Saxitoxin (STX) is a potent neurotoxin and the best-known paralytic shellfish toxin (PST). Ingestion of saxitoxin by humans, usually by consumption of shellfish contaminated by toxic algal blooms, is responsible for the illness known as paralytic shellfish poisoning (PSP). The term saxitoxin originates from the genus name of the butter clam (Saxidomus) from which it was first isolated. But the term saxitoxin can also refer to the entire suite of more than 50 structurally related neurotoxins (known collectively as "saxitoxins") produced by protists, algae and cyanobacteria which includes saxitoxin itself (STX), neosaxitoxin (NSTX), gonyautoxins (GTX) and decarbamoylsaxitoxin (dcSTX). Saxitoxin has a large environmental and economic impact, as its presence in bivalve shellfish such as mussels, clams, oysters and scallops frequently leads to bans on commercial and recreational shellfish harvesting in many temperate coastal waters around the world including the Northeastern and Western United States, Western Europe, East Asia, Australia, New Zealand, and South Africa. In the United States, paralytic shellfish poisoning has occurred in California, Oregon, Washington, Alaska, and New England. Source in nature Saxitoxin is a neurotoxin naturally produced by certain species of marine dinoflagellates (Alexandrium sp., Gymnodinium sp., Pyrodinium sp.) and freshwater cyanobacteria (Dolichospermum cicinale sp., some Aphanizomenon spp., Cylindrospermopsis sp., Lyngbya sp., Planktothrix sp.) Saxitoxin accumulates in "planktivorous invertebrates, including mollusks (bivalves and gastropods), crustaceans, and echinoderms". Saxitoxin has also been found in at least 12 marine puffer fish species in Asia and one freshwater fish tilapia in Brazil. However, the ultimate source of STX is often still uncertain. The dinoflagellate Pyrodinium bahamense is the source of STX found in Florida. Recent research shows the detection of STX in the skin, muscle, viscera, and gonads of "Indian River Lagoon" southern puffer fish, with the highest concentration (22,104 μg STX eq/100 g tissue) measured in the ovaries. Even after a year of captivity, Landsberg et al. found the skin mucus remained highly toxic. The concentrations in puffer fish from the United States are similar to those found in the Philippines, Thailand, Japan, and South American countries. Puffer fish also accumulate a structurally distinct toxin, tetrodotoxin. Structure and synthesis Saxitoxin dihydrochloride is an amorphous hygroscopic solid, but X-ray crystallography of crystalline derivatives enabled the structure of saxitoxin to be determined. Oxidation of saxitoxin generates a highly fluorescent purine derivative which has been utilized to detect its presence. Several total syntheses of saxitoxin have been accomplished. Mechanism of action Saxitoxin