Brain size variation in extremophile fish: local adaptation versus phenotypic plasticity

The brain is a plastic organ, and so intraspecific studies that compare results obtained from wild individuals with those from common‐garden experiments are crucial for studies aiming to understand brain evolution. We compared volumes of brain regions between reproductively isolated populations of a neotropical fish, Poecilia mexicana, that has locally adapted to perpetual darkness (Cueva Luna Azufre), toxic hydrogen sulphide in a surface stream (El Azufre) or a combination of both stressors (Cueva del Azufre). Wild fish showed habitat‐dependent differences: enlarged telencephalic lobes and reduced optic tecta were found in fish living in darkness and sulphidic waters, in darkness without hydrogen sulphide or exposed to light and sulphide; fish from the sulphidic cave additionally showed enlarged cerebella. Comparison with common‐garden reared fish detected a general decrease in brain size throughout populations in the lab, and little of the brain size divergence between lab‐reared ecotypes that was seen in wild‐caught fish. The pronounced differences in brain region volumes between ecotypes in the wild might be interpreted within the framework of mosaic evolution; however, the outcomes of common‐garden experiments indicate a high amount of phenotypic plasticity. Our study thus highlights the importance of combining the investigation of brain size in wild populations with common‐garden experiments for answering questions of brain evolution.