Plankton are microscopic life forms fed on by shellfish. The toxins produced by some forms of plankton can accumulate in certain marine animals, affecting humans who consume them.
What is Red Tide?
Red tide is a toxic algal bloom of red-brown dinoflagellates that occurs after a population explosion due to specific water conditions. The blooms are usually seen when warm surface temperatures, low salinity, high nutrient content, and calm waters are present. Warm, sunny days after much rain often create these conditions, and is likely the reason for the issues with paralytic shellfish poisoning along the coast of Maine in April 2010, which resulted in the closing of mussel, clam, and snail harvest areas. Sometimes the bloom can discolor the water, giving the phenomenon its name.
Organisms that Cause Red Tide in North America
The toxins from algal blooms that cause paralytic shellfish poisoning tend to accumulate in shellfish that live in the cool coastal waters of the Pacific U.S. and New England. The specific organisms that cause red tide in the United States and along the Canadian Maritimes are:
- Alexandrium fundyense – Atlantic coast
- Alexandrium catenella – Pacific coast
- Karonia brevis – Gulf of Mexico coast
The shellfish affected by the toxins usually do not appear different. The toxin also has no taste or smell. So the safest method to avoid toxicity during red bloom is to avoid eating certain seafood, specifically molluscan shellfish.
Unsafe seafoods during toxic algal bloom include the following:
- clams (both hard and soft shell)
- oysters
- mussels
- whelks
- moon snails and other gastropods
- cockles
- oysters
- geoduck
- lobster tomalley (the liver)
Safe seafoods during toxic algal bloom include the following:
- scallops (only the adductor muscle is safe)
- lobster (only the meat)
- crab (only the meat, properly clean and discard the guts)
- shrimp
- finfish
Paralytic Shellfish Poisoning
Within minutes to 10 hours of eating shellfish contaminated with saxitoxin, which attacks the nervous system, the lips will feel numb and tingling. The amount of toxin consumed will determine the severity and extent of symptom progression. The tingling can extend to the fingers and toes, eventually resulting in a loss of use of the arms and legs. Paralysis can progress to the muscles throughout the trunk of the body, including the chest and respiration. In severe cases of poisoning, death can occur in two to 25 hours after consumption. Headache, dizziness, and nausea may also appear as muscle coordination is lost.
Preventing Human Poisoning
Shellfish are monitored year round for toxin levels. Cases of paralytic shellfish poisoning are reported to state health departments for appropriate monitoring and require treatment by a doctor. Spring, summer, and fall are prime monitoring seasons due to weather factors that influence algal blooms. Shellfish that make it to seafood markets are only taken from clean waters.
Sources:
Massachusetts Department of Health and Human Services, “ Red Tide Fact Sheet ” (accessed April 14, 2010).
U.S. Centers for Disease Control and Prevention, “Marine Toxins” (accessed April 14, 2010).
Washington State Department of Health, Division of Environmental Health, Office of Shellfish and Water Protection, “Paralytic Shellfish Poisoning (“red tide”)” (accessed April 14, 2010).
Alicia Mae Prater - Alicia received her doctorate in Experimental Pathology in 2007. She has been a freelance writer and scientific editor since 2008.
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