CONTEMPORANEOUS DEPOSITION OF ANNUAL GROWTH BANDS IN MERCENARIA-MERCENARIA (LINNAEUS), MERCENARIA-CAMPECHIENSIS (GMELIN), AND THEIR NATURALHYBRID FORMS

We analyzed the periodicity of annual band deposition in the shells of each of three hard clam genotype classes (Mercenaria mercenaria (Linn aeus), Mercenaria campechiensis (Gmelin), and hybrid forms of the two species) collected from the Indian River lagoon, Florida. Terminal gro wth bands from each of 396 genetically identified specimens were studi ed, using the translucent-opaque staging method, to determine the seas onality of rapid vs. slow shell growth. The pattern of terminal growth -band formation that we describe for M. mercenaria from the Indian Riv er is similar to patterns observed for conspecifics collected from Geo rgia and North Carolina; the translucent growth band (indicative of re latively slow shell growth) is formed during summer and fall, and the opaque growth band (indicative of relatively rapid shell growth) is fo rmed during winter and spring. During summer and fall, growth-band for mation in M. campechiensis and hybrid forms is similar to that observe d for M. mercenaria. In contrast, during winter and spring subtle but significant deviations from the M. mercenaria growth pattern were note d. For M. campechiensis, relatively equal proportions of the transluce nt and opaque growth stages were recorded among specimens collected du ring both the winter and spring seasons. Hybrid forms exhibited a seas onal pattern of terminal growth-band formation reflecting characterist ics of each of the two parental species. During winter, translucent an d 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 consistenc y that we describe among genotype classes in the seasonal pattern of g rowth-band formation suggests that previously described geographic dif ferences in those patterns are primarily mediated by environmental fac tors rather than by inherent genotypic differences among populations. Nevertheless, subtle differences in the proportions of opaque vs. tran slucent 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 seasona lity of growth has occurred during the evolution of these two species. (C) 1998 Elsevier Science B.V.