You might recall that some species of clam live for as long as four centuries, and possibly longer. Others do not. Such differences between closely related species are an opportunity for researchers to uncover important mechanisms of longevity. From Ouroboros: "This invertebrate group includes species with the longest metazoan lifespan approaching 400 y, as well as species of swimming and sessile lifestyles that live just for 1 y. Bivalves from natural populations can be aged by shell growth bands formed at regular intervals of time. ... Extreme longevity of some bivalve models may help to analyze general metabolic strategies thought to be life prolonging, like the transient depression of metabolism, which forms part of natural behaviour in these species. ... One of the great advantages of bivalves is their variety: even though they're anatomically quite similar, they occupy a wide range of niches and consequently exhibit a large variation in aspects of their natural histories, including longevity. This makes clams and oysters excellent candidates for comparative biogerontology: studying organisms with basically identical body plans but wildly different lifespans allows us to focus more tightly on the features (molecular, cellular, systemic) that might explain the change in longevity. This theme is currently being developed - outside the mollusk community - into a large-scale project that will study dozens of species in four or five vertebrate clades."