Ultrastructure of Gymnodinium aureolum (Dinophyceae): Toward a further redefinition of Gymnodinium sensu stricto

Examination of the ultrastucture of the unarmored dinoflagellate Gymnodiniu m aureolum (Hulburt) G. Han en (syn: Gyrodinium aureolum Hulburt) revealed the presence of nuclear chambers, which are specialized differentiations of the nuclear envelope, similar to those described in the type species of Gy mnodinium, G. fuscum (Ehrenberg) Stein and certain other Gymnodinium specie s. The nuclear pores were restricted to these chambers. In the flagellar ap paratus a nuclear fibrous connective linked the longitudinal microtubular r oot and the nucleus. This structure had so far been observed only in Gymnod inium spp. and in the heterotrophic species Actiniscus Pentasterias (Ehrenb erg) Ehrenberg, Nematodinium armatum (Dogiel) Kofoid et Swezy and Polykriko s kofoidii Chatton. Another unusual feature of G. aureolum was the presence of a striated fiber in the longitudinal flagellum, a feature previously on ly found in Ceratium furca (Ehrenberg) Claparede et Lachmann and C. tripos (O.F. Muller) Nitzsch. Gymnodinium aureolum also possessed a prominent vent ral protrusion associated with the peduncle and containing electron opaque material. It is concluded that G. aureolum belongs to the Gymnodinium sensu stricto group. This may be a temporary classification, however, because G. aureolum and its allies differ from the type species G. fuscum by the pres ence of a transverse striated root, striated collars, trichocysts, and a pe duncle.