Hazardous Marine Life

Hazardous Marine Life

Red Tide and Shellfish Poisonings

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 use of 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 microorganisms, certain species of phytoplankton may be present, which can produce harmful natural toxins that can become concentrated in tissues of filter feeders such as shellfish and other mollusks 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 of seafood in areas where HAB is known to have endemic outbreaks.

Shellfish Poisonings

Shellfish are bivalve (two-part shells) mollusks 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 mollusks contaminated with toxins produced by microscopic algae.


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.

Neurotoxic Shellfish Poisoning (NSP)
NSP is caused by a toxin called brevetoxin, naturally produced by a dinoflagellate known as Karenia brevis. Brevetoxin can cause a variety of neurological symptoms very similar to ciguatera. NSP is generally not life threatening, but hospitalization is recommended until all other possible causes have been ruled out. In the United States and the Gulf of Mexico, a blossom of Karenia brevis usually causes the phenomena known as HAB.


HABs occur throughout the world, killing millions of marine animals and affecting fisheries. Before harvesting your own seafood from coastal areas, research where HABs may occur, and avoid consuming self-caught shellfish and fish from areas known to have HABs. Commercial fisheries tend to be safer than small-scale artisanal harvesters.

The National Oceanic and Atmospheric Administration (NOAA) has a NOAA HAB (Red Tide) Watch page on Facebook. This system provides an operational forecast for harmful algal blooms. For those not on Facebook, NOAA's Tides and Currents portal also provides an operational forecast system for HABs.

The Florida Fish and Wildlife Conservation Commission offers an online resource with a current map of red tide counts in the state of Florida.