We investigated mitochondrial ageing in a temperate (Mya arenaria) and an Antarctic (Laternula elliptica) mud clam, with similar lifestyle (benthic filter feeders) but different maximum life spans (MLSP), 13 and 36 years, respectively. The short-lived temperate M. arenaria showed a more pronounced decrease in mitochondrial function (respiration, respiratory control ratio, proton leak, membrane potential) with age than the long-lived Antarctic L. elliptica. H2O2 generation rates at habitat temperature were far higher in the short-lived M. arenaria compared to L. elliptica. Reactive oxygen species (ROS) production as proportion of the mitochondrial oxygen consumption rate (%H2O2/O2) increased significantly with age in M. arenaria, whereas in L. elliptica the proportion remained unchanged. Lower rates of mitochondrial H2O2 generation were presumably due to mild uncoupling as L. elliptica mitochondria showed higher proton leak compared to M. arenaria mitochondria. The results are discussed in to the light of the "Free Radical-Rate of Living theory", (Pearl, R., 1928. The Rate of Living. Alfred Knopf, New York; Harman, D., 1956. Aging: a theory based on free radical and radiation biology. J. Gerontol. 11, 298-300) and the "Uncoupling to Survive" hypothesis (Brand, M.D., 2000. Uncoupling to survive? The role of mitochondrial inefficiency in ageing. Exp. Gerontol. 35, 811-820). © 2005 Elsevier Ireland Ltd. All rights reserved.