There are about 30 species of Alexandrium that form a clade, defined primarily on morphological characters in their thecal plates.

[1] The group was not well-defined in the past and thus, due to interest of its toxic behaviour, people have revised the group through formal tabulation and usage of molecular markers such as the toxic gene, stx4A, to better understand not just the relationship between each species; but more importantly, to tell something about the spread and barriers to the bloom.

Learning about the pathways of toxin synthesis can allow us to determine how species of Alexandrium have adapted and co-evolved to persist.

[2] In the 1960s, an Egyptian researcher by the name of Youssef Halim discovered a red tide along the coast of the Alexandria harbour in Egypt.

[3][4] Species currently included in Alexandrium have been known for a long time, but initially they were classified in different genera, notably, Gonyaulax.

His greatest work was the development of the universal "Kofoid System" that classified dinoflagellates based on plate morphology very detailed drawings and well-defined structures.

[6] The latter was studied in much more detail due to the fact that it caused paralytic shellfish poisoning (PSP) in humans, getting it from bivalve mollusks.

The "tamarensis group" was divided into two subgroups and were separated by whether or not one of its thecal plates had contact with the ventral pore or not.

The group included for example the genus: Gonyaulax, Goniodoma, Pyrodinium, Alexandrium, Gessnerium, Protogonyaulax, and Glenodinium.

What gave people interest in studying Alexandrium was because it produces paralytic shellfish toxins (PST).

Later studies have shown that indeed if you remove the symbiotic bacteria of specific Alexandrium species, the dinoflagellate produces less toxin.

Before molecular data, the main way to distinguish between Alexandrium species was to identify whether or not it made chains or if the 1' thecal plate touched the ventral pore or not.

By constructing using rDNA, researchers found that the phylogeny forms 5 different geographical clades in the Alexandrium tamarense species complex.

Although A. catenella is found in the same location as A. fundyense, it is suggested to not include it in the Alexandrium tamarense species complex.

ITS2 compensatory base changes (CBC) are used as genetic markers that is used for studying the diversity of eukaryotic species.

Scientists screened for presence or absence of the sxtA4 domain of the sxtA gene to determine the toxicity of each group.

[14] Alexandrium as a whole is wide spread across the globe and has been identified in different parts of the Northern and Southern Hemisphere.

There may be large numbers of cyst populations, but only 20% of them can germinate and develop into a bloom, because oxygen is present mostly a few millimeters inside the sediment.

However, in areas like the Bay of Fundy, bloom dynamics depend more readily on the mixing of tidal waters in the open region.

(3) The hydro-geographical barrier and the organism's behavior restrict blooms to occur in neighbouring waters, and thus, result in high concentration of toxicity accumulating in shellfish.

However, when wind is pushing the bloom offshore, the vegetative cells can encyst and sink to the bottom of the sediment.

However, even with optimal temperatures, Alexandrium populations can be declining and this has more to do with the life cycle than what was thought to do with things like predation and parasitism.

[15] The triggering of pellicle cyst formation is often due to turbulence, presence of parasites, and or passage through the gut of grazers.

It is suggested that forming chains is in response to high turbulence, to swimming faster than single cells allowing them to migrate farther or deeper, and a defense mechanism for reducing grazing.

In A. tamarense, researchers have identified 48 non-minicircle encoded photosystem genes which are found in the nuclear genome.

Some researchers have hypothetically calculated that the average age of Alexandrium is 77 million years old which is going back to the Cretaceous period.

The rise of the isthmus of Panama that happened between 3–13 MYA subdivided and separated many species into distinct geographical populations.

The first group of toxins it produces includes carbomoyltoxins: saxitoxin (STX), neosaxitoxin (NEO), and C-11 O-sulfated analogues gonyautoxins (GTX1-GTX4).

However, it seems that with distinct geographical locations, there may be differences in the PSP toxin composition of Alexandrium species or even strains.

A. catenella causes tissue damage in gills, by primarily producing reactive oxygen species or ROS.