Environmental dynamics influence scale cortisol levels in Mexican cavefish

Cortisol is a key hormone involved in stress responses, and its regulation reflects how organisms cope with environmental stressors. While acute stress responses are well studied, much less is known about how cumulative, long-term stress shapes the responses of populations adapted to different habitats. Here, we investigated scale cortisol - a biomarker of long-term stress exposure in fish - in wild and laboratory reared populations of Astyanax mexicanus, a species with surface and cave-dwelling ecotypes adapted to contrasting environments. We measured scale cortisol levels in six populations (four cave and two surface) over two consecutive years, alongside abiotic parameters in the respective habitats. Some cave populations exhibited lower scale cortisol levels than surface fish under both wild and laboratory conditions, suggesting a genetically based enhancement of stress-coping ability or altered cortisol-mediated metabolic and osmoregulatory processes. Moreover, populations experiencing more environmental variability - such as the lake surface habitat and hydrologically dynamic cave systems - showed higher scale cortisol levels and increased inter-individual variation. These results suggest that evolutionary adaptations to different habitats shape long-term stress physiology in A. mexicanus, and they highlight scale cortisol as a valuable tool for assessing how organisms physiologically cope with diverse and extreme environments.