1) Researchers studied the transcriptomic response of eelgrass (Zostera marina) to a simulated heatwave across sites with different temperature regimes in the Atlantic and Mediterranean. 2) They found that heat-responsive genes were more differently expressed between Mediterranean and Atlantic samples than between Northern and Southern Atlantic samples, indicating adaptive differentiation to local temperatures. 3) Some genes showed parallel adaptation between Northern and Southern Atlantic populations, but most transcriptomic differentiation was explained by adaptation between Mediterranean and Atlantic populations, likely driven by epigenetic regulation of gene expression.