A taxonomic study on Gyrodinium aureolum HULBURT (Dinophyceae) from the morphological viewpoint based on materials collected in Japanese coastal waters

For unarmored dinoflagellates which includes harmful species, observation of fragile morphology is indispensable in arriving at correct biological names. However, the difficulties in fixation and preservation of detailed characteristics brings about coufusion due to unstable taxonomic decision. This leads to the creation of new synonymous species name or innocent misuse of known species name. Repetition of this practice increases difficulty in species identification. Gymnodinium mikimotoi Miyake et Kominami ex Oda (abbreviated as Gym. mikimotol) and Gyrodinium aureolum Hulburt (Gyr. aureolum) are two major harmful unarmored species reported in several countries and these species are in state of taxonomic confusion. In Japan, Gym. mikimotoi is considered most noxious and had devastated the fishing and aquaculture industries since 1960's. In northern Europe, another unarmored species called either Gyr. aureolum or Gymnodinum sp. cf. mikimotoi is known to cause serious damages in coastal waters. Morphological similarities between these species have cause taxonomic confusion for the last two decades. This study was conducted to clarify the taxonomic problems by observation of materials collected from Japanese waters and re-interpretation of original and succeeding morphological descriptions. After careful LM and SEM observation of the two species which occurred simultaneously in the coastal waters of Tanabe Bay, central Japan, and literature survey of these species, we concluded the separation of the two species should be based on the the following morphological characteristics and behavior. 1) Thickness of vegetative cell: Gyr. aureolum is always thicker than Gym. mikimotoi. 2) Swimming manner: Cell thickness strongly affect the swimming manner of these two species. Gyr. aureolum invariably rotates forward and Gym. mikimotoi swims in butterfly motion. 3) Shape and position of nucleus: The nucleus of Gyr. aureolum is spherical to slightly elongate laterally and centrally located. However，G ym. mikimotoi has longitudinally ellipsoidal nucleus and found in the left lobe. Morphological characteristics of Gyr. aureolum were carefully examined using publications reported from North Atlantic Ocean plankton samples. Taking the morphological variabilities of the two unarmored species shown in this report into consideration，th ey are subdivided into four morphotypes，a lthough they are described under the same species name. Two of them are identical to the original description of Gym. mikimotoi and Gyr. aureolum. But the other two have different characters from the above two species