Ultrastructural characteristics of the vitellarium of basal polyopisthocotylean monogeneans of the family Hexabothriidae, with comments on glycan vesicle development and its phylogenetic significance

Abstract Transmission electron microscopical observations were made on the vitellarium of two basal polyopisthocotylean monogeneans, the hexabothriids Rajonchocotyle emarginata from the ray Amblyraja radiata in the Norwegian Sea and Callorhynchocotyle callorhynchi from the chimaera Callorhynchus capensis in the South-east Atlantic Ocean. The characteristics of the follicular vitellarium of these parasites are described and compared with those of other monogeneans and other groups of free-living and parasitic Platyhelminthes. A very close association between the vitellarium and intestinal diverticula is indicated by the presence of special intercellular junctions linking vitellocytes and intestinal epithelial cells. The absence of any sort of specialized sheath around the vitelline follicles and the presence of a single cell type appear to be discriminatory traits of polyopisthocotylean monogeneans. The characters of two types of vitelline inclusions, shell globules and glycan vesicles, have been considered. Differences in the amount of lipid droplets and glycogen in fully-developed vitellocytes of the two hexabothriids may depend on the different nature and life-styles of their hosts. Three steps in the development of glycan vesicles are apparent in these hexabothriids: (a) membrane-bound glycan vesicles ( ca. 0.2–0.3 μm in diameter) containing moderately dense particles; (b) glycan vesicles ( ca. 0.5–2.0 μm in diameter) with a condensed content in the form of two or three homogeneous glycan bodies; and (c) glycan clusters (up to 6 μm in diameter) containing small glycan bodies modified to exhibit a uniform, heterogeneous, mosaic pattern, within part or all of which glycogen-like particles later develop. The features of the vitelline inclusions within the vitellocytes of these two basal hexabothriids are considered in terms of the phylogeny of the phylum.