Correct answer: the plankton they’ve consumed
Everyone knows that a meal containing a bad oyster or mussel can have very unpleasant consequences. What is less well known is that you can suffer from not just one, but four kinds of shellfish poisoning: diarrheal, neurotoxic, amnesiac and paralytic. Each of these kinds of poisonings is caused by different chemical compounds and has distinct symptoms.
Shellfish are filter feeders; they circulate water through their shells, filtering out nutritive particles in the process. Oysters are thought to filter about five liters of water per hour. Their diet is largely made up of dinoflagellates, single-celled organisms, and plankton. Large concentrations of these organisms in the ocean can result in “red tides” and “algal blooms”. These proliferations can be damaging to marine life, and shellfish can be poisoned from the accumulation of toxins present in the dinoflagellates and algae that they absorb.
The least serious of the four kinds of poisoning is probably the one that causes diarrhea. It is also accompanied by nausea, stomach cramps and vomiting, which can last for up to three days. The toxin responsible, okadaic acid, produced by a particular species of dinoflagellate, need be present in quantities of only 48 micrograms in order to cause symptoms. It cannot be rendered harmless by cooking. It operates by making cells extremely permeable to water, which causes an imbalance in the exchange of water in the intestines, leading to diarrhea. Although its effects are certainly unpleasant, no one has ever died from this kind of food poisoning.
You’d most likely also survive a bout of neurotoxic food poisoning, even though it has led to many hospitalizations. In this kind of poisoning, at least ten compounds known as brevetoxins are responsible for symptoms that include nausea, vomiting, slurred speech, a tingling sensation in the mouth, tongue or lips, and in very rare cases partial and temporary paralysis. These compounds are also impervious to heat, and thus remain toxic even after cooking.
The third kind of shellfish poisoning, amnesic poisoning, is caused by domoic acid which is produced in algae and diatoms. In addition to nausea, vomiting and diarrhea, is causes neurological symptoms such as migraines, short-term memory loss and in the most severe cases irregular heartbeat or death. Once again, domoic acid is impervious to heat. There is no known antidote.
Paralytic poisoning, the last of the four types of shellfish poisoning, is potentially the most serious. It is mainly caused by saxitoxin, which is produced by dinoflagellates and cyanobacteria. Saxitoxin interferes with the transmission of nerve impulses, which can lead to paralysis. In addition to the usual symptoms, the gradual paralysis of muscle tissue can result in respiratory failure and cardiac arrest.
Because of these potentially fatal consequences, shellfish harvesting can be banned during red tides episodes. If you feel any of the symptoms of food poisoning after a meal that includes shellfish, it’s a good idea to consult a doctor immediately.
From the book “Pourquoi l'asperge donne-t-elle une odeur au pipi?” PPUR.