Ultrastructure of the archigregarine Selenidium vivax (Apicomplexa) – A dynamic parasite of sipunculid worms (host: Phascolosoma agassizii)

Abstract

Selenidium vivax is a large and unusual unicellular parasite that inhabits the intestinal lumen of the dotted peanut worm, Phascolosoma agassizii. Molecular phylogenies suggest that this archigregarine lineage diverges near the nexus of the apicomplexan radiation and could shed light on to the early evolution of parasitism within the group. The behaviour and ultrastructure of the trophozoites were described using digital videography and scanning and transmission electron microscopy. The trophozoites were extremely flat and capable of dynamic cellular deformations. An intimate association between a superficial layer of mitochondria and longitudinal clusters of subpellicular microtubules formed a distinct functional configuration that helped explain the mechanism behind the cellular motility. Although inconclusive, the presence of small mitochondria-like profiles and narrow connections between larger mitochondrial profiles suggested that an expansive mitochondrial reticulum might surround the trophozoites. The nucleus was highly convoluted and gave rise to blebs of different sizes. The nuclear blebs were connected to the nucleus proper and surrounded by one cisterna of endoplasmic reticulum, giving the impression of four membrane-bound organelles that were misleadingly reminiscent of apicoplasts. The novel attachment apparatus consisted of a transverse ridge, a linear arrangement of pores that contained thread-like structures and a network of dense bodies and endoplasmic reticulum.