Algal toxin impairs sea lion memory
SAN FRANCISCO – There is no locally caught crab on the menu here. The state of California warned earlier this year that the crustaceans may have high levels of domoic acid, a powerful neurotoxin, and should not be eaten. Domoic acid is produced by algae in the genus Pseudo-nitzschia, and there’s a huge bloom of them off the coast. The toxin accumulates in creatures that eat the algae, such as crabs, and on up the food chain. A person who eats one of these critters can get a potentially deadly case of amnesic shellfish poisoning.
Unfortunately, sea lions can’t read the warning notices. They, too, can get domoic acid poisoning after eating fish laced with the toxin. And since 1998, California sea lions have been washing up along the Pacific Coast during blooms of the algae, experiencing seizures and disorientation, which are signs that they have been exposed to the toxin.
Scientists had known that domoic acid poisoning led to changes in the hippocampus, a brain region involved in memory. The changes led to seizures and then damage to the hippocampus. Now researchers report that this brain damage impairs the animals’ spatial memory, which could affect their survival. And the toxin could be causing even more global deficits in brain function, Peter Cook, a psychologist and neuroscientist at Emory University in Atlanta, said at the 21st Biennial Conference on the Biology of Marine Mammals.Cook and his colleagues studied 30 California sea lions that had been rescued from the wild and undergone rehabilitation between April 2009 and November 2011. Some had signs of domoic acid poisoning. Others needed care for other reasons. The researchers imaged the animals’ brains with MRI and measured the volume of their hippocampi (there are two, left and right). Then they put them through two tests of spatial memory.
The first test used a simple maze, where an animal has to choose right or left. Food is alternately placed on the left or the right, and a sea lion quickly learns this simple pattern, easily making the correct choice again and again. A healthy sea lion will follow the pattern even if there is a small delay — say, a researcher puts a board at the front of the maze for seven seconds. But sea lions that had damage in the right hippocampus had trouble remembering the pattern and finding the fish.
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In the second test, a sea lion was presented with four buckets placed at separate locations each day for 12 days. One bucket always had fish, and animals tend to find the fish faster every day, eventually learning that one bucket always has the food. But again, sea lions with damage in the right hippocampus weren’t as good at this task.
“The guys with the hippocampal damage were really lost at sea,” Cook said. The animals’ spatial memory was clearly impaired, and in the wild, that could make it difficult for the animals to navigate and find food. This would explain why some animals become stranded on land and others veer off in odd patterns in the ocean, he said. Cook and his colleagues also report their findings in the December 18 Science.
In humans, the right hippocampus is involved in spatial memory and the left is needed for verbal memory. But scientists aren’t quite sure what the left hippocampus does in sea lions, Cook said. And damage to the left hippocampus didn’t seem to have any effect on the sea lions’ performance — at least not in these tests.
The damage may go beyond spatial memory, however. In the brains of healthy sea lions, brain scans showed that there was neural chatter between the hippocampus and the thalamus. But in the animals exposed to domoic acid, “the hippocampus and the thalamus are not talking to each other,” Cook said. That could be a sign that there are more global deficits to brain function.
The algal blooms that cause domoic acid poisoning naturally occur off the California coast in spring and fall, but they have been getting more common. With ocean waters warming and sea ice disappearing, the blooms are also moving farther north, noted Kathi Lefebvre, a marine biologist at the Northwest Fisheries Science Center in Seattle. “The blooms are getting bigger,” she said. “It seems to have a relationship with warming waters.”
Scientists had never seen a sea lion with domoic acid-related seizures outside of California, but this year, one showed up in Washington state, Lefebvre noted. And there is concern that many other animals may be affected, including birds, whales — and people. Though commercially caught fish are tested for the toxin, people can be exposed if they eat fish or other seafood they catch themselves.