238-1 Size matters: Investigating mechanisms behind body size trends in three species of Glycymerid bivalves from the Atlantic Coastal Plain

Abstract:

A major pattern over the Phanerozoic is that of increasing body sizes in marine organisms.  Larger body sizes could be driven by faster growth or longer lifespan, but so far little work has been done to investigate changes in life history strategy. Furthermore, such patterns could be the result of heterochronic processes, but understanding the nuanced patterns requires organisms which record age-at-size in their skeletal structures. Fortunately, bivalves offer an avenue to do just that. Here we present preliminary data using three species of Glycymerid bivalves: Glycymeris anteparilis (Oligocene), G. parilis (Miocene), and G. americana (Pliocene and Pleistocene). These species allow us to examine trends between an ancestor-descendant pair (G. anteparilis and G. parilis) and within a species over time (G. americana). To do this, we first measured the heights (anterior to posterior) of specimens from museum collections using a digital caliper to confirm reported body size increases. Next, we made polished thick sections to count and measure internal annual growth increments on selected individuals. Finally, we fit a von Bertalanffy growth equation to these data to compare the parameters k (a proxy for growth rate) and H∞ (body size), and to compare the ages of taxa at 50% of H∞, which is a proxy for reproductive maturity. Our results confirm increasing body size trends in these three species: G. anteparilis (n = 29, max = 56.75 mm, mean = 35.30 mm) is smaller than its descendant G. parilis (n = 53, max = 93.18 mm, mean = 40.66 mm), whereas Pliocene G. americana (n = 17, max = 73.96 mm, mean = 66.63) are smaller than Pleistocene individuals (n =40, max = 113.78 mm, mean = 75.72 mm). In both pairs, descendant populations grew slower and lived longer than ancestral populations (G. anteparilis, k=0.25, max lifespan = 23; G. parilis, k=0.08, max lifespan = 87; Pliocene G. americana, k= 0.08, max lifespan = 78; Pleistocene G. americana, k=0.03, max lifespan =93). In addition, the age at 50% of H∞, is reached at a later age in descendants. In the taxa studied here, larger body sizes are attained in descendant species through hypermorphosis whereby, descendants maintain juvenile growth rates longer, reach maturity later, and live longer than their ancestors.

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