Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Wolfgang K. Vogelbein


The heterotrophic dinoflagellates Pfiesteria piscicida and Pseudopfiesteria shumwayae (here referred to as Pfiesteria) have been reported to secrete potent toxins responsible for inducing lesions in Atlantic menhaden, causing deaths of fishes in natural systems and laboratory assays and impacting human health. Many aspects of Pfiesteria biology and ecology have been questioned, including its complicated life-cycle, its role in fish kills, fish lesion events and human health effects as well as its ability to produce toxins. Although the involvement of Pfiesteria in major estuarine fish kills and lesion events has been disputed, there is a demonstrated ability of these organisms to cause fish pathology and mortality in laboratory bioassays. Little evidence exists however, to support the conclusion that these effects are due to a potent exotoxin. Many other dinoflagellates are morphologically and genetically similarity to Pfiesteria and have been referred to as "Pfiesteria -like", although the pathogenic ability of these other species has not been investigated. In this work we address various aspects of the behavior and biology of Pfiesteria and related dinoflagellates in order to determine their ability to impact the health of fishes and to determine if any adverse effects are caused by a toxin or if of some other pathogenic mechanism is involved. Aquarium-format bioassays commonly used to detect and determine the toxin-producing status of Pfiesteria are subject to impediments that can make determination of the cause of fish mortality difficult. We developed a sensitive, 96 hr larval fish bioassay for assessing Pfiesteria pathogenicity using 2-10 mL volumes and larval cyprinodontid fishes. This assay was a highly effective method to verify and evaluate ichthyocidal activity in these dinoflagellates. Results from the larval assays, together with histopathological analysis, electron microscopy and direct observations of the interactions of Pfiesteria and fish, demonstrated the ability of dinoflagellates of the Pfiesteriaceae (including several Pfiesteria-like species) to cause pathology and mortality by feeding on the epidermal tissues of live fish. For some species, this process of micropredatory feeding resulted in fish mortality in laboratory bioassays comparable to that reported for purportedly "toxic" strains, but without the involvement of a toxin. Thus, the assertion that Pfiesteria produces a potent, fish-killing toxin is disputed. In laboratory assays with larval fish, P. shumwayae was consistently and significantly more ichthyocidal than P. piscicida. This differential pathogenicity was observed to be unrelated to chemoattraction of the dinoflagellates to fish tissues. Instead, as determined with comparative morphometric analyses, differences in ichthyocidal activity between these two species were related to their capacity to grow and reproduce after feeding on fish or algal prey. The results of these studies suggest that the perception of toxicity in these dinoflagellates is erroneous and related to different life history strategies and associated differences in growth rate in response to prey type. There is no direct evidence that these heterotrophic dinoflagellates impact fish health in the natural environment.



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