Deep within the dark caves of northeastern Mexico lives a fish that has spent hundreds of thousands of years adapting to a world without light. The blind Mexican cavefish (Astyanax mexicanus), has evolved in perpetual darkness, losing its eyes and pigmentation while developing remarkable adaptations that help it survive in nutrient-poor environments.
Now, scientists are using this extraordinary species to uncover how evolution rewires the brain and shapes behavior. Because Astyanax exists both as sighted surface fish and as more than 30 independently evolved cave populations, researchers can directly compare how life in darkness alters sensory systems, neural circuits and behavior.
With new genetic tools and advanced imaging technologies that allow scientists to watch brain activity in real time, this unique fish is providing unprecedented insights into how animals adapt to extreme environments – and how evolution transforms the brain itself.
To investigate how evolution changes the brain to produce new behaviors, Florida Atlantic University researchers and collaborators compared how surface fish and cavefish respond to changes in light. They combined behavioral experiments with advanced whole-brain imaging techniques that allowed them to visualize neural activity in living fish at cellular resolution.
Using genetically engineered fish that express fluorescent markers in neurons, the scientists tracked how different brain regions responded when fish were exposed to light and darkness. They also mapped these responses onto an established cavefish brain atlas and used targeted experiments to examine the role of dopamine-producing neurons in driving behavior.
The study, published in Science Advances , revealed a striking evolutionary reversal in behavior. While surface fish became more active when suddenly plunged into darkness – a response believed to help them search for light – cavefish did the opposite, becoming more active when exposed to light. This light-triggered response likely helps cavefish avoid illuminated areas such as cave entrances, where they would be more vulnerable to predators and environmental conditions outside their dark subterranean habitat.
By mapping activity across the entire brain, the researchers identified changes in a region known as the posterior tuberculum, as well as a previously unrecognized neuronal cell type linked to these behaviors.
"Remarkably, neurons that respond to darkness in surface fish were found to respond to light in cavefish, suggesting that evolution can repurpose existing neural circuits rather than creating entirely new ones," said Erik R. Duboué, Ph.D., senior author, an associate professor of biology in FAU's Harriet L. Wilkes Honors College , and a member of FAU's Stiles-Nicholson Brain Institute .
The team also discovered that dopamine signaling plays a central role in these responses, revealing a conserved brain pathway that has been modified over evolutionary time.
"Our discovery that cavefish have evolved light-evoked photokinesis allowed us to ask what brain regions are affected and which neuronal subgroups could contribute to behavioral variation," said Duboué. "The fact that all previously studied eyed fish exhibit dark photokinesis and that only cavefish exhibit light photokinesis suggests that this behavior evolved as an adaptation to cave life."
The findings expand scientists' understanding of how brains evolve in response to extreme environments and provide one of the clearest examples of how changes in neural circuits can drive behavioral adaptation. Because similar dopamine pathways are conserved across vertebrates, including fish, rodents and primates, the research may offer broader insights into how brains process sensory information and adapt to changing conditions.
The study also provides evidence that photokinesis is genetically inherited. By crossing surface fish with cavefish, the researchers observed a wide range of responses in hybrid offspring, demonstrating that the tendency to become more active in light or darkness is encoded in the genome. Future studies will explore the genes and developmental mechanisms responsible for rewiring these neural circuits and shaping behavior.
"Cavefish provide a unique model for studying how sensory systems evolve and how brains adapt to novel environments," said Duboué. "By understanding how evolution modifies neural circuits to process environmental information, we can gain deeper insights into the fundamental principles that shape behavior across the animal kingdom."
Beyond illuminating how animals adapt to extreme environments, this research may have far-reaching implications for understanding the human brain. Many of the neural pathways involved in sensory processing, movement and dopamine signaling are highly conserved across vertebrates, meaning they function similarly in fish and humans. By revealing how evolution rewires brain circuits to produce new behaviors, these findings could provide insights into neurological and neurodevelopmental disorders linked to altered sensory processing and dopamine function, including Parkinson's disease, schizophrenia, autism spectrum disorder and ADHD.
Study co-authors are Robert A. Kozol, Ph.D., an assistant professor, St. John's University; Ally Canavan, a graduate of FAU's Harriet L. Wilkes Honors College – this paper was part of her honors thesis; Bernadeth Tolentino, a Ph.D. student at the University of Southern California; Alex C. Keene, Ph.D., professor and department head of biology, Texas A&M University; and Johanna E. Kowalko, Ph.D., an assistant professor of biological sciences at Lehigh University.
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About Florida Atlantic University:
Florida Atlantic University serves more than 32,000 undergraduate and graduate students across six campuses along Florida's Southeast coast. Recognized as one of only 13 institutions nationwide to achieve three Carnegie Foundation designations - R1: Very High Research Spending and Doctorate Production ," " Opportunity College and University ," and Carnegie Community Engagement Classification - FAU stands at the intersection of academic excellence and social mobility. Ranked among the Top 100 Public Universities by U.S. News & World Report, FAU is also nationally recognized as a Top 25 Best-In-Class College and cited by Washington Monthly as "one of the country's most effective engines of upward mobility." To learn more, visit www.fau.edu .
