Saxitoxin is a potent neurotoxin produced by certain species of marine and freshwater algae, particularly those in the genus Alexandrium. It can accumulate in shellfish, leading to a condition known as paralytic shellfish poisoning (PSP) when consumed by humans. Saxitoxin blocks sodium channels in nerve cells, disrupting normal nerve function and potentially causing severe health issues. This toxin is of significant concern in coastal areas where shellfish harvesting occurs. Monitoring programs are often implemented to test for saxitoxin levels in shellfish to ensure public safety. Symptoms of PSP can include numbness, dizziness, and in severe cases, respiratory failure.