The fine structure of the parasitic dinoflagellateHaplozoon axiothellae

Summary The multicellular parasitic dinoflagellateHaplozoon axiothellae Siebert was studied with electron microscopy. The trophocyte (attachment cell) bears a suction apparatus with a movable protruding stylet that penetrates the epithelial cell of the host gut. The gonocytes are binucleate and divide frequently. Nuclear structure is similar to the mesokaryotic condition of other dinoflagellates although the chromosomes lack the helically coiled appearance of the DNA fibrils. During nuclear division the nucleus retains its envelope intact and cytoplasmic invaginations develop in which packets of parallel microtubules occur. The microtubules attach to the nuclear envelope opposite the site of chromosome attachment. The chromosomes remain condensed during interphase but the helically coiled DNA fibrils characteristic of the mesokaryotic condition are not evident.The theca which encloses all cells is composed of elements similar to those of typical free-living dinoflagellates, the outer cell membrane and flattened vesicles which contain either flat thin plates or larger spines. No subthecal microtubules are present. The theca grows inward following nuclear division and separates the daughter cells. Trichocysts, pusules, flagellar structures and chloroplasts are not present. The relationship ofHaplozoon to other free-living and parasitic dinoflagellates is discussed.