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ABSTRACT: Despite the widespread occurrence of mixotrophic dinoflagellates, most research oncyst formation in dinoflagellates has focused on phototrophic organisms or on factors affecting photo-trophic growth (i.e. light intensity and nutrient supply). Presumably, factors that stimulate cyst for-mation in mixotrophic organisms would be combinations of those factors that affect phototrophic and phagotrophic growth (such as limiting light intensity and limiting prey concentrations). The toxicdinoflagellate Pfiesteria piscicida is an interesting test case because it has a complex life history andis considered mixotrophic. Recently, a form of P. piscicida has been described that does not have theability to produce toxins (termed ‘non-inducible’). The objectives of this study were to identify whichlife stages are likely to be present in a mixotrophic culture of ‘non-inducible’ P. piscicida and to deter-mine the morphological differences based on fluorescent stain uptake. Also, we examined whichcombinations of adverse environmental factors (low light intensity and low prey concentrations)affect life stage transformations. In this culture, we observed 3 distinct life stages (a zoospore stage,and 2 cysts). One of the cysts (termed Cyst A) has a thick cell wall and appears to form from activelyfeeding zoospores regardless of light intensity. The second type of cyst (Cyst B) has a much thinnercell wall and only forms from recently fed zoospores maintained in complete darkness. During theexperiment, encystment rates to either cyst were low, suggesting that encystment will not dramati-cally affect bloom dynamics on small timescales.KEY WORDS:

Formation of two types of cysts by a mixotrophic dinoflagellate, Pfiesteria piscicida