In a study published in the journal Science Advances, scientists have described an unexpected behaviour of the Mexican tetra, also known as the blind Mexican cavefish.
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This fish is a gold mine of knowledge for scientists – since the same species evolved in such vastly different environments, comparing the two offers insights into how evolution and how behaviour is shaped.
The blind Mexican cavefish (Astyanax mexicanus), found mostly in the eastern waters off Mexico, is observed both as a sighted surface fish and as an independently-evolved blind cave form – with depigmented, pinkish-white scales, non-functional eyes and a highly developed sensory organs along its body.
Secrets of the brain
Researchers at Florida Atlantic University (FAU) wanted to investigate how evolution shapes the brain, so they looked into one specific behaviour – how both the surface fish and cavefish respond to changes in light. Using genetic tools, advanced imaging technologies and behavioural experiments, they tracked how various brain regions react to light and darkness.
The findings, which were published in the journal Science Advances, were quite unexpected. While surface fish became more active when plunged into darkness, cavefish did the exact opposite – they became more active when exposed to light.
Scientists think these two adaptations might help the fish search for light and avoid illuminated areas, respectively.
The research team also determined that this light-evoked photokinesis (change in movement speed or activity level in response to a shift in light intensity) depends on dopamine signalling – suggesting that a conserved brain pathway evolved over time to adapt to life in complete darkness.
“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,” says Erik R. Duboué, the study’s senior author and associate professor of biology at FAU.
“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 behavioural variation.
“The fact that all previously studied eyed fish exhibit dark photokinesis and that only cavefish exhibit light photokinesis suggests that this behaviour evolved as an adaptation to cave life.”
The study also provided evidence that photokinesis can be genetically inherited, since hybrid populations demonstrated the same tendencies – proving that these responses to light are encoded in the genome itself.
“Cavefish provide a unique model for studying how sensory systems evolve and how brains adapt to novel environments,” explains Duboué.
“By understanding how evolution modifies neural circuits to process environmental information, we can gain deeper insights into the fundamental principles that shape behaviour across the animal kingdom.”
This research doesn’t just shed light on this species’ anatomy and evolution – many neural pathways involved in sensory processing, movement and dopamine signalling are similar across vertebrates, meaning fish and humans share similarities in how they function.
By studying the Mexican tetra and the effects of evolution on its brain, scientists can gain insight into disorders such as Parkinson’s disease, schizophrenia, the autism spectrum and ADHD.
Read the full findings here.
Top image credit: antos777/Getty Images
