Ultrastructural features of the basal dinoflagellate Ellobiopsis chattoni (Ellobiopsidae, Alveolata), parasite of copepods.

Abstract

Ellobiopsis chattoni is the type species of the ellobiopsids, an enigmatic lineage of parasitic alveolates that branched between the syndinean dinoflagellates and the perkinsids. We have investigated the ultrastructure of four trophonts from three calanoid copepod hosts collected from the port of Valencia, northwestern Mediterranean Sea. The cell wall showed a thick and homogenous layer and flask-shaped mucocysts that excreted an electron-dense substance that forms the outer layer. The cell wall in the attachment peduncle of Ellobiopsis was thicker and with numerous invaginations. The inner section showed numerous longitudinal channels here interpreted as conduits for the transport of host fluids. Trophomere and gonomere were separated by a thin septum with a central pore. Before the mature gonomere detached from the trophomere, the area of junction became undulated. Deficiencies in the fixation of the membrane organelle preclude discussing onother ultrastructural features. To date the ultrastructure of three ellobiopsid genera have been examined. The trophonts of Ellobiopsis and Thalassomyces showed a high similarity in the cell wall, with characteristic flaskshaped mucocysts. The lack of flask-shaped mucocysts in Ellobiocystis and other morphological and ecological differences argue against the monophyly of the ellobiopsids.