Pfiesteria piscicida Steidinger et al., 1996
Species Overview:
Pfiesteria piscicida is a putatively toxic dinoflagellate species with flagellated and cyst stages. This species, dubbed the 'ambush predator', was first observed in the Pamlico Sound, North Carolina, USA, in 1991 after a massive fish kill. Pfiesteria piscicida has been associated with fish kills, and then feeds on the dead prey (Burkholder et al., 1992, Burkholder et al., 1995, Steidinger et al., 1996).
Taxonomic Description:
Pfiesteria piscicida is a polymorphic and multiphasic dinoflagellate species with a number of unicellular stages throughout its life cycle: bi- and triflagellated zoospores, and nonmotile cyst stages. Within the different life stage forms there is a wide range in size and morphology (Steidinger et al., 1996).
The flagellated stages are small and oblong thecate cells that resemble gymnodinioid cells, although they are actually small cryptic peridinioid cells (Figs. 1-3). The biflagellated stages, zoospores, have two size groups: 5-8 µm (gametes) and 10-18 µm (vegetative)(Fig. 3). The larger triflagellated stage, 25-60 µm, is a planozygote with the features of a vegetative cell along with one transverse and two longitudinal flagella (Fig. 4). Cyst stages, with highly resistant cell walls, range in size from 25-33 µm (Fig. 5). The flagellated forms are typically planktonic and ephemeral, whereas the cyst stages are benthic (Steidinger et al., 1996).
Thecal Plate Description:
The biflagellated stages of P. piscicida have thin thecal plates with a plate formula unique to the Dinophyceae: Po, cp, X, 4', 1a, 5'', 6c, 4s, 5''', 2'''' (Figs. 7-10). Raised sutures designate plate tabulation (Fig. 6). Thecal nodules border plate sutures (Figs. 7,9). Theca is smooth with scattered pores; trichocysts are present. The epitheca is equal to or exceeds the hypotheca in height (Figs. 1,6). The apical pore complex (APC) houses a broadly ovate apical pore plate (Po) and closing plate (cp) (Figs. 7,8). The elongate canal plate (X plate) is at a slight angle to the APC (Figs. 7,8). The first apical plate (1') is rhomboid in shape (Fig. 8). The broad and shallow cingulum is without lists, and descends almost 1 time its width. The sulcus is excavated, without lists, descends to the right, and slightly invades the epitheca via the anterior sulcal plate (s.a.)(Figs. 1,9) (Steidinger et al., 1996).
Morphology and Structure:
P. piscicida exhibits a number of different life cycle stages. This species uses both heterotrophic and mixotrophic nutritional modes depending on the life stage. Flagellated stages are mixotrophic: they use a peduncle (Figs. 1,2) to capture and ingest prey (myzocytosis), and kleptochloroplasts (chloroplasts retained from ingested algal prey) to photosynthesize when prey supply is low. Large food vacuoles are often found in the epitheca, the mesokaryotic nucleus is located in the hypotheca (Schnepf et al., 1989, Elbrächter, 1991, Fields and Rhodes, 1991, Stoecker, 1991, Steidinger et al., 1996, Lewitus et al., 1999).
Reproduction:
Biflagellated zoospores reproduce asexually via temporary cysts. Sexual reproduction has also been documented for this species: biflagellated zoospores produce anisogamous gametes (Fig. 3), which fuse to produce triflagellated planozygotes (two longitudinal flagella and one transverse) (Fig. 4). Sexual and asexual reproduction can occur on either a fish or algal diet (P.A. Tester, pers. com.).
Species Comparisons:
P. piscicida is a distinct free-living estuarine dinoflagellate (Fensome et al., 1993, Burkholder and Glasgow, 1995, Burkholder and Glasgow, 1997).