Red Tide and shellfish poisoning

Red tide is a colloquial term for a specific phenomenon known as harmful algal bloom. Occasionally, large concentrations of aquatic microorganisms naturally bloom in coastal areas. The rapid accumulation of algal blossom can be significant enough to cause a green, red or brown discoloration of estuarine and freshwater environments.
Scientists discourage the term red tide, because these phenomena are unrelated to tidal water movements and may not necessarily be red in color or present any discoloration at all. Instead, when these algal blooms are associated with potentially harmful toxins, a more precise and favored terminology is harmful algal bloom (HAB).
 
NEGATIVE IMPACT ON ECOSYSTEMS
Among the involved micro-organisms certain species of phytoplankton may be present, which can produce harmful natural toxins that can become  concentrated in tissues of filter feeders like shellfish and other molluscs and crustaceans. The whole food chain may be affected, and millions of fish may die as a result.

DANGER TO HUMANS
These toxins can affect commercial fisheries and represent a public health threat. People who consume contaminated shellfish may suffer a variety of shellfish poisonings, some of which are potentially lethal. Hazards related to HAB may not be limited to shellfish consumption, so avoid harvesting any type seafood on areas where HAB is known to have endemic outbreaks.
SHELLFISH POISONINGS Shellfish are bivalve (two-part shells) molluscs that capture nutrients by filtering water. During this process, these filter feeders can accumulate toxins and other contaminants. When humans consume these bivalves, they may be poisoned. These toxins are water soluble and heat- and acid-stable—they are unaltered by ordinary cooking methods. Shellfish poisonings are a group of four different syndromes caused by eating bivalve
molluscs contaminated with toxins produced by microscopic algae.

SYNDROMES
There are four different types of shellfish poisonings that are primarily associated with mollusks such as mussels, clams, oysters and scallops.

PARALYTIC SHELLFISH POISONING (PSP)
These mollusks can accumulate a toxin called saxitoxin, which is produced by phytoplankton (dinoflagellates, diatoms and cyanobacteria). Some shellfish remain toxic for several weeks, while others can store the toxin for up to two years. PSP blooms are associated with harmful algal blooms, which can occur in almost all oceans. PSP can be fatal, particularly in children. Symptoms can appear a few minutes after ingestion and include nausea, vomiting, diarrhea, abdominal cramps, numbness or burning around the mouth, gums, tongue and progressing to the neck, arms, legs and toes. Other symptoms may include dry mouth, shortness of breath, slurred speech and loss of consciousness. Signs of toxicity and mortality are also seen in wild animals.

AMNESIC SHELLFISH POISONING (ASP)
This rare syndrome is caused by consuming shellfish contaminated with a toxin called domoic acid produced by certain marine diatoms. Symptoms can appear 24 hours after ingestion of contaminated mollusks and may include nausea, vomiting, diarrhea, abdominal cramps and hemorrhagic gastritis. Neurological signs are severe and can take up to three days to develop. Neurological signs include dizziness, disorientation, visual disturbances, short-term memory loss, motor weakness, seizures, increased respiratory secretions and life-threatening dysrhythmias (irregular heartbeat). Death is rare. Resulting conditions due to permanent damage to the central nervous system may include short-term memory loss and peripheral neuropathy (weakness, numbness or pain as a result of nerve damage).

DIARRHEAL SHELLFISH POISONING (DSP)
Certain dinoflagellates produce a toxin known as okadaic acid that can cause a
diarrheic syndrome. This toxin can damage the intestinal mucous membrane making it very permeable to water, which causes significant diarrhea as well as nausea, vomiting and abdominal cramps. Symptoms can strike within a few minutes to an hour of ingesting contaminated shellfish and can last for about one day. No life-threatening symptoms have ever been recorded, but serious dehydration may occur.