Harmful Algal Blooms-Seafood and Aquatic Life

Harmful algal blooms (HAB's) are naturally occurringconcentrations of microscopic algae. You can find them in waters worldwide. About2% are known to be harmful or toxic of the 5000+ species of marine planktonthat exist worldwide. Blooms of harmful algae can have large and varied impactson marine ecosystems, depending on:

  • The species involved,
  • The environment where they are found, and
  • The mode of transport by which they exert harmful effects.

We’ve seen that harmful algal blooms can cause adverse effectsto a lot of aquatic organisms. The most harmful effects are on:

  • Marine mammals,
  • Sea turtles,
  • Seabirds, and
  • Finfish.

The impacts of HAB toxins on these groups can include harmfulchanges to their developmental, immunological, neurological, or reproductivecapacities.

Karenia brevis (red tide)

Karenia brevis is a planktonic marine dinoflagellate. The water takes ona reddish discoloration when K. brevis has explosive growth. That’s why thename, red tide, is used to describe these blooms.

This HAB produces a group of toxins called brevetoxins. Thesetoxins are lethal to fish. They can cause respiratory problems for people whoare near the bloom. It will also give people neurotoxic shellfish poisoning(NSP) if they eat molluscan shellfish (oysters, clams, and mussels) that feedson this algal species. Molluscan shellfish can become toxic when K. brevis is atlower concentrations than what causes water discoloration. This is becausemolluscan shellfish filter algae as they feed.

Which Seafood Are Safe to Eat When K. brevis is Present

Brevetoxins are not concentrated by:

  • Fish,
  • Shrimp, or
  • Crabs, so

these forms of seafood are safe to eat if they are caught aliveand acting normal. 

You should not ever eat any fish or shellfish that has:

  • Washed ashore sick,
  • Washed ashore dead, or
  • Found floating "belly up" in the water.

Also, any molluscan shellfish (oysters, clams, mussels, andscallops) legally available in markets and restaurants also should be safe toeat. They were harvested from areas that the bloom didn’t affect. 

Neurotoxic Shellfish Poisoning (NSP)

K. brevis produces at least two major heat-stable, lipid-soluble toxinsknown as brevetoxins A and B. These toxins appear to affect sodium transport inthe autonomic nervous system. They will inhibit neuromuscular transmission inskeletal muscle.

Many species of fish are sensitive to brevetoxins. They will"drown" in red tide waters because the toxins will paralyze theirgills.

Oysters, clams, and mussels are not susceptible to these toxins.They may appear perfectly healthy, but, when these shellfish feed for a time inwaters with high concentrations of K. brevis, they accumulate brevetoxins intheir body tissues. This makes them toxic to people who eat them. The toxin isonly slowly cleared from shellfish after the red tide disappears. So, if youeat oysters, clams, or mussels caught during (or even months after) a red tide,it can give you neurotoxic shellfish poisoning (NSP). We would close harvestingto any Texas bays with K. brevis bloom. 

  • Cooking will not eliminate the toxin. That’s why cooking willnot make contaminated seafood safe.

Characteristic Symptoms of NSP: Gastrointestinal and Neurologic

There is currently no specific diagnostic laboratory test toidentify NSP. The diagnosis is based on clinical evaluation of cases withrelevant exposures.

You usually have onset of NSP symptoms within three hours of eatingcontaminated shellfish (range 15 minutes to 18 hours after exposure).

Initial complaints typically include:

  • Abdominal pain,
  • Nausea,
  • Vomiting, and
  • Diarrhea,

accompanied by progressive paresthesia. Paresthesia firstaffects the area around the mouth. Then, it affects the pharynx, trunk, andlimbs.

Other common symptoms include:

  • Vertigo,
  • Malaise,
  • Generalized muscle weakness,
  • Ataxia,
  • Incoordination,
  • Chills,
  • Headache, and
  • Myalgia.

Some people have also reported a reversal of hot and coldtemperature sensation (like that seen in ciguatera poisoning).

In cases of severe poisoning:

  • Dilated pupils,
  • Bradycardia, and
  • Convulsions requiring respiratory support (a rare symptom).

 Differential Diagnosis

The geographical origin of the affected shellfish can helpidentify the probable toxigenic dinoflagellate. Alexandrium catenella (formerly Gonyaulaxcatenella) is the leading toxigenic dinoflagellate that we find along thePacific coast of North America. Alexandrium tamarense-excavatum (formerly Gonyaulaxtamarensis) is mostly in the northern Atlantic coast of North America.

K. brevis is the dinoflagellate that usually makes the red tidesin the Gulf of Mexico and the southern Atlantic coast of North America. NSP isa relatively mild illness. You should not confuse it with the much more seriouscondition, paralytic shellfish poisoning (PSP).

Saxitoxin elaborated by one of the Alexandrium speciesof dinoflagellates causes PSP. The symptoms of PSP are very similar to those ofNSP. The difference with the neurotoxic effects of PSP is that it can progress quicklyto respiratory paralysis and death without a medical respirator.

Treatment for NSP

Treatment is supportive in nature. This is because:

  • NSP is a relatively innocuous form of shellfish poisoning, and
  • No specific antitoxin is available.

Patients should be monitored closely for hydration status, especiallyif symptoms include a significant amount of vomiting or diarrhea. Patients whohave eaten a lot of the affected shellfish with more severe symptoms might needbrief hospitalization for observation. The illness is usually self-limiting. Symptomsusually subside in less than 24 hours with supportive therapy.

Other Symptoms Associated with K. brevis Blooms

NSP has gastrointestinal and neurologic symptoms because ofeating shellfish with high levels of brevetoxins. Some people have hadrespiratory, mucous membrane, and skin irritation simply by walking on thebeach during K. brevis red tides.

Karenia organisms release cellular endotoxins into the surroundingwater when they’re disrupted. Wind and surf action makes a fine aerosol that travelsonly a short distance from the beach. So, in areas with a great deal of surfaction, airborne exposure to the toxins can be a problem for some people.

Exposed individuals frequently report an acute but rapidlyreversible syndrome consisting of:

  • Conjunctival irritation,
  • Rhinorrhea,
  • Sneezing,
  • Cough, and
  • Respiratory distress like an asthma attack (in rare cases).

People swimming or wading in K. brevis redtides might have eye and skin irritation. They might also have redness anditching.

Symptoms usually go away after short-term exposure when you leavethe immediate beach area. Long-term exposure, though, can cause symptoms thatlinger for hours or days after the person leaves the affected area. 

  • We recommend that visitors to the affected areas avoid swimmingor wading in red tide waters.


Dinophysis ovum is a marine dinoflagellate. It makes okadaic acid (OA),and it can give you diarrhetic shellfish poisoning (DSP). This usually happenswhen OA is concentrated in the intestinal cavity of molluscan shellfish. DSPtoxins are heat stable. Cooking will not eliminate the toxin. So, cookingwill not make contaminated seafood safe.

DSP is a self-limiting diarrheal disease. There is no evidenceof neurotoxicity. There have been no fatal cases of DSP reported. Diarrhea isthe most common symptom, but some people also report nausea and vomiting. Symptomscan happen between 30 minutes to 12 hours. You will have complete recoverywithin 3 days.

These are approved laboratory methods to identify and quantify OAor Dinophysis toxins:

  • Mouse bioassay,
  • Direct detection with high performance liquid chromatography,
  • Electrospray ionization,
  • Multiple reactions monitoring, and
  • Mass spectrometry (HPLC/ESI/MRM/MS).

Most people have reported cases of DSP from eating mussels, butoysters have also been implicated. Recently, DSP has been reported from oystersharvested in Washington and British Columbia.


You get Ciguatera fish poisoning from ciguatoxins. These aremade by the dinoflagellate Gambierdiscus toxicus. This single-celledorganism attaches to marine algae during blooms of G. toxicus.Those organisms are eaten by various herbivorous fish.

These fish are then eaten by carnivorous fish. That’s how theciguatoxin is passed up the food chain into a many of the larger tropical reeffish, such as:

  • Red snapper,
  • Barracuda,
  • Amberjack, and
  • Grouper.

Humans can be affected if they catch and eat any fish with largeamounts of the toxin.

Ciguatera Advisory Issued for Fish Near Flower Gardens (CoralReef)

We and the U.S. Food and Drug Administration are askingrecreational fishermen and other people not to eat the fish from the FlowerGarden Coral reef system. This system is in federal waters off the northerncoast of Texas. Algae that grow on some coral makes the ciguatera toxin.

The Flower Garden is a coral reef system. The toxin is stored infish tissue from the natural food chain. Smaller fish eat the algae and pass itto larger fish who eat smaller fish. 

  • Cooking will not eliminate the toxin. Cooking will not makecontaminated seafood safe.

The fish species covered in this advisory are marbled, gag,scamp and yellowfin grouper; blackfin and dog snapper; and hogfish caughtwithin 10 miles of the Flower Garden. Also included are yellow, horse-eye andblack jack; king mackerel; amberjack; and barracuda from within 50 miles of thesanctuary.

Readthis article for more information: FDA advises seafood processors about ciguaterafish poisoning in the northern Gulf of Mexico.