Heat shocks during eggs incubation lead to development, morphological and behavioural differentiation in Arctic charr (<i>Salvelinus alpinus</i>)

Temperature variation is affecting fish biodiversity worldwide, causing changes in geographic distribution, phenotypic structure and even species extinction. Incubation is a critical stage for stenothermic species, which are vulnerable to large temperature fluctuations, and its effects on the phenotype at later developmental stages are understudied, despite phenotype is essential for organism ecology and evolution. In this study, we tested the effects heat shocks during the embryonic period on the phenotype of Arctic charr (Salvelinus alpinus). We repeatedly quantified multiple phenotypic traits, including morphology, development and behaviour, over a period of 4 months, from hatching to juvenile stage on individuals having experienced heat shocks (+ 5°C on 24h, seven times) during their embryonic stage or not. We found that heat shocks led to smaller body size at hatching, and a lower sociability. Interestingly, these effects weakened throughout the development of individuals, and even reversed in the case of body size. We also found accelerated growth rate and higher body condition in the presence of heat shocks. Our study provides evidence that heat shocks experienced during incubation can have long-lasting effects on an individual's phenotype. This highlights the importance of the incubation phase for the development of ectothermic organisms, and suggests that temperature fluctuations may have significant ecological and evolutionary implications for Arctic charr. Given the predicted increase in extreme events and unpredictability of temperature fluctuations, it is critical to further investigate their effects on development by examining fluctuations that vary in frequency and intensity.