Latitudinal life history gradients in two Pliocene species of Glycymeris (Bivalvia)

Modern bivalves display a latitudinal life history gradient (LLHG): tropical bivalves tend to grow fast and die young, whereas mid- and high-latitude bivalves typically grow more slowly and may live much longer. Environmental factors such as temperature and seasonal food availability, both of which affect metabolic rates, are thought to be partially responsible for this pattern. Given that temperature influences life histories, we predict that the expression of individual life history gradients should vary over time with changes in global climate. Here, we use internal growth increments to constrain lifespans and growth rates in populations of Glycymeris americana and Glycymeris subovata along the Pliocene and Pleistocene Atlantic continental shelf of North America. We find that G. americana was long-lived (up to 93 years) and follows the expected pattern of longer life and slower growth at higher latitudes, whereas the shorter-lived (up to 36 years) G. subovata does not. G. americana life history data lend preliminary support for a change in the slope of the LLHG from the Pliocene to the Pleistocene. While G. americana is currently living along the U.S. Atlantic Coast, we were unable to obtain sufficient samples for our analysis; this represents a future area of research.