An algae toxin is causing permanent brain damage in some California sea lions, according to new research that could help to explain strandings of these and other marine mammals.
The discovery reveals yet another deadly threat of the toxin, domoic acid, which has put the otherwise lucrative California crab season on hold now for weeks. The research is published in the journal Science.
Mammals of the Sea
Microscopic algae that occur naturally in coastal waters are the culprits, although prior research found that warmer than normal waters can fuel the toxin's prevalence. Blooms of the toxic algae along the California coast typically occur in the spring and fall, but during the last few weeks, the blooms have been increasing in frequency and severity. This year's massive bloom, the largest ever recorded, lasted through the summer and extended from Santa Barbara to Alaska.
"This is the first evidence of changes to brain networks in exposed sea lions, and suggests that these animals may be suffering a broad disruption of memory, not just spatial memory deficits," lead author Peter Cook said in a press release.
"It's rare to have this type of experimental evidence linking a naturally occurring neurotoxic effect to behavioral impairment in a wild animal," continued Cook, who led the study as a graduate student at UC Santa Cruz and is now at Emory University. "There are thousands of California sea lions with varying degrees of exposure to domoic acid out there. Sea lions navigate a complex and changing environment, and if their spatial memory is impaired, it's bound to affect their ability to survive in the wild."
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Cook and his team studied 30 California sea lions undergoing veterinary care and rehabilitation at the Marine Mammal Center in Sausalito. Researchers administered behavioral tests to assess spatial memory and performed brain imaging (MRI) to see the extent of brain lesions in the affected animals.
They found that sea lions suffering from domoic acid poisoning experienced damage to the hippocampus, a part of the brain involved in memory.
One of the behavioral tests was similar to the maze apparatus used to assess rat spatial memory, but it was adapted for these large marine mammals. Sea lions suffering from the algae poisoning showed impaired performance on both short and long-term spatial memory tasks.
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Further brain scans, using high tech equipment never applied before to marine mammals, found that in addition to the damage to the hippocampus, there were negative effects on interactions between the hippocampus and other brain structures, notably the thalamus.
"This is the first evidence of changes to brain networks in exposed sea lions, and suggests that these animals may be suffering a broad disruption of memory, not just spatial memory deficits," Cook said.
Sea Lion Strands on Busy San Francisco Street
Co-author Charan Ranganath of the Dynamic Memory Laboratory at the UC Davis Center for Neuroscience added, "We didn't know exactly why the algae lead to strandings. But sea lions are dynamically foraging - and for an animal like that, if you don't know where you are, you have a big problem."
The researchers believe that brain lesions may develop over time with repeated exposure to the algae toxin. This could help to explain why some sea lions and other marine mammals, such as whales, strand themselves on beaches all year round, even when there is no active bloom of the domoic acid.
While the East Coast has been experiencing warmer than normal temperatures recently, the West Coast has had very cold, wet weather. While this may help to temporarily curb the massive CA toxic algal blooms, the problem - like efforts to slow climate change - will require long-term solutions.