Ultrastructure of the oncospheral envelopes in the pseudophyllidean cestode Eubothrium salvelini [Schrank, 1790]

The aim of this study is to describe the ultrastructure of oncospheral envelopes in the pseudophyllidean cestode Eubothrium salvelini, a parasite of salmonid fishes. Our results indicate that the eggs of E. salvelini differ in their ultrastructure from those of the majority of the Pseudophyllidea. The entire embryonic development, including differentiation of the mature, infective oncosphere of E. salvelini takes place in the uterus and not in the aquatic environment, as is common for other pseudophyllideans. Egg maturation is not simultaneous; together with mature eggs containing fully differentiated oncospheres, can be found numerous small immature, nonfertilized and nonviable abortive eggs. The normally developing eggs of E. salvelini are large, oval and nonoperculated. Three envelopes surround the infective hexacanths: (1) the eggshell; (2) the outer envelope originating from macromere fusion; (3) the inner envelope formed by numerous mesomeres which usually persist in the mature eggs. Our observations confirm that both the outer and the inner envelopes of E. salvelini eggs are cellular in origin and syncytial in nature. The typical oncospheral membrane was not observed in this species. New data on the origin and ultrastructure of oncospheral envelopes may present useful criteria for phylogenetic analysis of lower cestodes. Ontogenetic characters, such as ultrastructural aspect of morphogenesis of infective larval stages, are proposed as phylogenetic indicators in studies of cestode evolution.