Species of the toxic Pfiesteria complex, and the importance of functional type in data interpretation.

We describe the two species of the toxic Pfiesteria complex to date (Pfiesteria piscicida and Pfiesteria shumwayae), their complex life cycles, and the characteristics required for inclusion within this complex. These species resemble P. piscicida Steidinger & Burkholder and also have a) strong attraction to fresh fish tissues and excreta, b) toxic activity stimulated by live fish, and c) production of toxin that can cause fish death and disease. Amoeboid stages were verified in 1992-1997 by our laboratory (various stages from toxic cultures) and that of K. Steidinger and co-workers (filose amoebae in nontoxic cultures), and in 2000 by H. Marshall and co-workers (various stages from toxic cultures), from clonal Pfiesteria spp. cultures, using species-specific polymerase chain reaction-based molecular probes with cross-confirmation by an independent specialist. Data were provided from tests of the hypothesis that Pfiesteriastrains differ in response to fresh fish mucus and excreta, algal prey, and inorganic nutrient (N, P) enrichment, depending on functional type or toxicity status. There are three functional types: TOX-A, in actively toxic, fish-killing mode; TOX-B, temporarily nontoxic, without access to live fish for days to weeks, but capable of toxic activity if fish are added; and NON-IND, noninducible with negligible toxicity in the presence of live fish. NON-IND Pfiesteria attained highest zoospore production on algal prey without or without inorganic nitrogen or inorganic phosphorus enrichment. TOX-B Pfiesteria was intermediate and TOX-A was lowest in zoospore production on algal prey with or without nutrients. TOX-A Pfiesteria spp. showed strong behavioral attraction to fresh fish mucus and excreta in short-term trials, with intermediate attraction of TOX-B zoospores and relatively low attraction of NON-IND cultures when normalized for cell density. The data for these clones indicated a potentially common predatory behavioral response, although differing in intensity distinct from a toxicity effect, in attack of fish prey. The data also demonstrated that functional types of Pfiesteria spp. show distinct differences in response to fish, algal prey, and inorganic nutrient enrichment. Collectively, the experiments indicate that NON-IND strains should not be used in research to gain insights about environmental controls on toxic strains of Pfiesteria spp.

Among the approximately 40 toxic dinoflagellate species reported within the past 15 years (1,2) are two species of ichthyotoxic Pfiesteria, Pfiesteria piscicida Steidinger & Burkholder and Pfiesteria shumwayae . Pfiesteria spp. are considered to be unusual toxic dinoflagellates in their predominantly estuarine rather than marine coastal habitat (1,5-7) because of their direct attack behavior toward fish (3,7); their wide array of potential prey ranging from bacteria and algae, to finfish and shellfish, to mammalian tissues (5,7,8); their chrysophyte-like cysts not previously found in dinoflagellates (3)(4)(5)7); and their complex life cycles with multiple amoeboid as well as flagellated forms with maximum cell dimensions ranging from 5 to 120 µm (5,(7)(8)(9). Pfiesteria produces bioactive substance(s) with neurotoxic activity. In the present article, we use the term "toxins" to describe this activity, in accord with the Pfiesteria Interagency Coordination Working Group (10). In so doing, we acknowledge that these substances are only partially characterized (11,12), which is true for various other toxic algae. In the present article, "algae" include heterotrophic dinoflagellates and cyanobacteria, as well as obligate photosynthetic eukaryotes (13,14). It should also be noted that in August 2001, J.S. Ramsdell and P.D.R. Moeller of the National Oceanic & Atmospheric Administration, National Ocean Service in Charleston, South Carolina, verified that a potent watersoluble neurotoxin has been isolated and purified from fish-killing, actively toxic Pfiesteria culture material in standardized fish bioassays from our laboratory [patent process initiated (11)].
Toxic dinoflagellates produce some of the most potent biotoxins known, including ichthyotoxins that can act as neurotoxins in mammals (13,14). Pfiesteria (type species, P. piscicida Steidinger & Burkholder) was unusual because it was the first toxic dinoflagellate found to be stimulated by the presence of live fish, whereas stimuli for toxin production in the other species are unknown (15). Exposure to toxic fish-killing cultures [(16); via water or aerosol contact] or toxic Pfiesteria outbreaks in estuaries (7,17) or exposure to toxic, fish-killing Pfiesteria culture medium [via subcutaneous injection in rats; filtered to remove the dinoflagellate population, or unfiltered (18)(19)(20)] have been linked to central nervous system impairment in mammals. Although Pfiesteria was unusual among toxic dinoflagellates, it actually is similar to some benign (nontoxin producing) dinoflagellates with complex life cycles, including freshwater and estuarine species with up to 38 stages [reviewed in (4); also see (5,(7)(8)(9)20)]; wide range in sizes in freshwater (21) and estuarine species (3)(4)(5)8); ambush-predator behavior in estuarine species (21); and use of various prey as food in freshwater and estuarine species (5,8,22,23)].
Like many other toxic algal species (1,(24)(25)(26)(27)(28)(29)(30), toxic Pfiesteria spp. have naturally occurring toxic as well as apparently benign strains. The latter are noninducible, that is, without toxic activity or capable of producing only negligible/undetectable toxin in response to live fish (5,6,9,31). Eventually as more is known about the effects of specific controls on toxin production, it may be possible to induce some or all of these strains to regain their toxicity, but at present the biochemical switches for toxin production apparently have been 'turned off' (31). More than 1,000 clones of Pfiesteria spp. have been isolated in our laboratory (1991-present). About 60% of the clones assayed with fish (6,(31)(32)(33)(34) have shown ichthyotoxic activity; the remainder have been noninducible. The proportion of toxic and noninducible isolates can vary substantially; occasionally for 1-to 2-year periods, most or all clones recently isolated from natural estuarine habitats have been toxic. Also like many other toxic algae (25,29,30), toxic strains of Pfiesteria spp. commonly lose We describe the two species of the toxic Pfiesteria complex to date (Pfiesteria piscicida and Pfiesteria shumwayae), their complex life cycles, and the characteristics required for inclusion within this complex. These species resemble P. piscicida Steidinger & Burkholder and also have a) strong attraction to fresh fish tissues and excreta, b) toxic activity stimulated by live fish, and c) production of toxin that can cause fish death and disease. Amoeboid stages were verified in 1992-1997 by our laboratory (various stages from toxic cultures) and that of K. Steidinger and co-workers (filose amoebae in nontoxic cultures), and in 2000 by H. Marshall and co-workers (various stages from toxic cultures), from clonal Pfiesteria spp. cultures, using species-specific polymerase chain reaction-based molecular probes with cross-confirmation by an independent specialist. Data were provided from tests of the hypothesis that Pfiesteria strains differ in response to fresh fish mucus and excreta, algal prey, and inorganic nutrient (N, P) enrichment, depending on functional type or toxicity status. There are three functional types: TOX-A, in actively toxic, fish-killing mode; TOX-B, temporarily nontoxic, without access to live fish for days to weeks, but capable of toxic activity if fish are added; and NON-IND, noninducible with negligible toxicity in the presence of live fish. NON-IND Pfiesteria attained highest zoospore production on algal prey without or without inorganic nitrogen or inorganic phosphorus enrichment. TOX-B Pfiesteria was intermediate and TOX-A was lowest in zoospore production on algal prey with or without nutrients. TOX-A Pfiesteria spp. showed strong behavioral attraction to fresh fish mucus and excreta in short-term trials, with intermediate attraction of TOX-B zoospores and relatively low attraction of NON-IND cultures when normalized for cell density. The data for these clones indicated a potentially common predatory behavioral response, although differing in intensity distinct from a toxicity effect, in attack of fish prey. The data also demonstrated that functional types of Pfiesteria spp. show distinct differences in response to fish, algal prey, and inorganic nutrient enrichment. Collectively ichthyotoxic activity in the presence of live fish when cultured for weeks to months (31). The environmental signals controlling toxin production have not yet been determined for any species where the phenomenon has been observed. However, loss of toxicity has been hypothesized to result from the lack of particular organic substrates, possibly including one or more bacterial cofactors, that occur in the natural habitat or from the lack of some other vital factor(s) in culture. If so, then strongly heterotrophic species or strains such as Pfiesteria spp. might be expected to be especially vulnerable to loss of toxin production in culture. Alternatively, loss of toxicity may arise from the loss of an essential gene over time in culture, as in certain toxic fungi (35). Other aberrations have been noted in cultured dinoflagellates, such as significant change in chromosome number over time (years) (36).
The objectives of this study were first to describe the species of the toxic Pfiesteria complex to date including their life cycles, with emphasis on stages found to date in both P. piscicida and P. shumwayae as well as further validation of amoeboid stages; and on the characteristics that must be manifested for inclusion within this complex. Following considerations used for other groups of toxic algae (Table 1) and the recommendations of a recent multifederal/multistate agency consensus document (10), we focused on the known toxic Pfiesteria species rather than considering all species of benign or unknown toxicity status and systematics that superficially resemble P. piscicida under light microscopy. All Pfiesteria look-alike species experimentally tested to date, for example, various strains of cryptoperidiniopsoid species, Karlodinium micrum [formerly Gyrodinium galatheanum (42)], and samples including an unnamed dinoflagellate informally referred to as "Shepherd's crook" (44) have shown no ability to grow, reproduce, or produce ichthyotoxins causing disease or mortality to fish under ecologically realistic conditions (tests of live cells in standardized fish bioassays) (5,9,31,32,34). Second, we tested the hypothesis that Pfiesteria differs significantly in response to environmental variables, depending on the functional type or toxicity status of the clone. We compared the response of the three functional types of P. piscicida and P. shumwayae to finfish materials, algal prey, and nutrient (N, P) enrichments (functional types: TOX-A, in actively toxic, fish-killing mode; TOX-B, temporarily nontoxic, without access to live fish but capable of toxic activity when live fish are added; and NON-IND, noninducible, with negligible/undetectable toxic activity in the presence of live fish) (5,6,8,31). We predicted that TOX-A Pfiesteria would be strongly attracted to fresh fish mucus/excreta, with low or negligible attraction shown by TOX-B and NON-IND Pfiesteria, respectively. We expected the opposite trend in response to inorganic nutrient enrichment and algal prey. PCOs (Pfiesteria complex organisms) is an incorrect acronym that should no longer be used. "Complex " has a specific meaning [that is improperly used in PCOs] and refers to a group of species within a genus or to an assemblage of species with certain behavioral characteristics (here, toxic activity, which is of critical importance when considering the Pfiesteria issue). Examples of the correct use of the term "complex " are the toxic Alexandrium complex, which refers to the species within the genus that produce saxitoxins and their derivatives; and the toxic Pseudo-nitzschia complex, which refers to the species within the genus Pseudo-nitzschia that produce the toxin, domoic acid. The TPC (toxic Pfiesteria complex) refers to dinoflagellate species with strains that resemble Pfiesteria when viewed under light microscope and produce toxins under appropriate experimental conditions (as demonstrated by a positive test with fish bioassays, see Glossary). To be included in the TPC, a species must have all of these characteristics: i) strong attraction to live fish prey, ii) ability to produce substances that are ichthyotoxic (i.e., cause erratic behavior, disease, or death in fish), and iii) toxic activity (production of toxic substances by some strains within the species) that is stimulated by the presence of secretions, excretions, or the presence of (usually large numbers of) live fish.

Culture and identification of
The consensus group also advised that dinoflagellates that are similar in appearance but not necessarily similar to Pfiesteria spp. in ability to produce toxin and other important behavioral characteristics should be called "pfiesteria-like organisms" (PLOs, which superficially resemble Pfiesteria under 400× in light microscopy), and that such organisms should (only) be of focus in management issues when completing presumptive counts in preliminary efforts to determine whether a toxic Pfiesteria or Pfiesteria-like species is actually present.
optical alignment and detector voltages, using fluorescent latex microspheres 6 µm in diameter (Molecular Probes, Inc., Eugene, OR, USA). Fish bioassay samples (200 mL) were gravity-filtered through 38-µm Nitex mesh (Aquaculture Research/Environmental Associates, Inc., Homestead, FL, USA) immediately prior to flow cytometric analysis. Electronic sort gates were based on detection of optical parameters defined to select the detectable subpopulations of interest. Sort recovery and purity were checked with light microscopy, and with the heteroduplex mobility assay (47). Isolation of highly purified cells via particle sorting yielded ultraclean cell preparations for polymerase chain reaction (PCR) and scanning electron microscope (SEM) analyses. From these subpopulations, a robotic Coulter AutoClone sorting system (Coulter, Miami, FL, USA) was used to establish multiple clonal isolates by directed deposition into multiwell microculture plates where the populations initially were grown for 2 weeks with cryptomonad prey. They were then inoculated into fish bioassays and monitored daily or more frequently to confirm toxic activity and to grow toxic clones. The autoclone sorting system allowed rapid cloning of single cells into multiwell microculture plates with 99.98% precision and purity. Once clonal sorting was complete, axenic (externally eubacteria-and cyanobacteria-free) prey were added to each well, and the dinoflagellates are allowed to grow and re-produce for several days under sterile conditions. To ensure that cultures did not contain prokaryotes, or eukaryotes other than Pfiesteria and the added axenic prey, 10 µL of each culture was plated onto agar growth medium (triptic soy agar [TSA]; Difco Laboratories, Detroit, MI, USA) following techniques described by (48). The plates consistently were evaluated as free from bacterial, fungal, or other contaminants.
One clone of each species (#101161 and # 410T for P. piscicida and P. shumwayae, respectively) was confirmed as toxic to fish in the standardized fish bioassay process (6,9,31,32,34). It is important to clarify that this standardized fish bioassay procedure, including cross-corroboration of the data from each step by one or more independent specialists, was developed and has been used by our laboratory throughout the past decade (31,34). The existence of this standardized procedure was recognized and endorsed by a national science panel that was charged by the CDC to review all previously published Pfiesteria research (49). In the present experiments including fish bioassays, Pfiesteria toxicity was cross-corroborated by Marshall et al. (9). Unidinoflagellate clonal quality was determined by the heteroduplex mobility assay (D. Oldach, University of Maryland, Baltimore, Maryland, USA) (47). Species identifications were made from sutureswollen cells with SEM of ≥100 zoospores analyzed per clone following the methods of Burkholder and Glasgow (8) and Glasgow et al. (5) and were cross-confirmed with PCR] probes and the fluorescent in situ hybridization (FISH) probes of P. Rublee of the University of North Carolina at Greensboro (46) and D. Oldach (47), as well as crosscorroborated by those laboratories. A second isolate of each species (#B93B-N and #270A-2 for P. piscicida and P. shumwayae, respectively) tested as NON-IND in repeated fish bioassays (6,9,31). TOX-A Pfiesteria spp. were grown with live fish for 3-4 months prior to the experiments [tilapia, Oreochromis mossambicus; total length (t.l.) 5-7 cm; 3-6 fish day -1 in 5-L microcosms (5,6,34)]. TOX-B subcultures were taken from the TOX-A cultures after being grown with live fish for 2-3 months. The subcultures were switched for 3 weeks to a diet of cryptomonad prey containing Cryptomonas LB2423 cloned from commercial material provided by a culture collection at the University of Texas in Austin, Texas, USA. These phytoflagellates were grown in f/100 medium (50) at a salinity of 15 (51) and fed to Pfiesteria zoospores in a 1:15 zoospore:prey ratio at 3-to 4-day intervals (5,6,31). NON-IND cultures (of necessity, from different clones) were also grown with cryptomonad prey. Cultures and experiments were maintained at 23°C, 12 hr:12 hr light:dark (L:D)cycle, and 80 µmol photons m -2 s -1 .
The medium was f/2-Si media (50) at a salinity of 15 made with sterile-filtered seawater [0.2 µm porosity, adjusted with sterilefiltered deionized water to a salinity of 15, which is optimum for TOX-A Pfiesteria spp. from North Carolina estuaries (5,52)] collected in the Atlantic Ocean, 2 km from Beaufort, North Carolina.
We estimated the chromosome number of P. piscicida using confocal laser scanning microscopy [(CLSM); Leica model TSC SP confocal laser scanning microscope equipped with a UV 351-and 363-nm laser, 63× water emersion optics, and a 1.2 numerical aperture; Leica, Solms, Germany] in combination with light microscopy (LM) and transmission electron microscopy procedures (5). CLSM was used to examine in more detail the morphology and localization of zoospore fluorescent cellular DNA structures (53). It provided high-resolution, threedimensional reconstruction of chromosomal DNA by recording and stacking a series of two-dimensional images taken at 1-µm increment depths from the surface of each zoospore examined (n = 12). In addition, the relative DNA content of P. piscicida (14 clones, 12-50 months after isolation from fresh estuarine samples) and P. shumwayae zoospores (9 clones, 8-36 months after isolation) was evaluated using flow cytometry, as mean DNA fluorescence, (compared to chicken red blood cell DNA standard; ≥10 4 zoospores analyzed from each clone) under the G1 peak (54). Samples were preserved with 1% paraformaldehyde, stored for ≥24 hr in darkness at 4°C, treated with 1 µg RNAse A mL -1 for 1 hr at 20°C, and stained with 5 µM SYTOX Green [Molecular Probes, S-7020] for 12-16 hr in darkness at 4°C (54)(55)(56).
Transformations to filose and lobose amoebae from the clonal zoospore cultures were induced in the presence of live fish or their fresh tissues, secreta and excreta. Transformations from filose to lobose amoebae were observed after temperature and salinity shock and by manipulating the type and abundance of prey (7,8), for example, removal of fish and provision of bacteria [Pseudomonas isolate from the mesohaline Neuse Estuary, or the Rhodomonas CCMP757 cloned from commercial material obtained from the Culture Collection for Marine Phytoplankton (CCMP) from Bigelow Laboratory, Bigelow, ME, USA; the cloning procedure removed contaminant eukaryotic algae and bacteria]. Amoeboid cultures were grown at 21°C and a salinity of 15, on a 0 hr:24 hr L:D or 14 hr:10 hr L:D cycle at 80 µmol photons m -2 s -1 .

The Complex Life Cycle and Behavior of Pfiesteria
Following the procedures of Glasgow et al. (5,6) and Burkholder and Glasgow (7,8) as indicated above, we used salinity shock (a sudden increase in salinity of 5-7, 1 hr, n = 10 populations of clonal zoospores, 50 cells population -1 ), temperature shock (a sudden change from 22 to 8-10°C or to 32°C, 1-2 hr, n = 12 populations of clonal zoospores, 50 cells population -1 ), the presence/absence of live fish (O. mossambicus), and the availability of different algal prey species to examine various stage transformations in these isolates (clones #B93BF1 and #416T of TOX-A and TOX-B P. shumwayae, respectively). We compared these observations from similar treatment of isolates of P. piscicida (7,8). As in Burkholder and Glasgow (7) for P. piscicida with finfish and Springer (57)  Filose and lobose amoebae that had transformed from the clonal TOX-A zoospore cultures (P. piscicida, 1993-present; P. shumwayae, 1996-present) were maintained on a diet of Cryptomonas LB2423 or Rhodomonas CCMP757 (8) (after cloning the commercial-source algae in axenic culture; ≤8 cryptomonads were consumed hr -1 amoeba -1 ), or bacteria (Pseudomonas isolate from the mesohaline Neuse Estuary). These amoebae initially ranged from 30 to 70 µm (major cell axis), with a nuclear diameter often approaching 20% of the total cell volume. Under preyreplete conditions, lobose amoebae averaged 17 ± 4 pseudopodia per cell (mean ± 1 standard deviation, n = 100 cells). Locomotion in these lobose amoebae typically occurred from steady cytoplasmic streaming, with slight anterior-lateral bulging. Clonal amoebae [clonal as defined in (5,10,31)] were grown with axenic cryptomonad algal prey at 21°C and a salinity of 15 (51), on a 0 hr:24 hr L:D or 14 hr:10 hr L:D cycle. They were monitored for changes in morphology and size over time in culture. We tested the influence of alterations in prey type/abundance in some subcultures (5,7,8). For example, change from the cyanobacterium Cyanothece [formerly Synechococcus (58); clone HP9001, diameter 1.5 µm, 10 6 cells mL -1 , 5000:1 ratio of prey:amoebae] to cryptomonad prey (e.g., 10 4 Cryptomonas LB2423 mL -1 , 40:1 ratio of prey:amoebae) has been shown to induce some amoeboid cells to produce zoospores (7). However, to retain pure cultures of amoebae without other life stages and thereby avoid the potential for uncertainty in interpretation, subcultures used for molecular probe analyses were maintained long-term (months to years) without altering the algal prey species used. Over these periods of observation, no zoospores were noted in the cultures.
FISH probes have been developed for P. piscicida and P. shumwayae zoospores (45,46). Specific Alexa Fluor fluorophores (Molecular Probes) were used to label an amine-modified oligonucleotide within the hypervariable region of the 18S ribosomal DNA gene (59,60) for the zoospores of each species (P. piscicida: Alexa Fluor 488, absorbance at 495 nm, emission at 519 nm, green fluorescence; P. shumwayae: Alexa Fluor 350, absorbance at 346 nm, emission at 442 nm, blue fluorescence). These probes were tested to assess for reactivity with amoeboid stages from our clonal cultures of P. piscicida and P. shumwayae. PCR testing was performed on amoebae isolates using 40 mL of sample from each clone (densities 800-1,200 cells mL -1 ). DNA was extracted from cultured cells using a DNeasy plant mini-prep kit (Qiagen Corporation, Valencia, CA, USA). Amplification of target DNA was accomplished using a PCR-based procedure (60) with 18S rDNA-based primers that have tested as specific to P. piscicida and P. shumwayae. The amplified DNA was loaded into a 1.5% agarose gel containing ethidium bromide, and gel electrophoresis was mediated by a Tris-borate-EDTA buffer. Electrophoresed products were visualized and archived using a Gel Doc 2000 gel documentation system (BioRad Corporation, Hercules, CA, USA). Analyses were repeated and cross-confirmed by the independent laboratory of P. Rublee.

Functional Types of Pfiesteria: Comparative Response to Algal Prey, Nutrients, and Fish
We examined zoospore production of the three functional types of each Pfiesteria species, using the above clones in three experiments. In the first experiment, we compared zoospore production of the three functional types of Pfiesteria (each Pfiesteria species tested separately) in response to cryptomonad prey: initially in batch culture mode for 6 days, 1:15 ratio of zoospores: prey, with initial zoospore densities of 150 ± 20 cells mL -1 ; algal prey, Cryptomonas sp. LB 2423 [isolated in clonal axenic culture from the multi-species commercial culture source; grown in f/1000 media (50), n = 3; note that a NON-IND isolate was not available for P. shumwayae (33)]. Controls were assessed as zoospores alone (each Pfiesteria sp. separately, no algal prey) and prey alone (no Pfiesteria). In the second experiment, we compared zoospore production of the three functional types of each Pfiesteria sp. in response to N i or P i enrichment + cryptomonad prey (500 µg NO 3 -N or PO 4 -3 P L -1 , 5 days, n = 4; otherwise, same initial conditions as above). Three types of controls were maintained in f/1000 media (50), including a) each Pfiesteria sp. without nutrient enrichment + cryptomonad prey; b) each Pfiesteria sp. + N i or P i enrichment but without cryptomonad prey; and c) each Pfiesteria sp. without nutrient enrich-ment and cryptomonad prey. Algal prey cell production without Pfiesteria ± N i or P i enrichment was also determined.
The third experiment tested Pfiesteria response to fresh fish mucus (minutes) using a microcapillary tube assay (61), wherein fresh mucus and excreta from several fish species were collected using consistent technique, sterile filtered (0.22-µm porosity), and added to replicate microcapillary tubes (aperture diameter ~30 µm; n = 3). The species included in the study were juvenile (all species) tilapia (O. mossambicus, t.l. 5-7 cm), Atlantic menhaden (Brevoortia tyrannus Latrobe, Neuse Estuary, t.l. 13-17 cm), juvenile hybrid striped bass Morone saxatilis × Morone chrysops Rafinesque; t.l. 15-20 cm), and bluegill (Lepomis macrochiris Rafinesque, t.l. 8-11 cm). The cultured fish (all species except Atlantic menhaden) had been fed Tetra Marine fish food once daily prior to collecting excreta and mucus. The mucus and excreta were sterile-filtered immediately upon collection (including filtration of the menhaden materials in the field) and were separated from the live animals ≤3 hr prior to testing with Pfiesteria spp. The responses of the three functional types of P. piscicida were assessed as net entry of zoospores into the tubes over 10min trials [10 3 -10 5 cells mL -1 , normalized to 10 4 cells mL -1 for comparative purposes using a linear normalized term determined by least-squares regression and verified by ANOVA (62)], from analysis of videotapes [Olympus AX-70 research light microscope, 600×, water immersion objective (Olympus); and a cooled-chip CCD video camera (Optronics Corp.); S-VHS video recorder (model SVO-9500MD, Sony); and 53-cm video monitor (Sony)]. Control tubes contained sterile-filtered seawater at a salinity of 15. For each experiment, one-way ANOVA was used to test for differences between controls and treatments, and multifactor ANOVA was used to compare replicates and test for differences among functional types. All probability values (p) were considered significant at p < 0.05 (62).

Toxic Pfiesteria Complex Species and Life Stages
The toxic Pfiesteria complex currently includes two species, P. piscicida and P. shumwayae (4,5). Both are heterotrophs (6,7,8,31), but zoospores are capable of photosynthesis when they retain kleptochloroplasts from algal prey (5,63). These species can be distinguished by the plate tabulation of the zoospore stages (Figure 1). The sutureswollen zoospores of the P. piscicida isolates (toxic and noninducible strains) in this study, as in previous research, yielded a plate tabulation of Po, cp, X, 4´, 1a, 5´´, 6c, 4s, 5´´´, 2´´´´, with a three-sided anterior intercalary plate ["a"; as in (4)] (Figure 1). The plate tabulation for zoospores of the P. shumwayae isolates (toxic and noninducible strains) was Po, cp, X, 4´, 1a, 6´´, 6c, 4s, 5´´´, 2´´´´, with a distinct four-sided "a" plate because of the additional precingular "c" plate (5). There are probably other toxic species of Pfiesteria not yet described (7,31). For example, a tropical species with a plate formula identical to that of P. piscicida, but with certain morphological differences has been reported from aquaria with fish kills (64). However, in that case, parasitic dinoflagellates were also present, which could have caused the fish death, and the culture was not cloned or formally tested for ichthyotoxicity.
Zoospores of Pfiesteria spp. varied considerably in size and shape (diameter usually 7-14 µm, but with a known range of ~3-24 µm, n = 2125), depending on the stage of origin and feeding activity (5,7,23). The epitheca and hypotheca were equal, subequal, or distinctly unequal in size, with the epitheca slightly to substantially larger. The flagella had a similar construction and insertion as has been described for other peridinoid species (65), including a helical transverse flagellum and a thick longitudinal flagellum (length ~20 µm, width ~0.1-0.2 µm, n > 420; mastigonemes were sometimes observed).
Other research has shown that the two Pfiesteria morphospecies are closely related. The isolates of P. piscicida and P. shumwayae examined in this study and in previous research differed by approximately 45 base pairs in their 18S rDNA sequence (5,47). The chromosome number of P. piscicida TOX-A and TOX-B zoospores (from clones isolated within ≤3 months from estuarine habitats) was 23 ± 2, as determined using light and epifluorescence microscopy, confocal microscopy, and transmission electron microscopy procedures (31). The small size and the thin but tough and highly impermeable theca have prevented use of stains and Environmental Health Perspectives • VOLUME 109 | SUPPLEMENT 5 | October 2001  (36), and Dodge (67). Flow cytometric procedures indicated that the DNA content of recent isolates of P. shumwayae TOX-A and TOX-B zoospores (clones isolated within ≤3 months from estuaries) was more than 2-fold higher than that of P. piscicida zoospores (23 clones tested; >10 4 zoospores per analysis) (54).
Thus, the chromosome number of Pfiesteria spp. is intermediate between the chromosome numbers of parasitic dinoflagellates (4-8 chromosomes) and some free-living species (≥20), on the one hand, and certain photosynthetic, free-living dinoflagellates with ≤270 chromosomes on the other (68). Over time in culture, some clonal zoospore cultures have become more variable in chromosome content. Although we have noted increasing ploidy over several months in TOX-A zoospores (31) [as observed in photosynthetic dinoflagellate species (36)], comparisons of DNA content of nine P. piscicida clones of varying age (1-4 years) have indicated that the DNA content can significantly decrease over longer periods in culture. This point is of interest, as toxin-producing capability of many toxic Pfiesteria clones has been lost over time in culture. We hypothesize that heterotrophic dinoflagellates, which likely require a more complex diet than obligate photosynthetic dinoflagellates, may more commonly lose than gain DNA when cultured for extended periods, as the artificial media probably is missing required substrates. It is not yet known whether the decrease in DNA content of cultures maintained for years reflects loss of genes involved in toxin production, as has been found for certain toxic fungi (35).
TOX-A zoospores are the most lethal stage in the life cycle of both Pfiesteria spp. (5-7,31) ( Figure 2). In standardized fish bioassays (34), TOX-A zoospores have been lethal to fish in densities ≥300 cells mL -1 , with time to death of juvenile tilapia species (O. mossambicus, O. aureus, Tilapia nolotica, t.l. 5-7 cm) ranging from 20 min (>3 × 10 2 to 10 3 toxic zoospores mL -1 ) to 12 hr (10 3 -10 4 toxic zoospores mL -1 ), indicating highly to weakly toxic isolates, respectively (6,7,33,52). In most Pfiesteria-related estuarine fish kills, zoospore densities have been at > 3 × 10 2 to 5 × 10 3 cells mL -1 (range up to 1.1 × 10 5 cells mL -1 ) (7,16,31). Whereas some Pfiesteria isolates have been tested as capable of killing fish whether allowed direct contact with the prey (Figure 3) or maintained within dialysis membrane (molecular weight cut-off 12,000-14,000 Da) to prevent direct contact (7,57), others have killed only when allowed direct contact with the prey. The two species thus far have been shown to produce analytically comparable toxin (12), but considerable intraspecific differences among isolates apparently occur in toxin potency and in the extent to which toxin is released versus retained within the cells. A mechanism for Pfiesteria toxin impacts on fish and mammals has been described from experiments with clonal, toxic cultures (crosscorroborated by independent specialists) wherein the toxin mimics an ATP neurotransmitter that targets P2X 7 receptors (11). The cultures used for that research were tested as capable of killing fish when prevented from direct contact with prey (7,57). The mechanism of targeting P2X 7 receptors and the cascade of impacts (including extreme response to inflammation) that followed would be optimized with physical abrasion or damage (11). Thus, physical attack by toxic Pfiesteria zoospores may help to promote entry and damage by the toxin in fish tissues. Alternatively, for some Pfiesteria isolates, close proximity to fish may be required to stimulate toxin release, and/or external tissue damage or wounding may create areas where the toxin enters the fish.
TOX-A zoospores were observed to be produced by chrysophyte-like cysts (stage #11A), coccoid cysts (stage #6A), coccoid cells (stage #7A), benthic or suspended palmelloid masses (stage #5A, ranging from 6 to ≥32 coccoid cells), and planozygotes (stage #4) in the presence of live fish or their fresh materials (tissues, excreta, secreta) ( Figure 2). They were produced following detection of materials from live fish by TOX-B zoospores. TOX-B zoospores were produced by stages to which they can directly [chrysophyte-like (chryso) cyst, temporary cyst, palmelloid mass-all haploid stages] or indirectly transform [from coccoid cells excysted from another cyst stage (both haploid) produced by TOX-B zoospores]; by large diploid lobose amoebae; and by diploid hypnozygotes. TOX-B zoospores were also formed from gametes through reversion of "-" anisogamous gametes (Figure 2) or of isogamous gametes (possible in both Pfiesteria spp.) when the signal from live fish that stimulates toxicity is no longer detected. In asexual reproduction TOX-A and TOX-B zoospores of P. piscicida either divide while swimming or form palmelloid masses where the cells undergo mitosis and cytokinesis. The latter pathway is more common in P. shumwayae. Most amoeboid stages in the complex life cycles of Pfiesteria spp. occur in actively toxic cultures, or in cultures that recently (hours to days) were in actively toxic mode. TOX-A zoospores can produce haploid filose, lobose, and rhizopodial amoebae, whereas thus far TOX-B zoospores have been verified to produce only filose and lobose amoebae (Figure 2). TOX-A and TOX-B zoospores were identical in appearance to isogamous gametes (diameter 7-9 µm; n = 320), although their size varied four-fold depending on feeding activity. In strains with anisogamy, the smaller "+" anisogamous gametes were 4-5 µm in diameter (n = 280) and had a much longer longitudinal flagellum than the larger "-" anisogamous gametes (diameter 7-9 µm; n = 280). Gamete fusion in the presence of live fish or their fresh excreta, secreta, and/or tissues produced a triflagellate, diploid planozygote (diameter 14-60 µm, n = 125) with one transverse and two longitudinal flagella. The planozygote produced four zoospores or, alternatively, formed a hypnozygote (sexual resting cyst). Following a period of dormancy, four haploid zoospores emerged from the hypnozygote in P. piscicida or, alternatively in (P. shumwayae), two triflagellate cells (each with one transverse and two longitudinal flagella) emerged from the hypnozygote, and these each rapidly produced two biflagellate zoospores (each with one transverse and one longitudinal flagellum).
Amoebae in both Pfiesteria spp. were also produced by gametes, planozygotes, or cysts, resulting in considerable size variation (5,7) ( Figure 2). The fine structure of the amoebae of Pfiesteria spp. is currently under examination and is less well known than that of the flagellated stages (which include the most toxic forms) or the cysts derived from them (31). Pfiesteria spp. amoebae had a normal eukaryote nucleus (diameter 3-11 µm, n = 75), with a double-membrane envelope. The chromosomes were not condensed during mitotic interphase (4,5), also reported for other dinoflagellate amoebae (21,71-79) (below). Depending on the clone, the food source, and environmental conditions, Pfiesteria amoebae included filose and/or lobose forms (both species) as well as rhizopodial forms [found thus far in P. piscicida (7)], with a smooth or rough outer covering (4,5,7,8). Transformations to amoeboid stages were common in some clones with live fish prey, but were rare in algal-fed clones. Amoebae were produced mostly during or following exposure to fresh fish materials, although filose amoebae were also sometimes produced by nontoxic cultures. The amoeboid stages generally ranged from 5 to 120 µm (n = 1,830) in maximum cell dimension, depending on the stage of origin (Figure 2), but like certain Sarcodinian amoebae (80), they sometimes grew much larger within ≥12 months [from ~40 µm initially to a maximum of ~750 µm in P. piscicida (n = 2 cultures) and from ~40 µm initially to a maximum of ~250 µm in P. shumwayae (n = 3 cultures); 70-90 cells measured initially in each culture and at ~6-month intervals]. By comparison, the largest Pfiesteria amoebae that have been observed in fish-killing cultures 4-20 hr after fish death in this as well as previous research were approximately 120 µm on the major cell axis (7,8). The cultures also sometimes became multinucleate over time, probably as a cultural aberration; this phenomenon of uninucleate amoebae becoming multinucleate over long-term culture has also been reported for various Gymnamoebae (80). Filose amoebae generally were observed as ephemeral stages lasting minutes to hours. However, some clonal Environmental Health Perspectives • VOLUME 109 | SUPPLEMENT 5 | October 2001 Figure 2. Schematic of the complex life cycle of Pfiesteria shumwayae as presently understood, showing stages and pathways that have been verified (solid lines) for toxic strains of P. shumwayae, and shared in common by P. piscicida. It should be noted that P. piscicida has been under study for a longer period, and several additional stages and pathways have been verified for that species (7). The pathways indicate the presence (+) versus the absence (-) of live finfish; AL = presence of cryptomonads and certain other algal prey; N = nutrient enrichment as organic and/or inorganic N and P; S = environmental stressor such as sudden shift in temperature or salinity, physical disturbance, or prey depletion). Dashed lines = hypothesized pathways. Stages have been conservatively numbered to facilitate description. Also note that varied approaches have been used for numbering stages. For example, complex life cycles of other dinoflagellates have sometimes numbered morphologically identical stages at each pathway or sequence of occurrence (21) which, if applied in this schematic, would result in at least nine additional stages. TOX-B zoospores [haploid; ploidy confirmed as in Burkholder (31)] are the temporarily nontoxic functional type in the absence of live fish prey [referred to as nontoxic zoospores in the life-cycle schematic previously published for toxic strains of P. piscicida (7)], although they can carry residual toxicity (57). TOX-B zoospores become TOX-A zoospores and produce toxin when sufficient live fish are added (7,32). As TOX-A and TOX-B zoospores are actually the same cells in the presence versus the absence of live fish, to stress that point we have designated morphologically identical stages as TOX-A versus TOX-B zoospores, or derived from those zoospores, with the same number followed by "A" or "B" (stages #1, 2, 5-12). TOX-B zoospores produced from diploid amoebae (stage #17) or hypnozygotes (stage #20) (rather than from stages directly derived from TOX-A or TOX-B zoospores) have been numbered as additional stages (stages #19, #21) in recognition of their distinct origin. TOX-A zoospores (stage #1A) can transform to filose (stage #8A) and lobose (stage #9A) amoebae (maximum cell dimension 15-60 µm). TOX-B zoospores (stage #1B) can transform to filose (stage #8B) and, less commonly, to lobose state #9B amoebae of similar size as those transformed from TOX-A zoospores. Planozygotes (stage #4) can transform to larger filose (stage #16) and lobose (stage #17) amoebae (maximum cell dimension 40-120 µm). Small filose (stage #13) and lobose (stage #14) amoebae (length 5-10 µm) can also be produced by gametes. Cysts include stages with a) roughened or reticulate covering (from amoeboid stages; haploid except when derived from diploid amoebae; stages #10A,B, #15, #18; diameter 4-30 µm); b) scaled covering ± bracts; from TOX-A and TOX-B zoospores; these chrysophycean-like cysts (stages #11A,B) are 4-25 µm (rarely 30 µm) in diameter and can lose their bracts (stages #12A,B) and scales over time so that they have a smooth covering. Also note that transitional forms to these cysts are not shown but occur in P. shumwayae as in P. piscicida (7); and c) hyaline covering [small cysts that can divide as in Spero and Morée (69)], with darkened contents; and hypnozygote (diploid cyst at lower right). Zoospores and gametes also form temporary cysts with thick mucus covering, which may settle out of the water column (arrows not shown).  (4,5,7,31). Thus, overall, the stages observed so far in the complex life cycle of P. shumwayae were similar to those of P. piscicida (5)(6)(7)(8)(9) and include an array of flagellated (biflagellated zoospores and gametes, triflagellated planozygotes; diameter of flagellated stages ranging from 8 to 24 µm; n = 4,050 cells measured), amoeboid {lobose, filose, and less commonly observed rhizopodial stages [but see Marshall et al. (9)]; maximum length 7-120 µm observed thus far [n = 1,200], depending on the stage of origin}, and cyst forms (diameter 4-25 µm; n = 620) (Figure 2). Similarly, as shown for P. piscicida in previous research (7,61), P. shumwayae (at typical field densities) exhibited strong attraction to live fish (here, sheepshead minnow larvae) and their fresh tissues, followed by extension of the peduncle and feeding via myzocytosis (70) (Figure 3). P. shumwayae was also similar to P. piscicida (7,8) in that sudden salinity or temperature shock sometimes promoted transformations of TOX-A zoospores to cysts; sexual reproduction with planozygotes was observed in the presence of live fish; and removal of live fish in the presence of high abundance of algal or bacterial prey caused most TOX-A zoospores to transform to amoebae within 24 hr (5) (Figure 4). Alteration of algal prey type, especially a change from cryptomonad to cyanobacterial prey, induced some TOX-B zoospores from the clone tested (that had been in TOX-A fish-killing mode only 3 weeks previously) to transform to filose and lobose amoebae. When TOX-B zoospores were given different algal prey, the amoebae:zoospore ratio remained low with cryptomonads ( ~95% of the dinoflagellate population as zoospores), intermediate with Thalassiosira (70% zoospores), and lowest with Cyanothece (42% zoospores) ( Figure 5).
Other estuarine Gymnamoebae [families Paramoebidae, Thecamoebidae according to Patterson (80); isolates 272A, 471A, 472A, 480A, 574A, 598A, 612A, 617A, 666A, and 872A from the Neuse and Pamlico Estuaries in North Carolina, and the Pocomoke Estuary in Maryland] were identical in appearance to certain Pfiesteria amoebae.  . These amoebae [confirmed in our laboratory with PCR and FISH probes specific to P. shumwayae and developed from zoospores; cross-corroborated with PCR and FISH testing by P. Rublee and co-workers (45,46)] were photographed from clonal cultures derived from clonal zoospores of P. shumwayae. They were maintained consistently on cryptomonad prey for 6 months, over which time no zoospores were observed. These amoebae had been isolated and cloned from the Neuse Estuary (mesohaline segment near Minnesott Beach). They had not been derived from clonal Pfiesteria spp. zoospores isolated from the same area in the Neuse Estuary, and they did not react with the Pfiesteria species-specific PCR and FISH probes (Figure 7).

Response of Functional Types of Pfiesteria to Nutrients, Algal Prey, and Fish
In experiment 1 testing the response of functional types of Pfiesteria spp. to algal prey, controls of all functional types of both Pfiesteria spp. (without cryptomonad prey) showed negligible zoospore production, and most (~95%) of the zoospores encysted (33). With cryptomonad prey, zoospore production was highest in the NON-IND culture; Figure 8). TOX-B cultures of both Pfiesteria spp. were intermediate in zoospore production, and there was only a slight increase in zoospore abundance above that of controls in the initial TOX-A cultures (P. piscicida and P. shumwayae, tested separately; p < 0.05).
In experiment 2 with N i or P i , additions ± cryptomonad prey, after 5 days all three functional types of P. piscicida and P. shumwayae zoospores significantly increased in abundance within the (+ cryptomonads -nutrient) controls [from ~150 zoospores mL -1 initially to 1.5-2.2 × 10 3 zoospores mL -1 (recently TOX-A) to 1.1-4.2 × 10 4 zoospores mL -1 (TOX-B, NON-IND); p < 0.05]. In the (-cryptomonads + N i or P i ) controls, zoospores of TOX-B and NON-IND functional types also increased (to 0.9-4.2 × 10 2 cells mL -1 ; p < 0.05). However, in the (-cryptomonads -nutrients) controls, all functional types of both Pfiesteria spp. showed Environmental Health Perspectives • VOLUME 109 | SUPPLEMENT 5 | October 2001  Influence of algal prey type on the ratio of amoebae to zoospores in clonal cultures of P. piscicida (TOX-B functional type, potentially toxic but without fish prey, previously grown on Cryptomonas LP2423 for 3 months; batch culture mode). Algal prey (single-species trials; normalized by total biovolume) included the cyanobacterium Cyanothece (Cyano, clone HP9001), cryptomonads Cryptomonas sp. (Crypto, cloned from commercial, multialgal species culture #LP2423) and Rhodomonas sp. (Rhodo, cloned from commercial, multialgal species culture CCMP757), and centric diatom Thalassiosira weissflogii Fryxell & Hasle (Thalas, cloned from commercial, multialgal species culture CCMP 1335). Note that when grown on cryptomonad prey, P. piscicida populations from this clone were dominated by zoospores, which comprised approximately 95% of the cells. In contrast, when given the cyanobacterium as prey, the proportion of the Pfiesteria populations as amoebae increased so that there were nearly twice as many amoebae as zoospores. An intermediate amoeba: zoospore ratio was obtained when P. piscicida was fed the centric diatom. . These clonal cultures were evaluated to assess whether they were Pfiesteria spp. life stages, using 18S rDNA molecular probes that were developed by P. Rublee and co-workers (45,46). The positive control was a lobose amoebae culture derived from clonal zoospores of P. piscicida. Of the 19 amoebae clones tested, 4 showed positive bands using PCR amplification (small white arrows). Analysis of all PCR-positive cultures was repeated in-house and was cross-confirmed by P. Rublee with identical results. negligible zoospore production and most of the populations encysted. In treatments with cryptomonad prey + N i or P i enrichment, negligible increase in zoospore production over that of the (+ cryptomonad -nutrient) controls was detected for the initially TOX-A functional type in both Pfiesteria spp. ( Figure  9). The greatest response to cryptomonad prey + nutrient enrichment was shown by the NON-IND functional type (both Pfiesteria spp.; p < 0.01), with intermediate zoospore production by TOX-B zoospores (p < 0.05) in response to N i or P i enrichment with cryptomonad prey.
In experiment 3 testing short term, response of TOX-A zoospores to fish (as net entry of into microcapillary tubes filled with fresh, sterile fish mucus and excreta; each Pfiesteria sp. tested separately) was significantly higher than that of either the TOX-B or NON-IND zoospores (data normalized for cell density; p < 0.01) ( Figure 10). The data indicate significantly stronger chemosensory attraction of the TOX-A zoospores toward fish materials in these 10-min trials (61). The behavioral attraction response of each functional type to the sterile-filtered mucus and excreta was similar regardless of the fish species of origin.

Discussion
The Pfiesteria issue (1,7,16,17,82,83) has led to recent focus on various small, poorly described, cryptic estuarine gymnodinioid-appearing (actually including thinly armored peridinoid) dinoflagellates as pfiesteria-like because of superficial resemblance to Pfiesteria spp. Some of these look-alike species have been found to have complex life cycles similar to those of Pfiesteria, with amoeboid stages and ambush-predator behavior toward algal or ciliate prey (22,76).
In previous research with actively toxic cultures, we confirmed an array of amoeboid stages in the complex life cycles of P. piscicida (7,8) and P. shumwayae (5). We noted that flagellated stages (zoospores, gametes and, to a lesser extent, planozygotes) from TOX-A cultures were most active in transforming to filose, lobose, and rhizopodial stages. Filose, lobose, and rhizopodial stages of P. piscicida to Rhodomonas prey in 6-day trials, additionally compared to the response of a cryptoperidiniopsoid species, which has not yielded ichthyotoxic activity in repeated fish bioassay tests as whole or sonicated cells (5). NON-IND P. piscicida and cicida the cryptoperidiniopsoid species attained highest zoospore production on algal prey, with less cell production by TOX-B and TOX-A Pfiesteria spp., respectively. Note that controls (each Pfiesteria species tested without algal prey) showed negligible zoospore production in the absence of an abundant prey source. Data are given as means ± 1 SE; n = 3 [modified from Parrow et al. (33)]. Figure 9. Zoospore production of the three functional types of P. piscicida and P. shumwayae (each species and each functional type tested separately) in response to N i or P i enrichment + N-and P-limited Cryptomonas prey (500 µg NO 3 -N or PO 4 -3 P L -1 , 5-day trials in batch culture mode; 1:15 ratio of zoo-spores:prey, with approximately 150 zoospores mL -1 at T 0 ). In treatments with N i or P i enrichment + algal prey, TOX-A zoospores (both species) showed negligible increase in production over that of the [+ algal prey -nutrient] controls. Highest response to nutrient enrichment along with cryptomonad prey was shown by NON-IND Pfiesteria, with TOX-B zoospores intermediate in nutrient stimulation (p < 0.01). Data are given as means ± 1 SE (n = 4). Time (days) Relationship between the rates of entry into micro-capillary tubes containing fish materials, and zoospore culture densities (10-min trials, salinity 15). The three functional types of each Pfiesteria species were tested separately in response to an extract of sterile-filtered mucus and excreta from tilapia (Oreochromis mossambicus) in microcapillary assays (see text; n = 129). TOX-A and TOX-B zoospores showed 3-to 5-fold higher attraction to the fish materials in comparison to their response to microcapillary tubes filled with sterile-filtered seawater (salinity 15). NON-IND zoospores of both Pfiesteria spp. were least responsive to the fish materials. The data indicate that the chemosensory attraction by zoospores toward fish materials is a biological phenomenon, independent of zoospore cell density within each functional type. (B) Attraction of zoospores of the three functional types of each Pfiesteria species (tested separately) to a mixture of sterile-filtered mucus and excreta from several finfish species (tested separately) in microcapillary assays [10-min trials, salinity 15; n = 4 per isolate tested; modified from Cancellieri (61)]. TOX-A and TOX-B zoospores consistently exhibited significant attraction to the fish materials relative to zoospore activity toward sterile-filtered seawater controls, whereas NON-IND populations showed much lower attraction to the fish excreta and mucus (p < 0.01). The chemosensory attraction by zoospores toward fish materials decreased with increasing duration of separation from live fish prey. Modified from Cancellieri et al. (61).  and an amoeboid protoplast emerged that moved across the culture dish, changing from spherical to oval in form. Rhizopodial amoebae of varying size were observed to form inside the common theca of a vegetative cyst from this dinoflagellate, and rhizopodial amoebae were released upon rupture of the theca. These amoebae were motile, consumed bacteria, and underwent asexual reproduction. Lobose amoebae of various sizes were also observed in this life cycle (78,79). Overall, Buckland-Nicks and co-workers (77)(78)(79) reported lobose, rhizopodial, and spheroid amoeboid stage, a vegetative dinokaryon, dinospores (zoospores) with condensed chromosomes, and amoeboid resting cyst. The biochemical and ultrastructural mechanisms whereby the dinoflagellate zoospore mesokaryote nucleus with permanently condensed chromosomes can change [in Pfiesteria, sometimes within minutes (7)] to the eukaryote nucleus of amoeboid stages has not been examined in detail. Fensome and co-workers (73) noted that the condensed spiral structure of the chromatin varies during the life cycles of certain other dinoflagellate species. Chromosomal features in the heterotrophic dinoflagellates Noctiluca (84), Blastodinium (84,85), and Oodinium fritillariae (74), and in the mixotrophic predaceous dinoflagellate Stylodinium sphaera (75) have been observed to change from eukaryote to mesokaryote as swarmers are produced or as the reproductive cyst divides. For example, Timpano and Pfiester (75) described at least three distinct life stages in S. sphaera, including a phagocytic amoeba, an immobile reproductive cyst that could be produced by the amoeba, and gymnodinioid swarmers. Although the nuclear status of the amoeba stage was not ascertained, the nuclei of the multinucleate reproductive cyst from which the mesokaryotic gymnodinioid swarmers emerged were eukaryotic with dispersed chromatin material. In H. ichthyophilum ectoparasite of sticklebacks, Buckland-Nicks et al. (77,79) reported that the typical dinokaryon nucleus of the vegetative cyst transformed to a eukaryotic nucleus in the lobose amoebae. From research with the dinoflagellate, O. fritillariae (marine ectoparasite of Appendicularians), Cachon and Cachon (74) reported that the nucleus of young, attached ectoparasites had condensed, rodlike chromosomes similar to those of the free-living gymnodinioid stage. However, as growth of the trophont (attached parasitic) stage progressed, the nucleus became increasingly homogenous as in eukaryotes. When Oodinium left the host, nuclear reorganization processes occurred rapidly, corresponding to a peculiar prophase of the first sporogenic division. A conspicuous fusorial system appeared between two archoplasmic areas that were responsible for offspring chromosome segregation. The nuclear envelope remained intact, while the fusorial microtubules were attached to the nucleus at distinct, kinetochore-like structures. As the chromosomes became more condensed, the kinetochore-like formations disappeared. In ongoing research in our laboratory, we have begun to document in detail the ultrastructural changes in the nucleus of Pfiesteria spp. during zoospore-amoeba transformations.
We additionally hypothesize, as in previous work (7,8), that certain amoebae previously described as Sarcodinian or Gymnamoebae (80), without linkage to dinoflagellates, are actually dinoflagellate amoebae or are morphologically identical to dinoflagellate amoebae. As noted, we have maintained filose and lobose amoeboid cultures of both Pfiesteria species for months to years. Thus, although filose and lobose stages may sometimes function as transitional forms, they are not merely ephemeral but, rather, can be sustained stages of Pfiesteria. Moreover, throughout the extended periods in which we have maintained amoeboid cultures of Pfiesteria spp., we have not observed any zoospores in the cultures as mentioned. Zoospore production can be induced in some Pfiesteria amoebae by suddenly altering the available prey. For example, production of TOX-A zoospores has been induced in some Pfiesteria amoebae by adding live fish (7). As another example, when amoebae transformed from recently TOX-A zoospores are switched from Cyanothece prey to cryptomonads, we have observed large lobose amoebae (length ~50-70 µm or more) each produce four zoospores, followed by disintegration of the remainder of the amoeba cell. With increasing duration in culture, however, Pfiesteria amoebae appear to lose the ability to produce zoospores.
The array of amoeboid stages in the complex life cycles of Pfiesteria spp. would be identified within at least eight different genera of Sarcodinian amoebae using standard amoebae keys [e.g., Patterson (80)], indicating a need for reevaluation of the systematics of estuarine amoebae to include consideration of dinoflagellates. Molecular-based identification assays will be valuable in such efforts, as SEM often cannot discern among species of dinoflagellates or of amoebae (dinoflagellate or otherwise), within the same genus (31,80). Subtle differences in cell covering (e.g., scales) may be perceptible with SEM, but in attempts to differentiate among amoeboid species, nonmorphological characteristics (e.g., mode of locomotion, granuloplasmic inclusions) have been used in traditional amoebae keys [(80)]. A recent quote from amoebae specialists (86) succinctly framed the overall problem: The classification of the free-living amoebae is a most contentious area. . . . It is now evident that the free-living amoebae have evolved along many different lines. . . . There is striking evidence that various taxa regarded as species are in fact polyphyletic.
Ultimately, reevaluation of the systematics of estuarine amoebae to include dinoflagellates of similar appearance as various Gymnamoebae will likely depend in large measure on molecular and biochemical techniques.
Of approximately 3,000 dinoflagellate species, only about 55 are known or Environmental Health Perspectives • VOLUME 109 | SUPPLEMENT 5 | October 2001 VOLUME 109 | SUPPLEMENT 5 | October 2001 • Environmental Health Perspectives suspected as toxic (1,2,24). Repeated tests (standardized fish bioassays) on estuarine samples from eight states (New York to Alabama) thus far have revealed only one additional toxic Pfiesteria / Pfiesteria-like species, P. shumwayae (5)(6)(7)9,31,32,52), indicating, as expected, that complex life cycles and attack behavior are more common than toxin-producing capability. We predict that additional toxic Pfiesteria / Pfiesteria-like species will be detected, although there probably will be few toxic forms among the many species of small, cryptic estuarine dinoflagellates that remain to be described.
This study has shown that functional types of Pfiesteria spp. are strikingly different in response to nutrient enrichment, algal prey, and fish prey. Within each of the two species known to date, the strains have shown high variability in physiological characteristics, and range from highly toxic to benign. Some strains produce potent ichthyotoxins that are lethal to fish [patenting process initiated on a water-soluble Pfiesteria toxin that has been isolated and purified (12)]. Some toxic strains can kill fish when prevented from direct contact with the prey, whereas others apparently require close contact. All three functional types can cause death of larval fish by physical attack as well, although this behavior is much more pronounced in toxic strains (31).
The data have important implications for policy considerations. Because of the high variability in toxicity among Pfiesteria strains [also characteristic, but unfortunately overlooked, for many other toxic algae (25)(26)(27)(28)(29)(30)], federal and state agencies charged with managing natural resources and public health should require that reports of toxic Pfiesteria from any laboratory are both replicated (for internal confirmation) and, importantly, also cross-corroborated by an independent specialist with demonstrated expertise in culturing toxic Pfiesteria (7,9,11,31,63) as standard quality control/assurance procedure (34). This scientifically sound practice of crosscorroboration by another independent laboratory should also be followed for reports of other toxic algae. Agencies additionally should note that a national science panel (49) recently critically reevaluated the peerreviewed literature on actively toxic Pfiesteria and supported as valid published findings (7,9,11,63) on the ichthyotoxicity and toxic, fish-killing activity of verified, cross-corroborated toxic Pfiesteria strains. As has been clarified for findings of toxic Pfiesteria after independent cross-corroboration (7,9,11,31,63), findings of "no toxic Pfiesteria" should be clarified as valid for [only] the strains tested, rather than being applied as a general statement regarding Pfiesteria spp. (49).
As additional policy considerations from these data and related studies (e.g, 7,22,87,88), both Pfiesteria species thrive in eutrophic estuaries, and both can be stimulated by nutrient enrichment (49). TOX-A zoospores of these heterotrophic dinoflagellates, found where excreta and leached fish materials are abundant [with high dissolved organic N and P; (7,22,23,31)], would not be expected to be stimulated directly by inorganic nutrient enrichment (31). TOX-B zoospores retain kleptochloroplasts from algal prey and thus can adopt a plant-like nutritional mode including stimulation by inorganic nutrients (6,87). Among the three functional types, NON-IND zoospores may have highest reliance on kleptochloroplasts (33). Although nutrient stimulation effect is highest for NON-IND Pfiesteria, use of such cultures would significantly bias in favor of a higher nutrient stimulation effect, relative to the response of toxic strains. Therefore, TOX-B Pfiesteria should be used in tests to determine influences of anthropogenic nutrient sources on these dinoflagellates. Toxic Pfiesteria strains cannot be maintained in commercial culture clearinghouses (lacking biohazard BSL-3 facilities). The data from this study and from related research (5,6,9,31) indicate that NON-IND strains should be avoided in research to gain insights about environmental controls on toxic Pfiesteria and about impacts of (toxic) Pfiesteria on fish and mammalian health.