Surface morphology of the marine parasite Haplozoon axiothellae Siebert (Dinoflagellata)

The genus Haplozoon contains a group of intestinal parasites of marine worms with a bizarre cellular organization. Although the phylogenetic relationship of these parasites to other eukaryotes was once controversial, a few studies using light and transmission electron microscopy (TEM) and small subunit rDNA confirmed that they are aberrant dinoflagellates. Microscopical studies also suggested that haplozoans possessed a number of surface features that remained to be characterized. We have conducted a finescale examination of the surface of Haplozoon axiothellae Siebert, a parasite of the maldanid polychaete Axiothella rubrocincta, collected from two sites in the northeastern Pacific Ocean. Our findings indicated that the parasites are covered with a theca of barbs and small polygonal plates and possess a longitudinal row of ventral pores that resemble a kinety. Most surprisingly, we provide evidence that the entire organism is bounded by a single, continuous membrane, suggesting that haplozoans are not really multicellular but a syncytium compartmentalized by sheets of alveoli. We also demonstrated distinct clusters of unusual concave bodies over the surface of roughly half of the parasites observed. Septum-like junctions between individuals, distinctive threads reminiscent of pili, and their size ranges suggest that the concave bodies might be unusual episymbionts.