Pseudo-nitzschia is a chain-forming diatom that produces domoic acid, a neurotoxin. Filter-feeding shellfish and small fish accumulate it, and Dungeness crabs concentrate it further, chiefly in the hepatopancreas, the soft organ often eaten as crab butter. Muscle meat carries far less, which is why advisories frequently permit the legs and claws while warning against the viscera and cooking liquid.
Blooms are not evenly distributed in time. The unusually warm north-east Pacific conditions of 2015 produced an exceptional bloom along the west coast of North America and forced protracted delays and closures of Dungeness and rock crab fisheries in California, Oregon and Washington. Testing programmes now sample crabs before openings and reopen areas piecemeal as levels fall.
The toxin is not destroyed by cooking or freezing, which is the point most often misunderstood. Domoic acid is heat-stable, so a crab that tests above the action level does not become safe on the stove. Commercially sold crab in regulated fisheries is tested; crab taken recreationally in a closed area is not.