There have been a number of potential contenders for this crown including Giant tortoises, large whales, Asian elephants and some reptiles and fish but none of these animals has been shown to live for more than 200 years. This is not the case with the Ocean quagog (Arctica islandica), a bivalve that frequently lives for several centuries. The oldest known specimen from Iceland was shown to have lived for 507 years. How can we be so precise about its age? The answer is that bivalves, like trees, deposit annual growth increments that can be counted and crossmatched enabling the construction of long chronologies extending back hundreds or even thousands of years. The fact that the same pattern of growth increments is recognisable within populations, and even between species, demonstrates conclusively that the bivalves are responding to identical environmental signals. Such signals include water temperature and nutrient availability, as well as other factors. Knowledge of these controlling factors enables the reconstruction of past oceanic conditions at annually-resolved resolutions, so providing critical archives for the study of climate change and variations in the position and strength of ocean currents and in water chemistry. The fact that these bivalves live so long has also attracted attention from gerontologists interested in understanding why these particular animals do not die in the usual way.