A Answers (2)
Bruce Pfuetze, MD, Allergy / Immunology, answered on behalf of Overland Park Regional Medical CenterMany people are allergic to shellfish and can have life threatening reactions. It would be extremely important that they avoid all shellfish (shrimp, crab, lobster and crayfish). People allergic to these foods are usually not allergic to clams, mussels and abalone which are in a different family.
Discovery Health answered
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.