The ocean quahog Arctica islandica is the longest-lived of all bivalve and molluscan species on earth. Animals close to 400 years are common and reported maximum live span around Iceland is close to 400 years. High and stable antioxidant capacities are a possible strategy to slow senescence and extend lifespan and this study has investigated several antioxidant parameters and a mitochondrial marker enzyme in a lifetime range spanning from 4-200 years in the Iceland quahog. In gill and mantle tissues of 4-192 year old A. islandica, catalase, citrate synthase activity and glutathione concentration declined rapidly within the first 25 years, covering the transitional phase of rapid somatic growth and sexual maturation to the outgrown mature stages (approximately 32 years). Thereafter all three parameters kept rather stable levels for > 150 years. In contrast, superoxide dismutase activities maintained high levels throughout life time. These findings support the 'Free Radical-Rate of Living theory', antioxidant capacities of A. islandica are extraordinarily high and thus may explain the species long life span.