GYRODINIUM UNDULANS HULBURT, A MARINE DINOFLAGELLATE FEEDING ON THE BLOOM-FORMING DIATOM ODONTELLA-AURITA, AND ON COPEPOD AND ROTIFER EGGS

The marine dinoflagellate Gyrodinium undulans was discovered as a feed er on the planktonic diatom Odontella aurita. Every year, during winte r and early spring, a certain percentage of cells of this bloom-formin g diatom, in the Wadden Sea along the North Sea coast, was regularly f ound affected by the flagellate. Supplied with the food diatom O. auri ta the dinoflagellate could be maintained successfully in clonal cultu re. The vegetative life cycle was studied, mainly by Light microscopy on live material, with special regard to the mode of food uptake. Food is taken up by a so-called phagopod, emerging from the antapex of the flagellate. Only fluid or tiny prey material could be transported thr ough the phagopod. Larger organelles like the chloroplasts of Odontell a are not ingested and are left behind in the diatom cell. Thereafter, the detached dinoflagellate reproduces by cell division, occasionally followed by a second division. As yet, stages of sexual reproduction and possible formation of resting cysts could not be recognized, neith er from wild material nor from laboratory cultures. Palmelloid stages (sometimes with a delicate wall) occurring in ageing cultures may at l east partly function as temporary resting stages. The winter species G . undulans strongly resembles Syltodinium listii, a summer species fee ding on copepod and rotifer eggs. Surprisingly, in a few cases this pr ey material was accepted by G. undulans as well, at least under cultur e conditions. When fed with copepod eggs, the dinoflagellate developed into a large trophont, giving rise thereafter by repeated binary fiss ion to 4, 8 or 16 flagellates, as a result of a single feeding act. 4 re-examination of both species under simultaneous culture conditions i s planned.