Alexandrium

A dinoflagellate genus with an outsized role in seafood safety. When Alexandrium species — including A. catenella, A. minutum and A. tamarense — bloom in coastal waters, they manufacture saxitoxins, the poisons behind paralytic shellfish poisoning. Filter-feeding mussels, clams and oysters concentrate the toxins while remaining apparently healthy, turning dangerous to whoever eats them. Coastal monitoring programmes track these blooms with plankton counts and flesh testing, closing harvest areas until toxins clear. A single-celled alga thus dictates the opening and closing of shellfish fisheries in many parts of the world.