Trophic Controls on Stage Transformations of a Toxic Ambush‐Predator Dinoflagellate 1

ABSTRACT. The toxic dinoflagellate, Pfiesteria piscicida, was recently implicated as the causative agent for about 50% of the major fish kills occurring over a three‐year period in the Albemarle‐Pamlico Estuarine System of the southeastern USA. Transformations between life‐history stages of this dinoflagellate are controlled by the availability of fresh fish secretions or fish tissues, and secondarily influenced by the availability of alternate prey including bacteria, algae, microfauna, and mammalian tissues. Toxic zoospores of P. piscicida subdue fish by excreting lethal neurotoxins that narcotize the prey, disrupt its osmoregulatory system, and attack its nervous system. While prey are dying, the zoospores feed upon bits of fish tissue and complete the sexual phase of the dinoflagellate life cycle. Other stages in the complex life cycle of P. piscidia include cryptic forms of filose, rhizopodial, and lobose amoebae that can form within minutes from toxic zoospores, gametes, or planozygotes. These cryptic amoebae feed upon fish carcasses and other prey and, thus far, have proven less vulnerable to microbial predators than flagellated life‐history stages. Lobose amoebae that develop from toxic zoospores and planozygotes during colder periods have also shown ambush behavior toward live fish. In the presence of abundant flagellated algal prey, amoeboid stages produce nontoxic zoospores that can become toxic and form gametes when they detect what is presumed to be a threshold level of a stimulatory substance(s) derived from live fish. The diverse amoeboid stages of this fish “ambush‐predator” and at least one other Pfiesteria‐like species are ubiquitous and abundant in brackish waters along the western Atlantic and Gulf Coasts, indicating a need to re‐evaluate the role of dinoflagellates in the microbial food webs of turbid nutrient‐enriched estuaries.