Seasonal timing of growth cessation in fossil Mercenaria spp. shells from warm vs. cold climates, Mid-Atlantic Coastal and Gulf Coastal Plains

The northern and southern hard clams, Mercenaria mercenaria and M. campechiensis respectively, are economically and ecologically important bivalves harvested along the eastern seaboard of the United States. Despite their economic importance, the impacts of climate and environmental change on Mercenaria remain unclear. In part because of their ecological and economical importance, growth patterns (sclerochronology) in modern Mercenaria have been long studied throughout their biogeographic range. Modern Mercenaria shells from the Mid-Atlantic and Gulf Coastal Plains record changes in the timing of seasonal growth cessation with latitude. Shells from the warm-temperate biogeographic province (low latitudes) tend to have slow growth in the summer and fast growth in the winter, whereas the opposite pattern has been observed in specimens from the cold-temperate zone (mid-high latitudes). It is unknown, however, if these patterns persist in fossil Mercenaria spp. from climate conditions warmer and cooler than today. This study utilizes isotope sclerochronology to identify the timing of seasonal growth cessation in fossil shells from mid latitudes (North Carolina) and low latitudes (Florida) during warm (Mid-Pliocene Warm Interval) and cool (early Pleistocene) climate intervals. We test the hypotheses that: (1) mid Pliocene Mercenaria shells from North Carolina (Duplin Fm) and Florida (Tamiami Pinecrest Beds) exhibit summer growth cessation (warm-temperature stress); (2) early Pleistocene Mercenaria shells from North Carolina (Waccamaw Fm) exhibit winter growth cessation (cold-temperature stress); and (3) like the mid Pliocene shells, early Pleistocene shells from Florida (Caloosahatchee Fm) exhibit summer growth cessation. These findings contribute to a larger conservation paleobiology context that will further our understanding of the biological responses of marine bivalves to future seawater warming.