Closure signs such as these are posted when shellfish contains dangerously high levels of toxin. This image is from the massive 2005 New England red tide. (WHOI / J. Kleindinst)

This humpback whale died from saxitoxin which accumulated in the mackerel it was eating. (G. Early)

Alexandrium cysts (WHOI, D. Anderson)

Alexandrium stained with Calcafluor White (G. Usup)

A high concentration of Alexandrium cells. (D. Anderson)

Alexandrium cysts (WHOI, D. Anderson)

Clams affected by an Alexandrium bloom in South Africa. (G. Pitcher)

Alexandrium tamarense (WHOI, D. Anderson)

Alexandrium tamarense (L. Fritz)