Domoic acid is a naturally occurring neurotoxin produced by certain species of marine algae, most notably diatoms in the genus Pseudo-nitzschia. It accumulates in shellfish and fish that feed on these algae, and when humans eat contaminated seafood, it can cause a condition called amnesic shellfish poisoning, named for the memory loss it produces. The toxin is one of the most potent threats to emerge from harmful algal blooms, affecting not just people but sea lions, dolphins, pelicans, and other marine wildlife along coastlines worldwide.
How Domoic Acid Works in the Brain
Domoic acid is structurally almost identical to glutamate, the brain’s primary excitatory chemical messenger. Because of that resemblance, it binds to the same receptors that glutamate normally activates, but with much higher affinity. It essentially outcompetes glutamate and locks onto the receptor, forcing ion channels to stay open far longer than they should.
When those channels stay open, sodium and calcium flood into nerve cells. At low concentrations, domoic acid first activates one set of receptors, which depolarizes the cell membrane enough to unlock a second, normally blocked set of receptors. This cascade triggers a massive surge of calcium into neurons. That sustained calcium overload causes the cells to swell and eventually die, a process called excitotoxicity. The hippocampus, the brain region critical for forming new memories, is especially vulnerable to this damage, which is why memory loss is the hallmark symptom of poisoning.
Where Domoic Acid Comes From
The primary producers are microscopic diatoms in the genus Pseudo-nitzschia, which are found in oceans around the world. A few species of red algae also produce domoic acid, but Pseudo-nitzschia blooms are responsible for the vast majority of contamination events that affect seafood safety. These diatoms are a normal part of ocean plankton communities. They become dangerous when environmental conditions cause their populations to explode into large blooms that produce high concentrations of the toxin.
Along the California coast, the triggers vary by region. In Monterey Bay, blooms tend to intensify during periods of unusually low upwelling, when waters are relatively nutrient-poor. Further south, in the Santa Barbara and San Pedro Channels, the pattern flips: blooms favor cold, nitrogen-rich conditions during stronger upwelling. Across the coast as a whole, domoic acid outbreaks correlate strongly with upwelling patterns and the availability of silicic acid relative to other nutrients.
Filter-feeding shellfish like mussels and razor clams concentrate the toxin in their tissues as they feed on the algae. Crustaceans, including Dungeness crab, also accumulate it, particularly in their internal organs. Fish that eat the diatoms can carry the toxin as well, passing it further up the food chain.
Symptoms of Amnesic Shellfish Poisoning
In most cases, gastrointestinal symptoms appear first, typically within 24 hours of eating contaminated shellfish. These include abdominal pain, diarrhea, and vomiting. Within 48 hours, neurological symptoms can follow: headache, confusion, disorientation, and the characteristic memory loss. The amnesia can involve difficulty forming new memories, recalling recent events, or both.
Most people recover within hours to days. Severe cases, though rare, can involve dangerous drops in blood pressure, heart rhythm abnormalities, paralysis of eye muscles, coma, and death. Survivors of serious poisoning may be left with permanent short-term memory deficits that persist long after the initial illness resolves.
Diagnosis relies on a combination of symptoms, a history of eating shellfish from a temperate coastal area, and laboratory testing. Urine samples can be tested for domoic acid, and food samples can be analyzed using several techniques to confirm contamination.
There is no antidote for domoic acid poisoning. Treatment is entirely supportive, focused on managing symptoms like seizures, dehydration, and cardiovascular instability while the toxin clears the body.
Impact on Marine Wildlife
Domoic acid takes a far heavier toll on marine animals than on humans, largely because wildlife has no way to avoid contaminated prey. Neurological symptoms have been documented in pelicans, cormorants, loons, grebes, sea otters, dolphins, and California sea lions.
Sea lions are particularly hard hit. In 1998, more than 400 sea lion deaths were attributed to domoic acid in the first documented large-scale marine mammal die-off from the toxin. Affected animals show a distinctive set of symptoms: weaving and bobbing of the head, seizures, bulging eyes, disorientation, and uncoordinated “drunken” movements. Because the neurological effects make it difficult for exposed animals to stay afloat and breathe in water, many drag themselves onto beaches in an effort to survive.
One of the more alarming findings is that the damage can be delayed. Sea lion pups exposed to domoic acid before birth show an increased risk of developing seizures in adulthood, sometimes after a long period of apparently normal health. A similar pattern of latent toxicity has been suggested in humans as well, where seizures or other neurological effects may surface well after the initial exposure.
Seafood Safety Limits
Regulatory agencies monitor domoic acid levels in commercial shellfish to prevent contaminated products from reaching consumers. The FDA sets the action level at 20 parts per million (mg/kg) for all shellfish. Dungeness crab viscera get a slightly higher threshold of 30 parts per million, reflecting both the biology of how the toxin concentrates in crab organs and practical considerations for the fishery. When testing shows levels above these thresholds, fisheries are closed and harvesting is halted until concentrations drop to safe levels.
These closures can last weeks or months during major bloom events, with significant economic consequences for coastal fishing communities. The monitoring systems in place along the U.S. West Coast, as well as in Europe, New Zealand, and Australia, are the primary reason that serious human poisoning events remain rare despite the growing frequency of toxic blooms.
Warming Oceans and Rising Risk
Research supported by NOAA has linked warmer ocean conditions off Oregon and Washington to hazardous levels of domoic acid in shellfish for the first time. A team led by Oregon State University scientists connected domoic acid concentrations to warm-water phases of natural climate cycles like the Pacific Decadal Oscillation and El Niño. Their work produced a climate-based risk model that can predict where and when domoic acid levels in shellfish are likely to exceed safety limits.
The 2015-2016 West Coast domoic acid event illustrated the scale of what warmer conditions can produce. A massive toxic bloom stretched from California to Alaska, triggering prolonged fishery closures and widespread wildlife mortality. That event coincided with an unusually persistent patch of warm ocean water in the Pacific. As ocean temperatures continue to rise and climate variability increases, researchers expect these large-scale outbreaks to become more common and harder to predict using traditional monitoring alone.