Loss of behavioral stress response in blind cavefish reduces energy expenditure.

The stress response is essential for animal self-defense and survival. However, species may exhibit stress response variation depending on their specific environmental and selection pressures. Blind cavefish dwell in cave environments, which differ markedly in stressors and resource availability compared to surface aquatic environments. However, whether blind cavefish exhibit differences in stress response as an adaptation to their cave environments remains unclear. Here, we investigated differences in stress response in six closely related species, including three blind cavefish (, , and ) and three normal-sighted river fish (, , and ). Results showed that blind cavefish exhibited a range of distinct behavioral responses compared to sighted river fish, including greater levels of activity, shorter duration of freezing, absence of erratic movements or thrashing behavior, and opposite behavioral trends over time. Furthermore, the cavefish species demonstrated attenuated increases in metabolic rate in response to stressors related to novel environments. Cave-dwelling also exhibited lower basal hypothalamic-pituitary-inter-renal (HPI) axis-related gene expression levels and stress hormone concentrations compared to river-dwelling . These results suggest that blind cavefish may have lost their behavioral stress response, potentially mediated by a reduction in basal activity of the HPI axis, thus enabling the conservation of energy by reducing unnecessary expenditure in energy-limited caves.