You may have heard the advice, "Never eat seafood in months that don't have the letter R." That's May through August, and because toxic algae blooms occur more frequently during warm months, that may really be good advice.
There are four ways shellfish — seafood with two shells, including clams, mussels and oysters — can make your vacation miserable: amnesic shellfish poisoning (ASP), diarrheal shellfish poisoning (DSP), neurotoxic shellfish poisoning (NSP) and paralytic shellfish poisoning (PSP).
Each type of shellfish poisoning is caused by a different toxin. ASP is caused by domoic acid, produced by Pseudo-nitzschia spp (species pluralis, indicating multiple species with one group). DSP is caused by okadaic acid, produced by Dinophysis spp. The neurotoxin brevetoxin is known to cause NSP and is also associated with the "red tide" (red-hued toxic algal blooms) phenomenon.
There are 20 toxins responsible for paralytic shellfish poisonings (PSP), and all 20 are derivatives of saxitoxin (STX), a toxin produced by the algae bloom Alexandrium fundyense.
Shellfish are filter feeders, which means they eat by filtering microscopic particles from the water. And they don't know the difference between plankton and toxic algae. The toxins accumulate in the body of the clam, mussel, or oyster, and your body absorbs them when you eat the contaminated shellfish, causing gastrointestinal and neurological symptoms.