*Result*: Chronic heat exposure fails to trigger lasting thermal imprinting in juvenile black rockfish (Sebastes schlegelii) revealed by growth, histological, and transcriptomic analyses.

*Global climate change increasingly exposes marine species to thermal extremes, posing significant challenges to their survival and development. Fish, as ectothermic organisms, are especially vulnerable, with body temperature and metabolic processes dictated by the external environment. This study investigated whether chronic high-temperature exposure (27 °C) during early juvenile stages imposes a lasting "thermal imprint" on black rockfish (Sebastes schlegelii) once temperatures return to ambient conditions. Fish were tracked from 26 days post-parturition (dpp) through a 60-day exposure to elevated temperature and a subsequent 200-day period of normal rearing. Growth performance, muscle histology, and transcriptome-wide gene expression were comprehensively analyzed. Although elevated temperature initially suppressed growth metrics and elicited short-term transcriptomic changes related to myogenesis, stress response, and metabolic processes, these effects dissipated over time and did not persist once the fish reacclimated to ambient conditions. Consistent with these molecular findings, muscle histology showed no irreversible structural damage, despite a transient shift from hyperplastic to hypertrophic muscle fiber growth under heat stress. Together, these data indicate that chronic exposure to 27 °C in juvenile black rockfish does not result in long-term thermal imprinting. These findings improve our understanding of the species' resilience to sub-lethal heat challenges and inform best practices for cultivating black rockfish under conditions of increasing climatic variability.
(Copyright © 2025. Published by Elsevier Ltd.)*