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Sea Lions Exposed To Toxic Algae At Risk Of Brain Damage and Memory Loss, Study Shows

Dec 15, 2015 10:54 AM EST
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An algal toxin known as domoic acid is causing brain damage and loss of spatial memory in California sea lions, according to researchers from the University of California, Santa Cruz. This may help explain why so many of these marine mammals are getting stranded along the West Coast.

Domoic acid is a deadly neurotoxin that is produced by naturally occurring marine algae. What's worse is the algal blooms that produce this toxin thrive in warmer waters brought on by climate change. High levels of this neurotoxin were also recently detected in Dungeness crabs, prompting officials to delay the opening of crab season along the West Coast and warn seafood lovers to steer clear of Dungeness and Rock crabs. Essentially, when domoic acid accumulates in these shellfish, other marine animals, such as sea lions, that feed on these crustaceans are exposed to dangerously high levels of the neurotoxin, researchers explain. 

A record-high number of sea lions have found themselves stranded along the California coast this year, and while previous studies have characterized the clinical effects of the toxin, until now researchers were in the dark about the behavioral effects of domoic acid, according to a news release.

"One thing that had been known and fairly well demonstrated is that DA (Domoic Acid) leads to fairly reliable neurologic conditions. While other neurotoxins might cause general brain damage, DA seemed to cause reliable, specific lesions on the hippocampus. In humans, this results in an illness called amnesiac shellfish poisoning when shellfish who've eaten the algae are ingested," Peter Cook, who led the study as a graduate student at UC Santa Cruz and is now at Emory University, told the Washington Post. "It hadn't really been studied in marine mammals. People didn't see it as a large-scale ecological concern."

For their study, researchers worked with 30 Californian sea lions that were found stranded and are currently receiving rehabilitative care from the Marine Mammal Center. These animals were tested for spatial memory and behavior for abnormalities, and their brains were scanned so that researchers could better assess the extent of brain lesions and domoic acid-related damage in the affected animals.

Compared to healthy individuals, sea lions exposed to domoic acid had severe damage to their hippocampus, which is the part of the brain responsible for memory. Additionally, the brain pathway between the hippocampus and the thalamus - a network that is essential for the formation of memories of events or experiences - was damaged in sea lions exposed to domoic acid. (Scroll to read more...)

(Photo : The Marine Mammal Center)
Damage to the hippocampus, a part of the brain involved in memory, is often seen in sea lions with domoic acid poisoning. Left: a normal California sea lion brain section. Right: California sea lion brain that has been affected by domoic acid exposure -- notice the shrunken hippocampus in the center of the brain section.

When correlating hippocampal damage to specific behavioral impairments, researchers found sea lions with hippocampal lesions exhibited impaired performance on short-and long-term spatial memory tasks. Researchers say this is likely to impact a sea lion's ability to find food and may explain why they end up in unusual places, far outside their normal range.

However, brain lesions and permanent brain damage from domoic acid exposure develops over time. Therefore, those sea lions exposed to less-lethal doses of this neurotoxin may only experience seizures and disorientation.  

"What isn't well understood yet is the dose response," Cook added in the university's release. "We don't know how heavy the exposure needs to be, or how often repeated, to cause this kind of brain damage, and we don't know the effects of repeated low-dose exposure."

Their findings were recently published in the journal Science.

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