COMPARING LIFE HISTORY PATTERNS IN SHELLS OF CO-OCCURRING MERCENARIA MERCENARIA AND M. CAMPECHIENSIS FROM FLORIDA AND NORTH CAROLINA, USA

Mercenaria mercenaria and M. campechiensis are ecologically and economically important bivalves found along the US Mid Atlantic and Gulf Costal Plains. Morphological characteristics have been used to distinguish between the two species. These characteristics are not always diagnostic of genotype because the two species can hybridize where they co-occur, and some morphologic characteristics may not be preserved in fossil specimens (e.g., pigmentation and ridges). Such uncertainty can potentially have important implications on life history studies of fossil and archaeological Mercenaria shells. This study tests the hypothesis that modern M. mercenaria and M. campechiensis shells from the same location will have similar life history patterns. We test this hypothesis using the von Bertalanffy growth (VBG) equation to assess growth rate and maximum lifespan (MLSP) between co-occurring species. We collected specimens alive from Wilmington, NC (n = 40) and Pineland Island Sound (PIS), FL (n = 8). Genotype analysis has not yet been performed. We also analyzed M. mercenaria and M. campechiensis shells from nearby Sanibel-Captiva Island (SCI), FL collected in the 1930s (provided by the Florida Museum of Natural History) (n = 14 for each species). Average MSLP of the NC and PIS populations are 15.6±4.6 and 11.3±1.4, respectively. MSLP of shells from the museum collections average 15.1±5.0 for M. mercenaria and 25.5±6.5 for M. campechiensis. Best fit VBG curves show differences in k values (a proxy for growth rate) between our live-collected specimens and those collected in the 1930s: NC (0.30); PIS (0.41); SCI M. mercenaria (0.19); SCI M. campechiensis (0.11). The similar k values of the two species from the museum collections suggest little impact of genotype on growth rate. Future work will identify species from our live-collected specimens using a combination of electrophoresis and morphologic characteristics allowing us to validate the assumption that species-level identification between M. mercenaria and M. campechiensis from fossil and archaeological deposits for life history studies is not required.