Environmental factors such as temperature and salinity regimes shape lifespan in marine ectotherms. We investigated whether the effect occurs through modification of metabolic reactive oxygen species (ROS)-producing processes and is thus in line with the rate of living-free radical theory of aging. We compared 6 biogeographically and climatically distinct populations of the extremely long-lived ocean quahog Arctica islandica for age-dependent differences in metabolic rates and antioxidant capacities (superoxide dismutase, catalase activity and total glutathione concentration). The temperature and salinity ranges covered by the sampling locations (Norwegian coast, White Sea, Iceland, Kattegat, Kiel Bay and German Bight) were 3.7 to 9.3°C and 20 to 35 ppt. Bivalve shells were used as age recorders by counting annual growth bands. Maximum determined age in different populations varied between 29 and 192 yr. Extreme longevity observed in some North Atlantic A. islandica populations seems to be based on their very low lifetime mass-specific respiration, in combination with stable maintenance of antioxidant protection throughout life in mature specimens. While the antioxidant capacity was similar among all populations, the shorter-lived populations exhibited the highest metabolic rates and showed no metabolic response (Q 10) when warmed to higher temperature. Low and fluctuating salinity in the Baltic may further exert stress, which enhances respiration rates and reduces longevity in the Baltic Sea population. The exceptionally long lifespan of A. islandica cannot be exclusively explained by a well-established antioxidant defense system, and the long lifespan of some populations may rather be a function of low ROS formation (low metabolic rate) and high damage repair/removal capacities. © Inter-Research 2012.