Pfiesteria piscicida (fee-STEER-ee-uh pis-kuh-SEED-uh) is a microscopic, polymorphic organism that belongs to a group of single-celled, free-swimming phytoplankton called dinoflagellates. Although many phytoplankton photosynthesize to generate the energy they need to survive, Pfiesteria does not—it is more animal-like in that it consumes other organisms such as bacteria and algae. It can transform in and out of the following stages within a matter of minutes: flagellated, amoeboid, and cyst. In the flagellated stage Pfiesteria is powered by two tiny flagella that resemble tails. The cyst stage lies dormant in the bottom sediments of estuaries and rivers.

Two species are known to release toxins that are harmful to aquatic and human life: Pfiesteria piscicida and Pfiesteria shumwayae.

When it was discovered in 1988, Pfiesteria was declared to be a new family (Pfiesteriaceae), genus (Pfiesteria), and species (piscicida), in the order Dinamoebales. It was named in honor of Dr. Lois Pfiester...