What is Pfiesteria piscicida?

Dinoflagellates are a group of microscopic one-celled microorganisms. Most are free swimming and are plant-like, that is, they have the ability to obtain energy by photosynthesis. A natural part of the environment, dinoflagellates are nontoxic organisms.

Pfiesteria piscicida, however, is an exception to that rule. Although it is classified as a dinoflagellate, it is toxic and can be both plant-like by performing photosynthesis and animal-like by consuming other organisms.

Although believed to have been in the environment for millions of years (http://www.epa.gov/region03/r3press/pr97-369.htm), Pfiesteria piscicida was only recently discovered. In 1988, Dr. JoAnn Burkholder, an aquatic ecologist at North Carolina State University, discovered the organism, and declared it a new species in a new genus and a new family in the order Dinamoebales. Burkholder and her associates named the new organism Pfiesteria after the late Dr. Lois Pfiester, a researcher of dinoflagellates. The "piscicida" part translates to "fish killer."

Burkholder theorizes that Pfiesteria piscicida has always been naturally present in the estuaries and tidal rivers of North Carolina, yet only in the last decade has something caused it to morph into a toxic fish killer (Barker, 1997). Since its discovery ten years ago, Pfiesteria piscicida has killed more than one billion fish (http://www.epa.gov/region03/r3lib/inthenews/archivol.htm).

Although Pfiesteria piscicida has been blamed for the deaths of so many fish, it is only a small organism. Throughout the course of its 24 different stages of life, Pfiesteria piscicida ranges in size from 5 to 450 micrometers (http://www2.ncsu.edu/unity/lockers/project/aquatic_botany/pfiest.html ); several of these stages may produce toxins. Click here to see some Pfiesteria piscicida photos.

Pfiesteria piscicida in its nontoxic forms is quite harmless; it may masquerade as a plant and appear to photosynthesize, or it may feed on bacteria and algae. In the presence of fish excreta and secretions, however, it is stimulated to metamorphose into a killer. Once triggered, Pfiesteria piscicida emits a neurotoxin into the water which subdues the fish and eats through their skin. Pfiesteria piscicida then feeds on the weak and exposed skin, blood, and tissue. The fish eventually die not by the invasion of Pfiesteria piscicida, but by suffocation (the toxins cause paralyzation of muscles) or by infection (bacteria and foreign objects can enter the fish through the lesions). After the fish die, the dinoflagellates may continue to feed on the fish or change forms and disappear, leaving as the only evidence of its presence open, quarter-sized lesions on the fish carcasses (http://www2.ncsu.edu/unity/lockers/project/aquatic_botany/pfiest.html ). Click here to see some lesions caused by Pfiesteria piscicida.

During a fish kill caused by Pfiesteria piscicida, fish exhibit peculiar behavior. They may swim abnormally, become disoriented, beach themselves, or hover near the surface of the water (http://www.dnrec.state.de.us/tpff1.htm). Fifteen minutes is long enough for Pfiesteria piscicida to complete the process of killing a fish (Hager, 1997), therefore, toxic outbreaks of Pfiesteria piscicida are altogether very short, normally lasting for the duration of only a few hours. Fish may continue to develop lesions or even die for days or weeks afterward, however, once weakened by the toxins (http://www.dnr.state.md.us/pfiesteria/facts.html).

Pfiesteria piscicida has been busy killing fish when the conditions are right for the past several years. This year, however, it has not been as active as in the past. Researchers hypothesize that Pfiesteria piscicida took a break after storms stirred waters and spread the populations, thus making the dinoflagellates less concentrated and less aggressive.