Fine structure and putative feeding mechanism of the archigregarine Selenidium orientale (Apicomplexa: Gregarinomorpha)

Archigregarines are considered one of the most plesiomorphic groups of Apicomplexa. Until recently, however, this viewpoint was based mainly on the results of the detailed investigation of a single species, Selenidium hollandei. The present study of the fine structure of trophozoites of another archigregarine species Selenidium orientale Bogolepova, 1953. (Apicomplexa, Archigregarinida, as proposed by Grassé and Schrével), with special reference to the forebody structure of the attached individuals, allows more confident discussion of the plesiomorphic status of the archigregarines. Specimens of S. orientale were collected from the midgut of the Pacific sipunculid Themiste pyroides Chamberlain, 1920. The ultrastructure of the trophozoites generally corresponds to that of other studied species of Selenidium. Differences in the forebody structure between S. orientale and S. hollandei do not conflict with Schrével's hypothesis on the feeding function of the apical complex in archigregarine trophozoites, although they suggest that, in S. orientale at least, the cytostome is not a persistent structure, but re-opens for each sucking event, and Selenidium trophozoites feed by intermittent sucking of host cytoplasm. Microtubules in the axial zone of the mucron neck may mediate the transport of food vacuoles.