Because of this property, dinoflagellates move up and down through the water column, usually coming near the surface during the daylight hours and moving down during the night. In some species of armored dinoflagellates, this flagella can be wound around the organism and lay in the girdle crease that encircles the cell.
The identification of toxic Alexandrium from other members of the genus and other dinoflagellates can be difficult and requires some degree of skill and training. These organisms are called "armored" dinoflagellates because the cell is covered by 'thecal' plates. These plates are used to distinguish one Alexandrium species from another. To further complicate identification and observation, particularly in regard to possible bloom of this HAB, the cell can shed the armor and also go into a 'cyst forming stage' that can be morphologically difficult to see and identify. Along the west coast of North America, the most common culprit involved in PSP outbreaks is Alexandrium catenella. Along the eastern coasts of Canada and the U.S., A. tamarense has been implicated in PSP events. However, A. tamarense has also been observed in waters of the west coast of Canada (British Columbia) and in northern Puget Sound in Washington State.