Reply To: Red tide in Florida Gulf (Tampa – pre-Katrina) – 08/22/2005

November 11, 2013 at 10:23 pm #943

red tide – 08/25/2005′, ‘from bridget:

Not every plant poisoning is caused by a large, terrestrial plant. Several microscopic marine algae are notoriously poisonous to hapless humans who consume them in shellfish.
These Algae are tiny, single-celled plants that, like plants on land, capture and use the sun’s energy to grow. The growth of algae is an essential life process, as it is the first step in transferring solar energy into aquatic food webs. The huge variety of marine algae are typically subject to annual cycles of growth & decay. These organisms thrive and multiply principally during the spring and summer, in response to increased light intensity and favourable levels of salinity & nutrients in ocean water. During the growth period, or bloom, each single algae cell may replicate itself one million times in two to three weeks.

During the reproductive riot of the bloom, warm, shallow seawater tends to become discoloured by the sheer concentration of algae seeking the sunlight. This discolouration is a result of the various pigments the plants use to trap sunlight; depending on the species of algae present, the water may reflect pink, violet, orange, yellow, blue, green, brown, or red. Since red is the most common pigment, the phenomenon has come to be called Red Tide.

Most species contributing to algal blooms are harmless, BUT (another big but!) some species are poisonous to animals which feed upon them directly or indirectly. Some of the toxins these species produce are seriously toxic. Often, the algae themselves are unaffected, as are the filter feeders, especially shellfish, for whom micro-algae are the principal diet. However, to carnivores further up the food chain, including humans, these toxins are potentially FATAL.

As in the case of poisonous fungi, scientific knowledge of poisonous algae is incomplete; indeed, new and alarming toxic algae are still coming to light. In Nova Scotia, however, three different algae are known to cause three, distinct, severe to deadly poisonings:

Alexandrium causes Paralytic Shellfish Poisoning (PSP)
Dinophysis causes Diarrhetic Shellfish Poisoning (DSP)
Pseudo-nitzschia multiseries causes Amnesic Shellfish Poisoning (ASP)
CLICK on the names above for a microscopic view and specific information.

THE GUTS AND INTESTINAL TRACT of shellfish and fish which have consumed toxic algae accumulate and store the toxins. As a result, fish, provided they are gutted in the usual manner, are not a threat; nor, ironically, are scallops- a shellfish whose "meats"- the muscles that open and close its shell- are the only parts harvested for food. All other infected shellfish, however- especially mussels, oysters & clams- may cause ASP, PSP, or DSP because they are eaten whole.

DOMOIC ACID is an amino acid-based toxin which is potentially lethal. Tiny amounts can have deadly results (see below) in humans. Algal blooms can also cause other problems besides toxicity. When an algae bloom dies back in a shallow bay, oxygen-depleting bacteria may thrive on the dead algae and so kill fish and other marine life. Sometimes floating algae cut eelgrass beds off from sunlight, robbing shellfish of habitat. On fish farms, concentrations of algae can simply clog the gills of the fish, causing respiratory failure.


Almost all incidents involve the accidental ingestion of mussels, clams, oysters or other shellfish affected by toxic algae. Since there is no visual method for determining whether particular shellfish are affected, Canada has been carrying out extensive laboratory monitoring of shellfish toxicity since 1943. As soon as these routine studies detect dangerous concentrations of toxic algae, Fisheries and Oceans Canada posts affected beaches and estuaries with orders closing them to shellfish harvesting.

It is dangerous to ignore these signs, which are commonplace in Nova Scotia during the warmer months. Little wonder that folklore warns us not to eat shellfish in months with no "r" in their names! By the mid-Eighteenth Century, shellfish poisoning was so commonplace as to give rise to the London proverb, "There is no venturing on oysters, Sir, or women of the town"…

Shellfish poisoning, of whatever sort, is UNPLEASANT AND OFTEN LETHAL. The onset of symptoms occurs as soon as the victim’s digestive system starts to work on the infected shellfish. In the case of paralytic shellfish poisoning (PSP), the toxin attacks the nervous system and causes paralysis. There is no antidote; death occurs from asphyxiation or respiratory paralysis.
PSP has been known since biblical times; what is thought to be PSP figures in the Book of Exodus, and the Red Sea is thought to have been named for the signs of frequent blooms of algae. Not for nothing did the Judaic Partriarchs proscribe shellfish…

The first recorded case in Canada, however, occurred in 1793, during Captain George Vancouver’s expedition to what is now British Columbia, when John Carter, a seaman, died from the effects of eating mussels, presumably infected with toxic algae. An eyewitness account of his death occurs in the June 17 diary entry of the expedition naturalist and surgeon, Archibald Menzies.

Diarretic Shellfish poisoning, as the name suggests, causes extreme gastrointestinal upset; DSP is less dangerous than PSP, but failure to treat the diarrhoea may lead to death from dehydration or other complications.

Domoic acid, which causes amnesic shellfish poisoning, binds to chemical receptors in brain cells and causes their dysfunction. The poisoning begins with stomach upset, rapidly followed by dizziness, disorientation, and memory loss- symptoms which often persist indefinitely. During a 1987 outbreak on PEI, 1% of the reported poisonings resulted in death from brain damage.