European green crabs (Carcinus maenas) are a prolific non-indigenous species (NIS) in North America that negatively impacts local species important for Indigenous food sovereignty and commercial shellfish production. Despite the low genetic diversity due to serial bottlenecks, green crab spread in North America is likely facilitated by high thermal tolerance and plasticity. How do C. maenas maintain thermal tolerance with seemingly low adaptive potential? To answer this question, this talk will explore environmental and genetic contributions to C. maenas thermal tolerance, and compare physiological and molecular mechanisms of plasticity between non-native populations in Massachusetts and Washington, USA. Crabs were exposed to cold (5ºC), ambient (15ºC), or warm (25ºC or 30ºC) temperatures for at least one month, and monitored weekly using time-to-right and respirometry. In both populations, crab righting response and rate of oxygen consumption slowed at 5ºC and sped up at 25ºC or 30ºC when compared to 15ºC. Metabolome and lipidome analyses elucidated biochemical pathways used to maintain homeostasis at experimental temperatures, with broad molecular shifts occurring as early as three days after exposure began. Crabs were also genotyped at a putative chromosomal inversion that contains a high proportion of amino acid-changing mutations and is strongly associated with sea surface temperature. Interrogation of individual crab responses to temperature revealed within-treatment physiological differences influenced by genotype, suggesting a genetic contribution to thermal plasticity. This work illustrates the importance of understanding factors that contribute to inter-individual variation in thermal tolerance, which may improve overall population fitness and influence spread of NIS.