Geukensia demissa, the ribbed mussel, is among many salt marsh species that are at risk of experiencing detrimental heat stress due to increasing temperatures. These mussels form large aggregates beneath patches of cordgrass (Spartina alterniflora) where they are less exposed to the sun’s rays; however, some mussels end up in areas that lack cordgrass and experience temperatures of 36°C and above during the summer months. If mussels in exposed areas are thermally stressed, we would expect an effect on their heart rates. To address this possibility, we collected 40 exposed and 40 less exposed mussels from a salt marsh on Tybee Island, Georgia and measured their heart rate with CNY70 infrared sensors. We also measured body temperature of 10 exposed and 10 less exposed mussels from each of 8 large aggregates in the field. The distance between aggregates varied from 5.5 to 9 meters. We predicted that exposed mussels will exhibit higher maximum temperature tolerance and thus have lower heart rates at temperatures of 36°C. As predicted, exposed mussels had higher body temperatures (30.1 ± 1.8 °C) and lower heart rates (53.5 ± 10.8bpm) than less exposed mussels (28.4 ± 1.0 °C, 66.2 ± 8.5 bpm). By linking temperature and heart rate, we concluded that not all mussels will be able to deal with rising temperatures in Georgia’s salt marshes. Those in exposed areas might actually be at an advantage. Decreasing their heart rate as temperature increases could be a strategy to improve energy conservation under stress.