It produces toxins that, when present in high concentrations as "red tides", can kill fish and reduce growth rates of shellfish.

Alexandrium monilatum is a planktonic species found in tropical to warm temperate coastal seas and estuaries of the Americas.

[1] Alexandrium monilatum are armored or thecate dinoflagellates with an outer cell wall covered with cellulose plates.

The cingulum or groove halfway between the top and bottom of the organism's single cell is where the pair of flagella are situated.

[1] When concentrations rise, algal blooms occur and fish can be killed by the ichthyotoxin, goniodomin A identified in both A. monilatum and A. pseudogoniaulax.