggest chemicals produced by the dinoflagellate cause an acute physiological response which renders the herbivore incapable of ingesting more than required for its respiratory needs. Secondary effects of starvation, suspended reproduction and mortality further reduce the predation pressure of both present and future generations of predators. These factors aid in development of the bloom. Chemical defense may confer a competitive advantage, but rejected dinoflagellates pay the price in the form of reduced growth rates.

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