326-11 A Tale of Two Clams: Investigating Body Size Changes in Glycymerididae and Lucinidae from the Caribbean Neogene

Abstract:

Paleontologists utilize body size as a proxy for productivity, as body size generally scales with nutrient demands; therefore, larger body sizes are generally found in higher productivity regions. Body size may also be an indicator of trophic position/ecological structure and is also a character on which natural selection acts. Despite the importance of body size data to paleontology, we still know little about how body size frequency distributions vary during periods of environmental change or across productivity gradients. The marine fossil record of the SW Caribbean, which captures the time of the formation of the Isthmus of Panama, provides a perfect opportunity to determine how changes in primary productivity impacted the body size of bivalves. After the Caribbean was isolated from the Pacific ~3.5 million years ago, upwelling in the Caribbean decreased, leading to lower planktonic productivity of the region and, therefore, less food sources for organisms, especially suspension feeders. We present body-size data from two families of fossil bivalves, Lucinidae and Glycymerididae, to understand how average body size has been impacted by planktonic productivity. Lucinidae are chemosymbiotic bivalves, as they have a symbiotic relationship with sulfur-oxidizing bacteria housed in their gills that provide them with energy. In contrast, Glycymerididae are suspension feeders, which means that their main source of energy comes from filter feeding. These specimens were collected from localities in Panama and Costa Rica and represent time periods before, during, and after the closure of the Isthmus of Panama. We obtained stacked digital images of individuals, then used the digital measurement software AutoMorph to obtain body size (length, height, width) from each image. Body size data were used to determine the mean body size and size frequency distributions for each family at each sample location. These size frequency distributions through time can be used to test our two hypotheses: 1) As productivity in the Caribbean decreased average body sizes of bivalves decreased and 2) The chemosymbiotic relationship may insulate Lucinidae from productivity changes, resulting in no change or even an increase in average body size. This research will help us to better understand the relationship between primary productivity and body size in marine benthic communities, which has implications for ecological and evolutionary responses of modern bivalves to a changing ocean environment.

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