Contemporaneous deposition of annual growth bands in Mercenaria mercenaria (Linnaeus), Mercenaria campechiensis (Gmelin), and their natural hybrid forms

We analyzed the periodicity of annual band deposition in the shells of each of three hard clam genotype classes (Mercenaria mercenaria (Linnaeus), Mercenaria campechiensis (Gmelin), and hybrid forms of the two species) collected from the Indian River lagoon, Florida. Terminal growth bands from each of 396 genetically identified specimens were studied, using the translucent-opaque staging method, to determine the seasonality of rapid vs. slow shell growth. The pattern of terminal growth-band formation that we describe for M. mercenaria from the Indian River is similar to patterns observed for conspecifics collected from Georgia and North Carolina; the translucent growth band (indicative of relatively slow shell growth) is formed during summer and fall, and the opaque growth band (indicative of relatively rapid shell growth) is formed during winter and spring. During summer and fall, growth-band formation in M. campechiensis and hybrid forms is similar to that observed for M. mercenaria. In contrast, during winter and spring subtle but significant deviations from the M. mercenaria growth pattern were noted. For M. campechiensis, relatively equal proportions of the translucent and opaque growth stages were recorded among specimens collected during both the winter and spring seasons. Hybrid forms exhibited a seasonal pattern of terminal growth-band formation reflecting characteristics of each of the two parental species. During winter, translucent and opaque terminal growth bands were equally represented among hybrids (as in M. campechiensis), whereas in spring significantly more hybrids were in the opaque growth stage (as in M. mercenaria). The consistency that we describe among genotype classes in the seasonal pattern of growth-band formation suggests that previously described geographic differences in those patterns are primarily mediated by environmental factors rather than by inherent genotypic differences among populations. Nevertheless, subtle differences in the proportions of opaque vs. translucent growth bands observed between species during the rapid-growth stage and the intermediate nature of the hybrid growth pattern at that time suggest that some genetically mediated divergence in the seasonality of growth has occurred during the evolution of these two species. (C) 1998 Elsevier Science B.V.