LIGHT AND ELECTRON-MICROSCOPIC OBSERVATIONS OF THE DINOFLAGELLATE ACTINISCUS-PENTASTERIAS (DINOPHYCEAE)

I examined the heterotrophic non-armored dinoflagellate Actiniscus pentasterias (Ehrenberg) Ehrenberg by light and electron microscopy. Actiniscus pentasterias contains an internal skeleton consisting of tao star-like siliceous elements. Special emphasis is given to the flagellar apparatus, the nucleus, and a new type of extrusome, named a docidosome. A three-dimensional model of the flagellar apparatus includes a fibrous nuclear connective, a posterior striated root, and a dorsal striated component of the longitudinal microtubular root. The nucleus is surrounded by a conspicuous fibrous lamina, also visible in the light microscope. The nuclear pores are situated in annulated invaginations of the nuclear envelope, increasing the nuclear surface area by 15-25%. The docidosomes are rod-shaped membrane-bound structures that terminate in a distinct proximal head. They show a very complex substructure, consisting of an inner medulla with highly ordered paired ribbons and an outer cortex.