Butter clams, Saxidomus giganteus, can hold the toxin for up to a year or more, so they are regulated separately. Razor clams, Siliqua patula, seem to also depurate slowly. We have Alexandrium catanella here.
Oysters are usually a little faster if they become toxic at all.
So as soon as the mussels show toxin we increase the testing for other species, and close the area to recreational harvest when any species reaches 80ug. Re-opening for all but butter clams occurs when we are assured that all species are below the closure level for at least 7 days.