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Abstract

Pfiesteria pisiccida is a microscopic, unicellular organism that is classified as both a mixotrophic and heterotrophic dinoflagellate, which has been associated with both fish deaths and a cause of human illness (Burkholder et al., 1992; Glasgow et al., 1995; Burkholder and Glasgow, 1997). This species possesses a complex life cycle that includes motile forms (e.g. zoospores, gametes, amoebae) and a cyst stage that may remain dormant in the sediment (Burkholder et al., 1995b). Pfiesteria piscicida is known to have toxin and non-toxin producing populations, where cyst transformation into the toxic motile zoospores may be initiated by the presence of certain unknown fish excretions, or secretions (Burkholder et al., 1995a; 1999). These zoospores can attack fish and cause their death, then pass through several vegetative and reproductive stages, including the formation of amoebae and cysts that may leave the water column and pass to the sediment (Burkholder et al., 1995b; Glasgow et al., 1998). These transitions occur over a relatively short time period, so their concentrations in the water after a fish kill are often greatly diminished. This rather rapid departure into the sediment resulted in this species to be initially labeled the "Phantom dinoflagellate" (Burkholder et al., 1992).

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