Widespread phagocytosis of ciliates and other protists by marine mixotrophic and heterotrophic thecate dinoflagellates

An electron microscopic examination of large amorphous inclusions located in a variety of photosynthetic thecate dinoflagellates (Alexandrium ostenfeldii (Paulsen) Balech et Tangen, Gonyaulax diegensis Kofoid, Scrippsiella sp., Ceratium longipes (Bailey) Gran, and Prorocentrum micans Ehrenberg) and a nonphotosynthetic thecate species (Amylax sp.) revealed each inclusion to be a food vacuole, the majority of which were ingested ciliate prey. Recognizable features of these ciliates included linear arrays of basal bodies and cilia consistent with oligotrich polykinetid structure, characteristic macronuclei, chloroplasts (evidently kleptoplastids), cup-shaped starch plates, and cylindrical extrusomes. Three species contained (apparent) nonciliate prey: Scrippsiella sp., whose food vacuoles consistently contained unusual and complex extrusome-like cylindrical bodies having a distinctive six-lobed, multilayered structure; P. micans, which contained an. unidentified encysted cell; and a single A. ostenfeldii cell, containing a Dinophysis sp, dinoflagellate cell. Several food vacuoles of ciliate origin had a red hue. This, together with the resemblance of A. ostenfeldii cells to planozygotes, suggests that similar structures previously identified as accumulation bodies may in fact be food vacuoles and that feeding may in some cases be associated with sexual processes.