Shellfish such as clams, oysters, mussels and scallops filter water through two shells to feed on algae, plankton and tiny life forms called flagellates. Sometimes the flagellates eat toxic alkaloids, which makes them toxic. A shellfish that eats those will accumulate toxins inside and then pass those on to whoever eats that shellfish. No amount of heat during cooking will destroy those bacteria once a shellfish becomes toxic. Whichever kind of alkaloids the flagellates consumed determines which toxins are present and which type of shellfish poisoning you'll have. Symptoms range from vomiting and diarrhea to short-term memory loss and dementia.