Alexandrium is a genus of dinoflagellates, with many species related to toxin production leading to Paralytic shellfish poisoning. The dinoflagellate cysts produced by this species are considered much more toxic than swimming cells. Enzyme electrophoretic data  and RNA genetic analysis  show two of the species, Alexandrium fundyense and A. tamarense to be closely related, while mating compatibilities even suggest them to be varieties of a single heterothallic species.
Recent molecular work demonstrates an Alexandrium tamarense complex (Atama complex, including A. tamarense, Alexandrium fundyense, Alexandrium catenella) and that none of the three original morphospecies designations forms monophyletic groups in the present SSU-based and previous LSU-based  phylogenetic trees, i.e. these species designations are invalid. This currently needs further investigation.
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