Age determinations of polar marine ectotherms using carbon isotope ratios and seasonal growth bands of hard structures, fish otholids or bivalve shells, indicate some polar species to live longer than their temperate relatives. Here we examined physiological ageing parameters in mantle tissue of different sized animals from two species of the same ecotype but with different maximum life spans (MLSP), the polar mud clam Laternula elliptica (MLSP: 36years) from the Antarctic Peninsula (King George Island) and the temperate mud clam Mya arenaria (MLSP: 10-12 years) from the North Sea, to characterize the physiological ageing process as a possible explanation for higher longevity in polar animals.We found lower mitochondrial oxidative capacity (nmol O2/ mg prot), energetic coupling and membrane potential and a higher % fraction of proton leak in older age clams of both species. Changes were more apparent in the temperate than the polar clam. In vitro H2O2 generation of succinate respiring mitochondria was 2 to 10-fold higher in the temperate compared to the polar clam and increased significantly with ageing. Antioxidant capacities were higher in the polar clam.Mitochondria of temperate mud clams show a pronounced loss of functioning, accelerated ROS release and their cellular redox milieu became more oxidized with ageing. Polar mud clams preserve cellular redox potential and mitochondrial functioning better in aged specimens and limit the production of mitochondrial ROS to lower levels. Together with a well developed AOX system, this may enable these animals to reach older ages than their temperate relatives.