Taxonomy and ultrastructure of a freshwater, heterotrophic Amphidinium (Dinophyceae) that feeds on unicellular protists

Amphidinium lacustre Stein, non sensu auctt, was identified by reference to original descriptions, which differ significantly from later interpretations. Mixed cultures containing A. lacustre, cryptomonads, Spermatozopsis exsultans Korshikov, and small chlorelloid cells were examined for feeding events. The dinoflagellate ingested the cytoplasm of cryptomonads, leaving the periplast, and completely ingested Spermatozopsis and chlorelloid cells. A peduncle was used in the initial stages of prey capture, although it was not visible during food uptake. The ultrastructure of A, lacustre was typical of unarmored dinoflagellates. A single pusule per cell, consisting of one long; convoluted tube, opened directly into one of the flagellar canals. An eyespot was present, composed of presumably crystalline, vesicle-contained units, similar to those of Gymnodinium natalense Horiguchi et Pienaar. The main components of the flagellar apparatus resembled those of other Amphidinium species, but differences were noted. The transverse basal body overlapped the proximal end of the longitudinal one at an angle of about 120 degrees Three connectives were distinguished between basal bodies, one of which consisted of radiating fibers linking individual triplets of the longitudinal basal body to one triplet of the transverse. Some flagellar apparatus components were closely associated to a vesicle and to a mitochondrion. A ventral ridge extended from the (incomplete) longitudinal striated collar to the peduncular striated collar. The peduncle was supported by a single strand of microtubules, which were surrounded by numerous vesicles with electron-opaque contents.