Subterranean Fishes of the World

3534

2013

Universidade de Lisboa

Understanding how the brain evolves in response to new environmental situations is key to comprehend not just the developmental mechanisms underlying the generation of variability in brain patterns, but also to understand how the animal’s perception and mental representation of their environment evolves. With this idea in mind, we described the differences in cranial nerve and brain morphology between the eyed surface form and the blind cave of Astyanax fasciatus mexicanus using micro-CT technology. This is, to our knowledge, the first time this method is employed for neuroanatomical studies. Comparisons were performed at both five day old and one year old fish. Our results show a topological conservation of cranial nerves in both morphotypes with significant reductions in visual related nerves in the cavefish along with a reduction in the optic tectum. We also found an increased robustness of the trigeminal and anterior lateral line nerves, responsible for somato- and mechanosensory input, respectively. At 5 dpf the surface fish brain is bigger than the cavefish and no differences were found between different regions. The overall adult brain volume of cavefish is approximately 20% smaller in the cavefish, which is also accompanied by a reduction of its telencephalic, hypothalamic and cerebellar regions. The olfactory bulb appears to circumvent this trend, showing signs of enhancement. This overall reduction shows that energetic constraints may be of significance in shaping the overall morphology of the brain the cave environment. Keywords Astyanax mexicanus; Evolution; Brain; Cranial Nerves; micro-CT

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