Mussels (Mytilus edulis) can show wide growth variability, even among same-aged individuals reared under similar conditions . Triploid y can influence growth rates as energy can be diverted from reproduction to somatic growth. Furthermore, the response of triploid mussels to heat stress remains poorly understood. This study investigated how growth phenotype (i.e., slow versus fast growers) and ploidy (i.e., diploids versus triploids) influence the response of mussels to thermal stress. Within each ploidy group, slow and fast growers (bottom and top 20% by size) were characterized across three mussel families. Each family was tested separately, where mussels were placed in individual respirometry chambers and exposed to thermal stress starting at an acclimation temperature of 18°C . The temperature was increased by 2°C every three days until it reached a sub-lethal temperature of 26°C. Shell lengths were measured at the start and end of the experiment, while feeding rates were assessed at 18, 22, and 26°C , and respiration rates were measured continually every 2h. Throughout the experiment, samples were collected for genotyping, RNA-sequencing, metabolomics, gill histology, and flow cytometry. This study aims to better understand how growth phenotype and ploidy influence mussel responses to thermal stress, with potential applications in mussel aquaculture through the selection of resilient growth lines.