Ultrastructure of oncospheral envelopes in Hymenolepidids (Cestoda) with aquatic life cycles

Abstract

The ultrastructure of the oncospheral envelopes of five species of hymenolepidid cestodes, namely,Fimbriaria fasciolaris, Dicranotaenia coronula, Sobolevicanthus gracilis, Diorchis elisae and Diorchis ovofurcata are described. These cestodes are parasites of aquatic hosts and utilise aquatic invertebrates as intermediate hosts. The ultrastructure of the oncospheral envelopes was essentially similar among these species. However, the structure of the components of the inner oncospheral envelopes displayed significant structural variation among the five species examined. The small eggs of D. coronula and S. gracilis have a thin embryophore and oncospheral membrane. The eggs of these species settle at the bottom of the water body and are eaten by benthic crustaceans. Eggs of F. fasciolaris, D. elisae and D. ovofurcata have thick multilayered embryophores with specific ornamentation. Moreover, in Diorchis species the eggs are elongate, and the oncospheral membrane is thick and striated. Zones of electron-dense aggregates penetrate the polar filaments in these species. Eggs of F. fasciolaris float passively in water whereas those of Diorchis species are attached by long filaments to aquatic plants and are eaten by crustaceans in the littoral zone. The interrelationships among the ultrastructure of the oncospheral envelopes, cestode life cycles and habitat of crustacean intermediate hosts are drawn and discussed.