Molluscan shellfish (shellfish with hinged shells such as oysters, clams, and mussels) are filter feeders and ingest phytoplankton in the surrounding water. When shellfish eat toxin-producing algae, the toxin remains in their system; large amounts of algae means more toxin can concentrate in their tissue. Biotoxins don't harm shellfish but can accumulate in shellfish over time and cause illness or death in humans and marine wildlife (seabirds, marine mammals) that eat them. Biotoxin produced by HABs are not affected by cooking and remain in the shellfish tissue.