That site documents filter-feeding mollusk toxicity levels.
The phenomena at work here is that filter-feeding clams, mussels and oysters work a tremendous amount of seawater through their systems in order to filter out enough phyto and zooplankton to make a living. Unfortunately, that large quantity of water also has some toxins that can be harmful to humans.
Crabs and shrimp are crustaceans that occupy a niche a bit higher in the food chain. In other words, drop those pots and enjoy your dungeness crab!