Saxitoxin is a paralytic poison from Alaska butter clams (Saxidomus giganteus), toxic mussels (Mytilus californianus), the plankton Gonyaulax cantenella and Protogonyaulax tamarensis. Causal agent of paralytic shellfish poisoning. Saxitoxin (STX) is a neurotoxin naturally produced by certain species of marine dinoflagellates (Alexandrium sp. , Gymnodinium sp. , Pyrodinium sp. ) and cyanobacteria (Anabaena sp. , some Aphanizomenon species, Cylindrospermopsis sp. , Lyngbya sp. , Planktothrix sp. ). Ingestion of saxitoxin (usually through shellfish contaminated by toxic algal blooms) is responsible for the human illness known as paralytic shellfish poisoning (PSP). (Wikipedia