ABSTRACT

Dinoflagellates represent a diverse group of both auxotrophic and heterotrophic protists. Most heterotrophic dinoflagellates are raptorial feeders that encounter prey using “temporal-gradient sensing ” chemotaxis wherein cells move along a chemical gradient in a directed manner toward the highest concentration. Using short-term “memory ” to determine the orientation of the gradient, dinoflagellates swim in a “run-and-tumble ” pattern, alternating directed swimming with rapid changes in orientation. As the extracellular concentration of the attractant increases, a corresponding increase in the ratio of net-to-gross displacement results in overall movement toward the stimulus. The dinoflagellates Pfiesteria piscicida and P. shumwayae are heterotrophic estuarine species with complex life cycles that include amoeboid, flagellated, and cyst stages, that have been implicated as causative agents in numerous major fish kills in the southeastern United States These organisms show documented “ambush-predator” behavior toward live fish in culture, including rapid transformations among stages and directed swimming toward fish prey in a manner that suggests the presence of a strong signalling relationship between live fish and cells of Pfiesteria spp