THE ROLE OF ENVIRONMENT IN THE EVOLUTION OF EXTREME LONGEVITY IN BIVALVES

What controls lifespan? Bivalves hold the record for longevity among animal species - Arctica islandica is the longest lived non-colonial animal on the planet, with a maximum reported lifespan of 507 years, and a number of taxa are known to live more than a century. This group may therefore provide significant insights into the factors that promote long life, a topic of great interest from our perspective as human beings. While a good deal of attention is being devoted to physiological aspects, very little is known about the evolution of extreme longevity as a viable life history strategy, either within or between species and virtually no work has been done on its fossil record. Paleontological research has the potential to contribute on two key fronts: the role of environment and the influence of phylogeny in facilitating long life. Here, we focus on the former. Anecdotal data suggest that environment is important because no centenarians are known within 40° of the equator. What is it about the high latitudes that allows for greater longevity? A compilation of maximum reported lifespans of modern A. islandica throughout its geographic range in the North Atlantic reveals a negative exponential relationship between mean annual sea surface temperature and maximum lifespan. As metabolism is exponentially related to temperature, and metabolic rate seems to inversely correlate with longevity, these data support a role for low temperatures in facilitating longevity. However, temperature cannot be the whole story. Multiple unrelated taxa from the high-latitude but warm paleoenvironments of the Eocene and Cretaceous of Seymour Island, Antarctica, also show surprising longevities, with several approaching centenarian status. In this non-analog setting, we attribute extreme longevity to low and highly seasonal availability of food associated with the high-latitude light regime. ‘Caloric restriction’ has been found to correlate with extended lifespans in modern taxa. Long life in each case could be ecophenotypic, simply a side consequence of reduced metabolic rate. Alternatively, extreme longevity could be an adaptive response to settings where reproduction is seasonal and recruitment unlikely, and where predation pressure is low enough to accommodate the delayed maturation generally associated with longer lifespans.