A matter of timing: How early-life stress shapes emergent coping styles in zebrafish.

Adaptive flexibility is essential for organisms to survive and thrive in the face of environmental pressures. Throughout their life cycle, animals experience diverse stressful conditions. Early life stages are considered highly sensitive to environmental stressors, which may, during these stages, affect brain function leading to persistent behavioural disruption. Notably, individuals often differ in their responses. Coping styles represent consistent, individual-specific behavioural and neurophysiological responses to stressors, shaped by both genetic and environmental factors, and play a key modulating role in how animals respond to stress. Coping styles fall along a shy-bold behavioural continuum, reflecting differences in behavioural responses to stressors, with bold individuals being proactive and risk-prone, and shy ones reactive and cautious. This work aimed to study how zebrafish early life stress (ELS) influences later-life coping behaviours. To achieve this goal, zebrafish larvae were acutely exposed, at three specific developmental stages, 4-, 7-, and 15 days post-fertilisation (dpf), to a sequence of non-chemical environmental stressors, and coping styles were assessed in the juvenile stage. Overall, the obtained results show that ELS modulates zebrafish later behavioural phenotype expression, altering the neurochemical profile and stress axis function. The developmental stage most sensitive to early-life stress was 15 dpf, with fish exhibiting lower activity than controls, regardless of their later-emerging coping style. Juveniles with a shy phenotype exhibited heightened susceptibility to ELS consequences over time, exhibiting a greater range of behavioural and physiological alterations than bold individuals, demonstrating that shy fish retain stronger biological imprints of ELS across development.