Pfiesteria piscicida has a complex life cycle that includes at least 24 flagellated, amoeboid, and encysted stages or forms. Both flagellated and amoeboid forms are known to be toxic to fish. The size range of these stages extends from 5 – 450 um along the major cell dimensions; the cyst (dormant) stages include an array of outer appearances or coverings (7-60 um in diameter), and they commonly occur among the bottom muds of North Carolina’s estuaries. Amoeboid stages can be found in the water column as well as among the bottom sediments; they feed on other organisms (bacteria, algae, small animals) or on bits of fish tissues by engulfing their prey. Flagellated stages (vegetative or asexual cells, sexual cells or gametes, and motile sexual products or planozygotes) can also engulf similar prey, but more often they feed, instead, by attaching to prey cells using a cellular extension called a peduncle and suctioning the prey contents.
Most of the time Pfiesteria survives as a nontoxic predatory animal, becoming toxic when it detects enough of an ephemeral substance that live fish excrete or secrete into the surrounding water. When fish (e.g., a large school of oily fish such as Atlantic menhaden) swim into an area and linger to feed, their excreta triggers encysted cells to emerge and become toxic. Active amoeboid and flagellated cells which are present also become toxic in the presence of the fish excreta. The small cells swim toward the fish prey and, in turn, excrete potent toxins into the water which make the fish lethargic so that they tend to remain in the area. The toxins also injure the fish skin so that they lose their ability to maintain their internal salt balance. As the skin is destroyed, open bleeding sores and hemorrhaging often occur. Once fish are incapacitated, Pfiesteria feeds on the sloughed epidermal tissue, blood, and other substances that leak from the sores. When the fish are dead, flagellated stages transform to amoeboid stages and feed on the fish remains or, alternatively, if conditions become unfavourable (e.g., sudden storm), Pfiesteria cells make protective outer coverings and sink out of the water column as dormant cyst stages. All of these changes can take place in a matter of hours.