These shellfish are filter feeders and accumulate neurotoxins, chiefly saxitoxin, produced by microscopic algae, such as dinoflagellates, diatoms, and cyanobacteria.

[1] Dinoflagellates of the genus Alexandrium are the most numerous and widespread saxitoxin producers and are responsible for PSP blooms in subarctic, temperate, and tropical locations.

Cyanobacterial saxitoxin biosynthesis has been studied in radioisotope tracing experiments, and turns out to be highly complex, involving many steps, enzymes and chemical reactions.

After two to three days of exposure to the cyanobacterium Anabaena circinalis it may contain upwards of 80 micrograms of neurotoxins per 100 grams of mussel, a level high enough to cause significant health risks to humans.

[citation needed] Symptoms typically appear within ten to 30 minutes after ingestion, and may include nausea, vomiting, diarrhea, abdominal pain, and tingling or burning lips, gums, tongue, face, neck, arms, legs, and toes.

[10] PSP has been implicated as a possible cause of sea otter mortality and morbidity in Alaska, as one of its primary prey items, the butter clam (Saxidomus gigantea) bioaccumulates saxitoxin as a chemical defense mechanism.

In the relatively rare case of clinically significant respiratory paralysis, symptomatic treatment in the form of oxygen supplementation and/or mechanical ventilation should be employed until symptoms subside.