Harmful Algal Blooms-Seafood and Aquatic Life

Harmful algal blooms (HAB's) are naturally occurring concentrations of microscopic algae. You can find them in waters worldwide. About2% are known to be harmful or toxic of the 5000+ species of marine  exist worldwide. Blooms of harmful algae can have large and varied impacts on 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 effects to 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 harmful changes to their developmental, immunological, neurological, or reproductive capacities.

Karenia brevis (red tide)

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

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

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

Breve toxins are not concentrated by:

  • Fish,
  • Shrimp, or
  • Crabs, so

these forms of seafood are safe to eat if they are caught alive and 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, and scallops) legally available in markets and restaurants also should be safe to eat. 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 toxins known as breve toxins A and B. These toxins appear to affect sodium transport in the autonomic nervous system. They will inhibit neuromuscular transmission in skeletal muscle.

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

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

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

Characteristic Symptoms of NSP: Gastrointestinal and Neurologic

There is currently no specific diagnostic laboratory test to identify NSP. The diagnosis is based on clinical evaluation of cases with relevant exposures.

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

Initial complaints typically include:

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

accompanied by progressive paresthesia. Paresthesia first affects the area around the mouth. Then, it affects the pharynx, trunk, and limbs.

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 cold temperature 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 help identify the probable toxigenic dinoflagellate. Alexandrium catenella (formerly Gonyaulaxcatenella) is the leading toxigenic dinoflagellate that we find along the Pacific 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 tides in the Gulf of Mexico and the southern Atlantic coast of North America. NSP is a relatively mild illness. You should not confuse it with the much more serious condition, paralytic shellfish poisoning (PSP).

Saxitoxin elaborated by one of the Alexandrium species of dinoflagellates causes PSP. The symptoms of PSP are very similar to those of NSP. The difference with the neurotoxic effects of PSP is that it can progress quickly to 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, especially if symptoms include a significant amount of vomiting or diarrhea. Patients who have eaten a lot of the affected shellfish with more severe symptoms might need brief hospitalization for observation. The illness is usually self-limiting. Symptoms usually subside in less than 24 hours with supportive therapy.

Other Symptoms Associated with K. brevis Blooms

NSP has gastrointestinal and neurologic symptoms because of eating shellfish with high levels of breve toxins. Some people have had respiratory, mucous membrane, and skin irritation simply by walking on the beach during K. brevis red tides.

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

Exposed individuals frequently report an acute but rapidly reversible 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 red tides might have eye and skin irritation. They might also have redness and itching.

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

  • We recommend that visitors to the affected areas avoid swimming or 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 happens when OA is concentrated in the intestinal cavity of molluscan shellfish. DSP toxins are heat stable. Cooking will not eliminate the toxin. So, cooking will not make contaminated seafood safe.

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

These are approved laboratory methods to identify and quantify OA or 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, but oysters have also been implicated. Recently, DSP has been reported from oysters harvested in Washington and British Columbia.


You get Ciguatera fish poisoning from ciguatoxins. These are made 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 the ciguatoxin is passed up the food chain into a many of the larger tropical reef fish, such as:

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

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

Ciguatera Advisory Issued for Fish Near Flower Gardens (Coral Reef)

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

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

  • Cooking will not eliminate the toxin. Cooking will not make contaminated seafood safe.

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