Past environments modulate response to fluctuating temperatures in a marine fish species, <i>Sebastes fasciatus</i>

Predicted ocean warming will impact the survival and structure of various marine organisms, in particular ectotherms. Phenotypic plasticity enables species to cope with environmental changes, providing a vital buffer for evolutionary adaptations. Yet, the dynamics and molecular mechanisms underpinning these plastic responses remain largely unexplored. Here, we assessed the impact of temperature acclimation on the capacity of organisms for thermal plasticity. We conducted a genome-wide transcriptomic analysis on the Acadian redfish, Sebastes fasciatus, exposed to four temperatures (2.5, 5.0, 7.5 and 10.0°C) over a long-term period (up to 10 months) followed on some individuals by a short-term temperature change (+2.5°C or -2.5°C for 24 h), simulating natural temperature variation the species could encounter. Our results showed a dynamic transcriptional response to temperature involving various gene functions. The rapid response to temperature shifts, coupled with the sustained expression of specific genes over an extended period, highlighted the capacity of this species for plasticity in response to temperature changes. We also detected a significant effect of the interaction between the long- and short-term temperature exposures on gene expression, highlighting the influence of the past environment on the response to short-term temperature changes. Specifically, fish acclimated to higher temperatures demonstrated an increased stress-related response to environmental fluctuations, as evidenced by both the shape of their reaction norms and the involvement of stress-related gene functions. This result suggests that temperature conditions predicted for the near future in the Northwest Atlantic will trigger reduced adaptive plasticity to environmental fluctuations, highlighting the vulnerability of this species to ocean warming.