N. pungens and P. australis are diatoms, a type of phytoplankton that has a cell wall composed of silica, called a frustule. The frustule has two parts that fit together like a petri dish. In N. pungens , a pennate diatom, the cell wall is stretched out giving the organism the appearance of a symmetrical flattened banana. P. australis is also a pennate diatom and has a shape similar to N. pungens . Many different kinds of phytoplankton exist, and only a few of them are toxic.
Most marine dinoflagellates are unicellular, photosynthetic organisms with two flagella.
One flagellum is projected for movement and the other lies in a transverse groove around the cell. Cellulose plates, often bearing horns or spines, house the organism. Dinoflagellates are in a different taxonomic division than diatoms, so discovering that a diatom, instead of a dinoflagellate, may kill humans is analogous to discovering that a slug, instead of a carnivore, kills humans. When symptoms of food poisoning and amnesia resulted in the death of several individuals, Canadian health authorities desperately needed to find the killer. Since mussels are a known vector of some marine toxins, as in the cases of PSP and DSP, they were immediately investigated.