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Red Tide in Maine

Life cycle of Alexandrium cell, by Don Anderson, WHOIRed tide closures of shellfish areas are different than shellfish area classifications and closures based on fecal coliform pollution, although both closures occur because conditions in the water make shellfish unsafe for human consumption. The term "red tide" as it is commonly used in Maine is a misnomer. What are commonly referred to as red tides are actually "harmful algal blooms," or HABs, and they do not necessarily turn the waters red when toxins are present. Red tide closures occur when high amounts of a toxin found in some types of naturally occurring, single-celled, microscopic algae in the ocean are measured in shellfish tissue.

The most prevalent HAB in Maine is the bloom of the phytoplankton dinoflagellate Alexandrium (a type of single celled green algae). The toxin in this phytoplankton (saxitoxin) causes paralytic shellfish poisoning (PSP). Alexandrium is common in the waters of Maine, but at certain times of year, it is not concentrated enough to pose a threat to public health. However, toxicity can become an issue when the Alexandrium cells “bloom,” meaning the cells reproduce and reach high densities. These cells get consumed by filter feeders such as mussels, clams, oysters and scallops. After the Alexandrium cells are digested, the saxitoxin remains behind and can accumulate if more Alexandrium is ingested. This toxin is transferred through the food web when other species, including humans, then eat the toxic shellfish.

Harmful Algal Blooms in Maine typically occur between April and October. When necessary environmental conditions (oxygen, light, temperature, etc) are present, the cysts which lie dormant in the ocean sediment over the winter begin to germinate (hatch). After germination, the swimming form of the cell is present. If the conditions allow, the cells divide. Under optimum conditions, the cells rapidly reproduce and a bloom may occur.

The State of Maine, Department of Marine Resources Biotoxin Monitoring Program tests coastal shellfish areas for red tide weekly, beginning in March and going through October or later when necessary. Shellfish are collected and their tissues are analyzed to determine how much of the saxitoxin that causes PSP is present. Different species concentrate the toxin at different rates and amounts, so mussels, clams, ocean quahogs, and other species are all analyzed in order to assess the levels of toxicity in each species. When levels rise above federally set public health standards, areas are closed to shellfish harvesting.

People can get PSP after eating a meal contaminated with the saxitoxin found in Alexandrium. Both shellfish and lobster tomalley have the potential to become contaminated with this toxin. Cooking or heating does not destroy the toxin or render it less potent. PSP symptoms are dependant on the amount of toxic seafood consumed, and the amount of the toxin in the food item.

People usually display PSP symptoms within 2 hours of consuming a toxic meal. Mild symptoms include numbness and tingling of the face, arms and legs, followed by headache, dizziness, nausea and muscular incoordination. Severe poisoning causes muscle paralysis and respiratory failure within 2 to 25 hours. There is no antidote for PSP and severe poisonings will require ventilator support.

Map of Maine PSP sampling stationsIt is important to know when shellfish are safe to collect and eat. PSP illnesses have occurred in Maine in 2007, 2008 and 2009. In each case, the contaminated shellfish were taken from an area that was closed to harvest due to high levels of saxitoxin. Toxic shellfish can be found in clear, clean and remote waters off the coast of Maine. Toxic shellfish do not look or taste any different from non-toxic shellfish. You can find out whether it is safe to harvest in a specific area by using one of the methods below. Alternatively, you can purchase your shellfish from certified dealers or enjoy your meal at a restaurant.