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Variation in environmental characteristics and divergent selection pressures can drive adaptive differentiation across a species' range. Astrangia poculata is a temperate scleractinian coral that provides unique opportunities to understand the roles of phenotypic plasticity and evolutionary adaptation in coral physiological tolerance limits. This species inhabits hard-bottom ecosystems from the northwestern Atlantic to the Gulf of Mexico and withstands an annual temperature range of up to 20 degrees C. Additionally, A. poculata is facultatively symbiotic and co-occurs in both symbiotic ('brown') and aposymbiotic ('white') states. Here, brown and white A. poculata were collected from Virginia (VA) and Rhode Island (RI), USA, and exposed to heat (18-32 degrees C) and cold (18-6 degrees C) stress, during which respiration of the coral host along with photosynthesis and photochemical efficiency (F-v/F-m) of Breviolum psygmophilum photosymbionts were measured. Thermal performance curves (TPCs) of respiration revealed a pattern of countergradient variation with RI corals exhibiting higher respiration rates overall, and specifically at 6, 15, 18, 22 and 26 degrees C. Additionally, thermal optimum (T-opt) analyses show a 3.8 degrees C (brown) and 6.9 degrees C (white) higher T-opt in the VA population, corresponding to the warmer in situ thermal environment in VA. In contrast to respiration, no origin effect was detected in photosynthesis rates or F-v/F-m, suggesting a possible host-only signature of adaptation. This study is the first to consider A. poculata's response to both heat and cold stress across symbiotic states and geography, and provides insight into the potential evolutionary mechanisms behind the success of this species along the East Coast of the USA.