GENOTYPE SPECIFIC GROWTH OF HARD CLAMS (GENUS MERCENARIA) IN A HYBRIDZONE - VARIATION AMONG HABITATS

Shell growth rate is an important component of fitness in bivalve moll uscs, Using the omega parameter computed from the von Bertalanffy grow th equation, we quantitatively compared rates of annual shell growth a mong the hard clams Mercenaria mercenaria, M. campechiensis, and their hybrids sampled from a variety of habitats in the Indian River lagoon , Florida, USA, a zone of species overlap and natural hybridization. O ur results indicate that the classical paradigm describing hard clam g rowth, in which growth rate is fastest in M. campechiensis, intermedia te in hybrids, and slowest in M. mercenaria, is not supported in the I ndian River lagoon. Instead, M. campechiensis has a growth advantage i n deep-water habitats in the northern section of our study area. In th e central and southern sections of our study area, hybrids have a grow th advantage over M. mercenaria in shallow-water habitats, but M. merc enaria has a growth advantage over hybrids in deep-water habitats. In all other sampled habitats, either growth rate among genotype classes is equal, or M. mercenaria has a growth advantage. This complex relati onship between genotype and habitat-specific growth provides a mechani sm for selection to act on hard clams in the Indian River.