Stranded sea lions? Blame toxic algae.

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Algae isn’t just the green goo in your fish tank. Toxic algal blooms in the Pacific Ocean, largely due to human activity, could explain why hundreds of California sea lions are stranded every year.

Researchers have observed that domoic acid, which is produced by these algal blooms, has neurotoxic effects on marine animals, including California sea lions. Sea lions that have been chronically exposed to domoic acid often exhibit seizures and unusual movement patterns. Scientists suspect this acid causes brain lesions and damage the sea lions’ hippocampus -- a brain region critical for memory and navigation. As sea lions are natural foragers, they rely heavily on spatial memory for survival. As a result, domoic acid could be severely inhibiting the animals’ ability to feed and survive in the wild.

To test this theory, researchers from Emory University, UC Davis, and UC Santa Cruz (among others) decided to conduct several experimental memory tasks on wild California sea lions – observing both a healthy group of sea lions and a group with suspected domoic acid toxicosis. They hypothesized that comparing the memory performance and brain structure to both acid-exposed and healthy sea lions could give insight into how domoic acid is altering their brains and behavior.

In the first task, a sea lion was placed in a T-shaped maze where a fish “reward” would be presented alternately from the left and right. The sea lions had to learn the pattern, remember which arm had yielded fish in the previous trial, and go to the opposite arm. In a test of longer-term memory, the sea lions performed the same task, but this time, there was a seven-second delay between trials. This required the sea lions to not only learn the pattern but to remember it during the pause.

In a second task, animals had to learn and remember which of four buckets contained food. All four bucket locations, as well as which bucket contained food, remained constant over a 10-12 day period. The animals simply had to navigate toward the correct bucket.

Across all the tasks performance was measured by the number of correct responses. Both the healthy and acid-exposed sea lions then underwent brain magnetic resonance imaging (MRI) in order to assess the size of the hippocampus. The researchers found a direct correlation between hippocampal volume and memory performance: sea lions with a larger right hippocampus performed better on the tasks while animals with a smaller right hippocampus performed worse.

Previous research provides strong evidence that exposure to domoic acid causes brain lesions in sea lions. And in other animals, lesions like these are shown to have severe effects on behavior. Here, researchers were able to show for the first time that the severity of lesions related to domoic acid exposure was correlated with specific memory deficits in California sea lions. 

These findings demonstrate compelling evidence that exposure to domoic acid may affect sea lions’ brain function and lead to severe memory impairments. For wild foragers, impairments like these could severely affect their ability to navigate and secure food. By comparing sea lions exposed to domoic acid to healthy specimens, scientists have come closer to understanding just how critically algal blooms are impacting wildlife.

These results have important implications for the future survival of California sea lions, and may play a critical role in future diagnostic and rehabilitation efforts for toxin-exposed wildlife.

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