Arctica islandica lays down a growth increment in its shell each year, thicker in good years and thinner in poor ones. Because the shell is dense and long-lasting, those increments can be sectioned, polished and counted, and cross-matched between individuals in the same way dendrochronologists cross-date tree rings.
A specimen dredged off Iceland in 2006 was initially aged at about 405 years from increments read at the shell edge. A later re-examination using the hinge region, where increments are less compressed, revised the age upward to 507 years, placing its settlement around 1499. The animal died during collection and processing, a detail that is frequently and fairly noted.
The record applies to individual, non-colonial animals. Colonial organisms and clonal systems reach far greater ages by different accounting, and some deep-sea corals and sponges are older as colonies. The quahog's longevity is not a freak: the species routinely lives for centuries, which is exactly why its shells are valuable archives of past North Atlantic sea temperatures.