Should you really only eat shellfish when there's an 'R' in the month?

By: Tom Scheve

That oyster might be delicious with a little lemon, mignonette or hot sauce, but is it safe to eat?
That oyster might be delicious with a little lemon, mignonette or hot sauce, but is it safe to eat?

If you've ever gotten seafood sickness, you may think shellfish should be eaten only when there's a "Z" in the month -- as in, never. But for seafood lovers who've never had a bad dining experience, the question may be, "Should I really only eat shellfish, and nothing else, always?"

Shellfish are edible, spineless, slimy and arguably delicious aquatic life forms that have protective shells that serve as external skeletons. Many people love eating them, some people swear they act as an aphrodisiac, and all people agree they can potentially make you incredibly sick.


Shellfish are bivalves (they have two shells), and include seafood delights such as oysters, clams, scallops and mussels. Shellfish filter water through their two shells and feed on the algae and plankton they find in it. Plankton in the water for a shellfish is like grass in a field for a cow. In this way, the shellfish are grazing upon the sea. What they're grazing on are tiny (as small as 1/50th of a millimeter) aquatic life forms called flagellates (which we'll more talk about shortly).

When shellfish themselves eat "bad" flagellates, the toxins accumulate in the "guts" of the shellfish. When you eat shellfish, you eat pretty much everything but the shell, guts and all. So if a shellfish ate bad plankton, it's now a bad shellfish, and if you eat it, you'll be in a bad way. (But take heart in a spiteful sort of way: If something then ate you, it too would feel nauseated for a day or so after eating some "bad human.")

Shellfish poisonings normally occur in warmer waters but are also frequent in New England and even in the chilly waters around Alaska. People have long associated shellfish poisoning with red tides, which are periods of massive reproduction of plankton that can give the waters in which they occur a reddish tint.

But are red tides solely to blame? Is shellfish safe to eat as long as there isn't a red tide? How sick will you be if you do get shellfish poisoning? And should you really only eat shellfish when there's an "R" in the month? Keep reading to find out.

Types of Shellfish Poisoning: How Bad Could It Really Be?

Red tides can cause widespread contamination and death among fish and other sea life.
Red tides can cause widespread contamination and death among fish and other sea life.

There are four different types of shellfish sickness. All four types share some common features (such as headache and nausea), but each one has a horrible feature all its very own:

  • When you think of seafood sickness in general, you probably imagine something much like diarrheal shellfish poisoning. It's the old-fashioned, one-day kind, and the preferred form among shellfish connoisseurs. It plays all the popular hits: vomiting, abdominal pain, date-ending diarrhea. It won't kill you, but it may cure you of your shellfish addiction.
  • Whereas diarrheal shellfish poisoning begins its ominous rumblings about half an hour after eating bad shellfish, paralytic shellfish poisoning instead causes numbness and tingling in your mouth. This is a good time to ask the waiter for the check. Soon, the tingling will spread to your arms and legs and you'll feel like you're floating. While some people pay good money for drugs to feel this way, this is the most serious form of shellfish poisoning -- it can paralyze your lungs and kill you in as little as two hours. The fatality rate is as high as 12 percent [source: Pledger].
  • Because doctors couldn't name this next form of poisoning "temporary insanity shellfish poisoning," they named it neurotoxic shellfish poisoning. Suddenly, an ice cube feels hot to you, and a candle flame ice cold. You'll feel a million tiny pinpricks on your skin, your muscles will ache and you'll become dizzy. You won't die, but interestingly, the toxin responsible for this form of poisoning can become aerosolized, meaning you can breathe it in and become sick while walking along the shoreline.
  • Shellfish poisoning isn't an experience you'll soon forget, unless it's the type that destroys your memory -- amnesic shellfish poisoning. It sounds like a plausible defense from a criminal attorney: "My client insisted on harvesting his own shellfish in July, got amnesic shellfish poisoning and can't remember where he was on the night in question." Amnesic shellfish poisoning is most common in the Pacific Northwest. The unfortunate consumer of the bad shellfish will first experience "normal" symptoms of seafood poisoning, but may soon become confused, disoriented and even comatose. The cognitive damage is permanent, affecting short-term memory and even causing dementia.

Now that we've clarified just how pleasant shellfish poisoning is, let's figure out how to avoid it.


How Toxic Shellfish Get That Way

Drakes Bay Oyster Co. workers harvest strings of oysters from racks on Schooner Bay in Point Reyes Station, Calif.
Drakes Bay Oyster Co. workers harvest strings of oysters from racks on Schooner Bay in Point Reyes Station, Calif.
Justin Sullivan/Getty Images

Dinoflagellates are a family of single-celled organisms that heavily populate the ocean. They clump together and float near the surface, allowing ocean currents to dictate their travels. Some dinoflagellates use photosynthesis to transform sunlight into energy, while others are capable of producing their own light. These tiny life forms are the perfect food for shellfish: delicious, nutritious and, most importantly, tiny enough to be filtered between shells.

Dinoflagellates reproduce in great numbers during the summer, and sometimes the population explosion produces so many of these tiny organisms that the water takes on a reddish tint. This is known as a red tide. People have long associated shellfish poisoning with red tides, even though researchers don't fully understand the connection between the two. Furthermore, toxic dinoflagellates can be present without the visual presence of a red tide, and there can be a visible red tide without the presence of toxic dinoflagellates, so it doesn't seem to be a reliable indicator.


What does seem to matter is what the dinoflagellates themselves have been dining on. Dinoflagellates sometimes consume toxic alkaloids called saxitoxins. The type of saxitoxin the dinoflagellate consumes determines what type of shellfish poisoning you'll receive at your lofty position atop the food chain.

The bad news is that once a shellfish becomes toxic, no amount of heat during cooking will destroy the bacteria. Because of this, you should definitely not harvest your own shellfish during months with an "R" in them, especially along the Pacific Coast. This activity is banned in the state of California (and restricted elsewhere) between May and October because the toxin responsible for paralytic shellfish poisoning is more prevalent during those months. Generally speaking, however, the major downside to summertime oysters is the disappointment you may feel upon eating them -- bivalves reproduce in the summertime and most of their energy goes toward this endeavor, leaving the meat somewhat scrawny.

The good news is that precautions taken during the raising and harvesting of commercial shellfish make them safe to eat any time of the year. So long as your shellfish has been commercially harvested and no advisories have been issued by state governments or the U.S. Department of Health, you should rest easy knowing your shellfish is safe to enjoy.

Want to see more old wives' tales debunked? See the next page.

Lots More Information

Related HowStuffWorks Articles

  • Arnold, Thomas, MD. "Toxicity, Ciguatera." June 7, 2007. (July 13, 2009)
  • Arnold, Thomas, MD. "Toxicity, Shellfish." June 7, 2007. (July 9, 2009)
  • Assure Controls. "All About Dinoflagellates." (July 14, 2009)
  • Fleming, Lora E. "Amnesic Shellfish Poisoning. NIEHS Marine and Freshwater Biomedical Sciences Center. May 7, 2008.
  • Harstine Oyster Company. (July 15, 2009)
  • Massachusetts Office of Health and Human Services. "Red Tide Fact Sheet." July 14, 2009.
  • MedlinePlus. "Poisoning - fish and shellfish." Jan. 30, 2009. (July 9, 2009)
  • Ohio Department of Health. "Fact Sheet: Shellfish toxins." (July 9, 2009)
  • Pledger, David, MD. "Toxicity, Seafood." June 21, 2006.
  • University of California Museum of Paleontology. "Introduction to the Dinoflagellata." (July 9, 2009)
  • U.S. National Office for Harmful Algal Blooms. "Harmful Algae."
  • Washington State Department of Health. "Paralytic Shellfish Poisoning ("red tide")." Mar. 5, 2009. (July 14, 2009)
  • Washington Department of Fish and Wildlife. "Shellfish." (July 16, 2009)