TAXONOMY AND ULTRASTRUCTURE OF A FRESH-WATER, HETEROTROPHIC AMPHIDINIUM (DINOPHYCEAE) THAT FEEDS ON UNICELLULAR PROTISTS

Amphidinium lacustre Stein, non sensu auctt, was identified by referen ce to original descriptions, which differ significantly from later int erpretations. Mixed cultures containing A. lacustre, cryptomonads, Spe rmatozopsis exsultans Korshikov, and small chlorelloid cells were exam ined for feeding events. The dinoflagellate ingested the cytoplasm of cryptomonads, leaving the periplast, and completely ingested Spermatoz opsis 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, o pened directly into one of the flagellar canals. An eyespot was presen t, composed of presumably crystalline, vesicle-contained units, simila r to those of Gymnodinium natalense Horiguchi et Pienaar. The main com ponents of the flagellar apparatus resembled those of other Amphidiniu m species, but differences were noted. The transverse basal body overl apped the proximal end of the longitudinal one at an angle of about 12 0 degrees Three connectives were distinguished between basal bodies, o ne 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 an d to a mitochondrion. A ventral ridge extended from the (incomplete) l ongitudinal striated collar to the peduncular striated collar. The ped uncle was supported by a single strand of microtubules, which were sur rounded by numerous vesicles with electron-opaque contents.