Esoptrodinium/Bernardinium dinoflagellates Selleckchem EGFR inhibitor were described in early field observations as containing relatively large brown or dark red spherical masses in the episome that were not interpreted to be food bodies at the time (Thompson 1951, Javornický 1962, 1997). In fact, the type species E. gemma was so named because of its possession in the episome of large (gem-like) red bodies described as “the most spectacular objects in an otherwise colorless, clearly transparent plasma” (Javornický 1997). These large pigmented bodies were most likely food vacuoles containing ingested red-pigmented cryptophytes

or other microalgae. Observations reported here demonstrate that Esoptrodinium can feed upon a variety of freshwater protists similar in size to itself, indicating a prey generalist strategy. Yeast and ciliate cells were only ingested (or partially ingested) after they were freeze-injured, suggesting prey detection can occur through a generalized chemosensory mechanism directed toward injured or dying prey (Spero and Morée 1981). This is more typically a behavior exhibited by dinoflagellate species that feed by myzocytosis, the suctioning of cell contents through an extensible feeding tube (Elbrächter 1991b, Hansen and Calado 1999), and may have represented an artifact of unnatural culture conditions. Among tested prey taxa, Esoptrodinium seemed to prefer as food

photosynthetic flagellates similar to somewhat smaller in size to itself ioxilan (e.g., Chlamydomonas MK-2206 and Cryptomonas). Although the tested diatom and heterotrophic flagellates (Chilomonas and Polytomella) were ingested, they did not sustain reliable growth of Esoptrodinium. The photosynthetic microalga C. ovata in particular was most suitable for promoting vigorous feeding and sustained growth of the dinoflagellates. It is unknown why Esoptrodinium died after incubation with the chrysophyte Ochromonas danica; reports of other dinoflagellates feeding on chrysophytes are rare, but it has been documented at least once previously (Ucko et al. 1989). In field samples or enrichments, Esoptrodinium-like cells have been observed to feed on chlamydomonad

and chlorelloid microalgae (Calado et al. 2006), cryptophytes, and euglenoid microalgae (our observations). Most Esoptrodinium cells in feeding populations contain several food bodies representing independent phagocytic events, the undigested remnants of which are often egested upon cell division (Fawcett and Parrow 2012). Although Esoptrodinium is rarely reported, feeding by these dinoflagellates in abundance could play a significant quantitative role in community structure and energy flow in freshwater microbial ecosystems (Elbrächter 1991a, Jeong 1999). The feeding process observed in Esoptrodinium appears congruent with the ultrastructure reported by Calado et al. (2006), who also observed that the peduncle of feeding cells exhibited a distinctly thickened outer edge (referred to here as the ABP).