Foods that harm: toxins in shellfish

Shellfish is the catchall term applied to molluscs and crustaceans — water-dwelling creatures that wear their skeletons on the outside. While they are a very low-fat, healthy addition to any diet, shellfish could also carry toxins that can be dangerous to eat.

If grown in polluted waters, shellfish may be contaminated with bacteria and carry a particular risk for hepatitis.

• Don't gather shellfish at the seashore or near wharf pilings or built-up areas.
• Instead, buy them from fish markets and food stores that keep shellfish well covered with ice or, in the case of lobsters, in tanks with circulating water aerated with oxygen.
• Shallow-water shellfish, such as clams and mussels, are the most susceptible to pollution; sea scallops and other deep-water varieties are less likely to be exposed to waste.
• Fresh shellfish may be covered with ice chips and stored for several hours at 0°C (32°F).
• Eat them on the day they are purchased.
• Oysters are available year-round, and cooked oysters can be safely eaten at any time.
• Raw oysters, particularly those harvested in the Gulf states, can present a problem because of contamination with the Vibrio vulnificus bacterium. Contamination is more likely in the warm months because the bacteria do not tolerate cold water well.
• Diabetics, people with liver disease and those with compromised immune systems have to be especially careful.
• From time to time, swarming plankton cause the phenomenon known as "red tide" in coastal waters. Shellfish exposed to the red tide ingest the micro-organisms, which produce a toxin that can survive cooking.
• The symptoms of red tide poisoning usually appear within 30 minutes of consuming contaminated fish; they include facial numbness, breathing difficulty and muscle weakness.
• Never take shellfish from red-tide areas. ("Red tide" doesn't necessarily mean there is a red tinge to the water).
• Mussels accumulate toxins more quickly than other types of shellfish. In fact, scientists who monitor waters for poisons use them as an indicator species.
• Mussels tainted with domoic acid, another toxin derived from algae, are undetectable by consumers.
• Scallops, on the other hand, pose less risk than other bivalves because we don't eat the animal itself but the muscle that holds the shell halves together.

• A low-fat source of high-quality protein.
• A rich source of minerals, including calcium, fluoride, iodine, iron and zinc.
• A good source of B-group vitamins.

• Some are high in cholesterol.
• Susceptible to spoilage and environmental contamination.
• Can provoke allergic reactions in some people.