A naturally-occurring algal toxin called domoic acid (DA) causes brain lesions in sea lions that can lead to a range of acute symptoms including disorientation and seizures. While this much was known, scientists in California have now identified the areas of the brain that are affected by DA, revealing how persistent exposure to the substance can lead to permanent neurological damage.
DA is produced by a number of algae belonging to the Pseudo-nitzschia genus, and therefore tends to become more concentrated when these algae are in bloom. During these times, a number of marine animals – including certain types of fish, birds and mammals – face an increased risk of suffering acute poisoning, although until recently, marine biologists had been unsure of exactly how this occurs.
Between April 2009 and November 2011, a team led by researchers from the University of California, Santa Cruz, studied wild sea lions undergoing rehabilitation following DA exposure, in order to learn more about its effects on the brain.
To do so, they devised a number of exercises that would examine impairments to the sea lions’ spatial memory. For instance, the animals were repeatedly put through a maze, enabling researchers to observe their ability to navigate and memorize their surroundings. Additionally, the sea lions were presented with buckets in four locations each day, one of which always contained food while the other three did not. By examining their ability to learn which location contained the food, the team was able to further assess the animals’ level of cognitive damage.
They then used functional magnetic resonance imaging (fMRI) to identify a correlation between brain structure and behavioral symptoms in the sea lions. In particular, the researchers focused on the right dorsal hippocampus, since previous studies in humans and rodents had indicated that this part of the brain is largely responsible for spatial memory.
Publishing their results in the journal Science, the team noted that sea lions displaying impaired navigational capacity tended to suffer from right dorsal hippocampal lesions, resulting in a decrease in volume of this part of the brain. Additionally, they discovered a reduction in communication between the hippocampus and a region called the thalamus in these subjects.
However, they also found that this type of damage does not necessarily occur every time that acute DA poisoning symptoms are seen, suggesting that permanent changes to the hippocampus only result from repeated exposure to the toxin. In a statement, study lead author Peter Cook explained that this raises a number of further questions that must be addressed in future research. “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,” he said.
The implications of this research are potentially significant for global sea lion populations. As the study explains, “because sea lions are dynamic foragers that rely on flexible navigation, impaired spatial memory may affect survival in the wild.”
Such risks are particularly prevalent at present, since a number of studies have indicated that DA-producing Pseudo-nitzschia algal blooms are currently increasing. Though this increase has been attributed to “natural climate variability” rather than man-driven changes to ecosystems, it presents a major challenge to conservationists working with sea lions and other marine species that may be susceptible to DA poisoning.