Diagnose_Genus: Gymnodinium F. Stein emend. G. Hansen & Moestrup: Unarmoured unicellular or colony-forming dinoflagellates with horseshoe-shaped apical groove running in an anticlockwise direction. Body circular to squarish in ventral view, truncate posteriorly with deep notch at sulcus, rounded anteriorly in anterior members of chains. The epicone of all other members of chains pointed where there is attachment to the sulcus of the cell ahead. In anterior view the body is subcircular.Nuclear envelope with vesicular chambers. Cingulum displacement one or more cingulum widths.

Diagnose_Species: Gymnodinium catenatum: The species occurs characteristically in chains of many individuals; as many as 29 units have been counted in a single chain. Many of the single individuals in preserved samples may represent broken chains. The cytoplasm is continuous throughout the chain. The apex of one individual is drawn out into a narrow extension which joins the posterior part of the sulcus of the next anterior member. Girdle median, without displacement, wide, about 0.16 transdiameter, deeply impressed. Sulcus extends from near apex to antapex. The nucleus is centrally located, moniliform, large, thick, and slightly lunate. Nuclear or dorsal fibrous connective present. Cytoplasm greenish-yellow, packed with irregularly rounded refractory bodies and food vacuoles. Ectoplasm forms a thin pellicle in some stages. No striae or other surface markings observed. Resting cyst spherical, about 50 µm in diameter, with microreticulate surface ornamentation. This species produces paralytic shellfish poisons, mainly sulfamate saxitoxin derivatives, which accumulate in shellfish such as oysters, mussels and scallops, and thus can cause neurological and gastrointestinal problems in human shellfish consumers. Shellfish containing more than 80 micrograms toxin per 100 g meat should be considered unsafe for human consumption (Hallegraeff, 1991).

Toxicity: Yes_PSP