A new study shows that a neurotoxin produced by algae disrupts the memory of California sea lions, animals that rely heavily on recall of food-rich locations to forage. The results may explain the impaired navigation observed in sea lions in recent years, which many believe has led to an increase in strandings. Off the coast of California, poisoning from the neurotoxin domoic acid (DA) is a major issue for marine animals. Exacerbating the problem, the size and frequency of DA-producing algal blooms have increased in recent years as a result of environmental change and human impacts on marine systems. Scientists know that exposure to DA causes seizures and even death in sea lions, though its non-lethal effects, including those on behavior, are less well-known. Here, Peter Cook and colleagues studied wild California sea lions admitted to rehabilitation clinics following exposure to DA. The researchers used magnetic resonance imaging to assess structural abnormalities in these animals' hippocampal regions, finding lesions throughout, including in the right dorsal hippocampus, a brain region known to play a role in spatial memory in other species. The researchers trained these sea lions on two spatial memory tasks, including one involving finding a food reward from among four potential locations. Greater right hippocampal damage corresponded to poorer performance on the memory tasks, the researchers report. They suggest measuring hippocampal volume in stranded sea lions may be a useful marker of how successful an animal would be after release from rehabilitation. Results of this study may be applicable to other wild animals naturally exposed to DA, including sea birds and cetaceans, the researchers say.