LATITUDINAL PATTERNS IN LIFESPAN AND GROWTH RATE ACROSS MODERN MARINE BIVALVES

Lifespans of fossil bivalves represent an almost untouched field of study despite the fact that the longest lived non-colonial animal on the planet today, Arctica islandica, is a bivalve with a maximum reported lifespan of 507 years and an extensive geologic history. At least a dozen bivalve taxa are known to attain lifespans in excess of 100 years. What factors might be involved in this impressive longevity? A handful of mostly anecdotal studies suggest that the longest lived individuals within a taxon tend to be found at the highest latitudes of their geographic range – implying a role for the environment. However, the pattern across the Bivalvia has not been fully documented. Here, we present the results of an extensive literature search for data on modern bivalve lifespans and growth rates across the globe in order to better understand the factors involved in extreme longevity. We completed exhaustive searches on the entire publication runs of over 35 biological and fisheries-related journals and compiled a database consisting of 1,211 entries spanning 242 marine species and fully half of the described living bivalve families and ranging from tropical to polar populations. Our results show a clear increase in the mean and variance of maximum reported lifespan with increasing latitude. Tropical bivalves typically live for less than ten years, whereas mid- and high latitude bivalves live significantly longer. In addition, growth rates are significantly higher and more variable than those in the mid- and high latitudes. Though sample size in the tropics is lower, these patterns are robust and cannot be ascribed to sampling inhomogeneities. Lifespan generally increases, and growth rate decreases, with latitude. An environmental parameter related to latitude and therefore insolation, such as temperature or primary production/food availability, acting through its effect(s) on metabolic rate, likely explains the observed trends. If this pattern so universally characterizes bivalve taxa today, it likely did in the past as well. To what degree does this spatial pattern in life-history traits hold true through time? As lifespan and growth rate are tied to metabolism, and metabolism has been implicated in a number of macroevolutionary trends, what are the implications of this pattern for the fossil record of bivalves?