Sea Lion Seizures May Result From Toxic Algae
for National Geographic News
|June 17, 2008|
With algal blooms on the rise in their habitat, sea lions in California are developing seizures and abnormal behavior, a new study of lab rodents shows.
The symptoms can result from low-dose fetal exposure to domoic acid, a naturally produced neurotoxin in algae that becomes concentrated in the sea lions' food supply, researchers say.
A new study follows an analysis earlier this year revealing that the symptoms comprise a new sea lion disease.
Domoic acid concentrations rise at times due to natural, climactic factors, the authors point out. But other human-made chemicals may be making the sea lions more susceptible.
"This is probably just the beginning to understand how not just a single chemical, but a complex mix of chemicals we start life with can leave us vulnerable to disease later in life," said study co-author John Ramsdell, a physiologist with the National Oceanic and Atmospheric Administration (NOAA).
He believes the new study, published in the current issue of the online journal Marine Drugs, also has implications for human health.
Poisoned Before Birth
Toxin-producing algal blooms are natural, cyclical events.
"What's fascinating about this work is how the natural rhythms of boom and bust in the sea can conspire to send a pulse of toxin through the food web " said Christopher Scholin, a scientist with the Monterey Bay Aquarium Research Institute, who was not involved with the study.
"If the timing is right, the effects of that pulse long outlive the immediate impacts of the bloom," he said, adding that "chronic exposure to [the toxin] clearly has lasting impacts on marine life."
Domoic acid has previously been blamed for die-offs of seabirds such as pelicans and cormorants.
Ramsdell, the study co-author, said sea otters, whales, and dolphins are also commonly poisoned by domoic acidnot only in California, but in nearly all coastal waters in the United States. More rarely, domoic acid has sickened and killed humans.
The poison enters the food chain when sea-lion prey—such as sardines, anchovies, and herring—eat the toxin-producing diatomic algae. In pregnant sea lions, domoic acid concentrates in the amniotic fluid, causing prolonged exposure to the fetus.
In their study, the researchers exposed female rodents to domoic acid during pregnancy and found that their offspring developed abnormal brain neurons, which caused usually nonfatal seizures and abnormal behavior—such as gnawing and head-shaking—later in life.
Though scientists have observed sea lions seizing on the beach, it's possible that if the animals were feeding offshore and a seizure occurred, they would drown, Ramsdell added.
Domoic acid isn't the first chemical assault on sea lions. The mammals were hit with the toxic industrial chemicals PCB and DDT in the 1960s and infectious disease in the 1970s, which scientists believe led to mass reproductive failures.
Domoic acid toxicity was first documented in sea lions a decade ago, when exposure caused acute neurological disease in sea lions and led to a mass stranding of the animals in California from Monterey Bay to San Diego.
The study authors believe female sea lions in California are highly susceptible to domoic acid because they forage year-round near rookeries that have become increasingly subject to blooms of domoic acid-producing algae.
And because sea lions stay in the womb for 11 months, unborn pups are exposed to the toxin for a long time, the researchers say.
One aspect of the new findings, which is still preliminary, suggests that sea lions' combined history of exposures may make them more vulnerable to domoic acid poisoning when they reach adulthood.
Ramsdell notes that because domoic acid occurs naturally, scientists aren't working to prevent it. But if human-made chemicals do turn out to be contributing factors, efforts could be made to address those.
Scientists in a separate NOAA initiative are already studying the impact of harmful algal blooms on marine mammals to determine if similar impacts could affect humans.
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