Abstract
The paper provides a summary of the current state of the taxonomy of harmful marine phytoplankton species and introduces new combinations for two fish-killing species, Heterosigma carterae (Hulburt) comb. nov. (for an organism thought to be conspecific with Heterosigma akashiwo (Hada) Hada) and Gyrodinium galatheanum (Braarud) comb. nov. Difficulties in the species concept when dealing with these organisms are briefly highlighted and the potential for significant genetic variability within “morphospecies” is emphasised.