Saxitoxin

Saxitoxin is a neurotoxin found in various marine organisms. Although it's chemical structure is completely different from that of tetrodotoxin, the biological action is similar: it is very efficiently blocking the sodium channels of nerve cells which results in paralysis. The disease it causes is called "paralytic shellfish poisoning" mostly resulting from the consumption of shellfish. Shellfish, but also some puffer fish, accumulate this toxin which is produced by microorganisms (dinoflagellates and cyanobacteria).