A tip everyone is given before going on holiday is to avoid eating the fish. People believe that their illness may be because the seafood is washed in the undrinkable water however the fish itself may be to blame.
Within the oceans shellfish feed upon algae. The algae themselves are responsible for the production of toxic substances. These substances are mainly derivatives of saxitoxin which shows similar effects to tetrodotoxin.
The accumulation of these toxins into shellfish is the cause of poisoning in humans, which can be fatal. The characteristic symptom of all shellfish poisoning is diarrhea. This being due to the presence of the naturally produced diarrheic substance known as okadaic acid.