Light and electron microscopical observations on the type species of Gymnodinium, G-fuscum (Dinophyceae)

Gymnodinium fuscum (Ehrenberg) Stein is the type species of Gymnodinium. on e of the largest genera of dinoflagellates. It has the typical fine structu re of dinoflagellates, but also possesses several unusual features. The nuc lear envelope is specialized by having vesicular chambers in which the nucl ear pores are located: the pusular complex includes an internal collection chamber: and a nuclear fibrous connective links the nucleus with the longit udinal microtubular root of the flagellar apparatus. A delicate horseshoe-s haped apical groove is visible by scanning electron microscopy. The same mo rphological features have been observed in a few other gymnodinioids. indic ating a close phylogenetic relationship, and have recently been used, toget her with large subunit (LSU) rDNA data, in redefining the genus Gymnodinium [Daugbjerg er nl. (2000) Phycologia 39: 302-317]. However, one of the most unusual features of G. fuscum is the absence of a transverse striated flag ellar root, a feature presently known only in the zoospores of the otherwis e very different genus Noctiluca. This feature, together with the lack of s triated collars around the flagellar canals, suggests a phylogenetically mo re isolated position for G. fuscum. Future investigations may reveal the ne ed for further 'splitting' of the genus Gymnodinium.